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MOSAIC DISEASE OF SU(;AR CANE
1. Typical mosaic on dark-green leaf. 2. Typical mosaic on
light-green leaf. 3. Effect developed on certain varieties.
4. Type of spotting often mistaken for mosaic.
Frontispiece
DISEASES OF CROP-PLANTS
IN THE
LESSER ANTILLES
BY
WILLIAM NOWELL, D.I.C.
Assistant Director of Agriculture, Trinidad and Tobago ;
late Mycologist on the Staff of the Imperial Department of
Agriculture for the West Indies
WITH A FOREWORD BV
PROFESSOR J. BRETLAND FARMER, F.R.S.
PUBLISHED ON BEHALF OF THE IMPERIAL DEPARTMENT
OF AGRICULTURE
BY
THE WEST INDIA COMMITTEE
14 TRINITY SQUARE, LONDON
/ n ^ "
FOREWORD
It is with no ordinary feeling of pleasure that I have fallen
in with my friend Mr. Nowell's suggestion that I should write
a foreword to his book, although his own established reputation,
based on brilliant elucidations of various difficult problems in
plant pathology makes the attempt perhaps less easy and
certainly more superfluous than it might otherwise have been.
Readers will find in this book much more than is often regarded
as sufficient to fill a volume purporting to deal with plant diseases.
It might, indeed, perhaps be more correctly designated as a
treatise on the principles of plant pathology with special
reference to the diseases occurring in the West Indies. These
islands, with their wide range of soil, with their varied climatal
and other environmental conditions, are peculiarly well fitted
to provide that broad outlook over the larger matters of pathology
which is too apt to escape those who are treating plant diseases
from a more purely practical and local standpoint. Mr. Nowell
has embraced the opportunity which lay to his hand, and has
produced a work which in my judgment constitutes a real land-
mark of progress in the science of the plant considered in relation
to health and disease. As might have been expected from one
who has successfully grappled with the interesting and important
jetiology of Red Ring in Coconut, with the real nature of the
damage wrought by the Cotton Stainer (Dysdercus) and its
remarkable parallelism to malarial infection in the human
subject, to say nothing of his work on RoseUinia and other pests,
the book will be found of great value to the practical agriculturist
as well as to the scientific investigator, who looks, or should look,
farther afield. But it is in the philosophical treatment of his
subject as a whole, as well as in the cautious way in which the
principles themselves are put forward, that the wider interest of
the author's work as a whole is to be attributed. As is well said
in the text, " In every case of parasitism there are two organisms
to be considered, the host and the parasite. ..." In a number
of instances it is shown how the incidence of a particular disease
depends on external conditions, and the way is thus paved for a
rational treatment of an infestation which depends upon a
knowledge of what might be called the agricultural physiology
of the plant itself. If I may be permitted to recall something of
what I have myself observed, while in the West Indies, I might
cite the froghopper attack on sugar-cane as an instance of the
V
vi FOREWORD
point here referred to. There are, in Trinidad, cane areas where
variations in agricultural practice have resulted in practical
immunity from " Froghopper Blight," yet, in one instance at
least, the immune area was contiguous with plantations, other-
wise similar, in which the pest had assumed serious dimensions.
Further illustrations are supphed, in the body of the work, by
the class of " debility diseases," where a sound knowledge of
agricultural physiology has been able to indicate satisfactory
methods of coping with the onset of the pests.
Of course the matter is not always so easily brought into line
with agricultural practice, and the careful reader will observe
the judicious attitude maintained by the author in respect of
'Otiier diseases in which the factors are not so simple or so suscept-
ible of control. But it is safe to say that no one can peruse the
work, more especially if he happens to possess a first-hand
acquaintance with the problems under discussion, without feeling
that in this book at any rate a successful attempt has been
made to grapple with the scientific principles of some of the
/larger issues that form the background of plant pathology.
It is only in this way, i.e. by an intelligent appreciation and
application of principles that the ravages caused by the many
and various diseases to which cultivated plants, especially in the
tropics, are continually liable can ever be satisfactorily controlled.
J. BRETLAND FARMER.
Imperial College of Science and Technology,
London.
AUTHOR'S PREFACE
This work was prepared as a handbook to the diseases affect-
ing crop plants in the Lesser Antilles at the request of the Imperial
Commissioner of Agriculture for the West Indies, Sir Francis
Watts, K.C.M.G., during the writer's tenure of the post of
Mycologist on the staff of the Imperial Department (1913-1920).
Full use has been made of the work of previous occupants of
this position, namely : Messrs. A. Howard (1901-1902), L.
Lewton-Brain (1902-1905), F. A. Stockdale (1905-1909), and
F, W. South (1909-1913). The large obligations due to the
published papers of plant pathologists in other parts of the West
Indies and the Tropics generally are indicated in the text.
The immediate aim has been to provide for the agricultural
officer and the planter a means of reference to the present state
of knowledge respecting specific diseases, and further to enable
him to compare his experience with the general body of knowledge
concerning plant pathology and its relation to agricultural
practice. The need for a local textbook of the subject in the
new West Indian Agricultural College has also been kept in
mind.
Certain diseases have been included which have not occurred
in these islands, but are of interest in view of the possibility of
their appearance or recognition in the future.
As the result of personal experience of which the whole
tendency has been to compel a mycologist to become an agri-
culturist, the writer is convinced of the need for a broad treat-
ment of his subject. The mycologist is most concerned with
the fungus, the pathologist with the disease, and the agriculturist
with the crop, but a combined view of the interests of aU three
is necessary for the production of an economic plant pathology.
The writer is conscious that the present work is more successful
in revealing the want of knowledge on very many subjects than
in imparting it, but has the hope that even this function may
be found useful as establishing a datum line for future investiga-
tions.
The thanks of the writer are due for critical perusal of the
manuscript to Mr. C. B. Williams, late Sugar-Cane Entomologist
in the Trinidad Department of Agriculture (the whole text).
Dr. S. C. Harland, late Assistant for Cotton Research in the West
Indies (Part I and the chapter on diseases of cotton), and Mr.
S. F. Ashby, late Microbiologist to the Jamaica Department of
viii AUTHOR'S PREFACE
Agriculture (Part II) : to Sir Francis Watts for valuable help in
preparing the chapter on fungicides ; to Miss E. M. Wakefield,
of the Royal Gardens, Kew, for kindly assisting in proof
reading, and Mr. S. F. Ashby, for assistance with the biblio-
graphy and illustrations ; to Dr. J. B. Farmer, F.R.S., Professor
of Botany at the Imperial College of Science and Technology,
not only for the immediate contribution of an introduction, but
in a special sense as the director of a course of biological
training in which information is held subordinate to essential
principles. The writer must further gratefully acknowledge
that without the constant assistance of his wife the labour
of writing could never have been faced in the time available
from other duties.
MAP OF THE LESSER ANTILLES.
CONTENTS
PART I
INTRODUCTION :
THE NATURE AND CLASSIFICATION OF PLANT DISEASES
SECTION I.— CAUSATION
CHAPTER I : DISEASES CAUSED BY FUNGI.
The Nature of Fungi. Page
The Mycelium. The Place of Fungi in Nature- - - 3
The Nature of Fungus Parasitism ----- 5
Reproduction and Infection ___-_- 6
The Host : Resistance and Susceptibility.
Effect of Agricultural Conditions on Resistance. Immunity
by Evasion. Resistant Species or Varieties. Susceptibility
to Exotic Diseases. Local Factors affecting Prevalence - 8
The Principal Types of Fungus Diseases.
Leaf Diseases. Flower and Fruit Diseases. Stem Diseases.
Root Diseases --------- 12
Notable Groups concerned in Fungus Diseases.
The Rusts (Uredinales). The Smuts (Ustilaginales). The
Powdery Mildews (Erysiphaceae). The Downy Mildews
(Phytophthora). The Anthracnoses. The Fusariums. The
Diplodias. The Rhizoctonias. The genus Marasmius. The
Fungi infecting Bug Punctures (Stigmatomycosis). The
Sooty Moulds (Capnodiae) - - - - - - 17
The Classification of Fungi ------ 34
CHAPTER II : DISEASES CAUSED BY BACTERIA - - 45
CHAPTER III : DISEASES CAUSED BY INFECTIVE VIRUSES 48
CHAPTER IV: PHANEROGAMIC PARASITES.
Love Vine, Vermicelle ------- 54
Cassytha _-___---- 56
Bird Vine, Mistletoe __---__ 56
Alectra, Cane-Killer ---____ 58
xi
xii DISEASES OF CROP-PLANTS
CHAFIER V : DISEASES DUE TO NEMATODES (EELWORMS).
Heterodera radicicola, Greef. Tylenchus devastatrix, Kuhn. Page
Tylenchus tritici. Tylenchus angustus, Butler. Tylenchus
ribes, Taylor. Aphelenchus cocophila, Cobb. Tylenchus
similis, Cobb and T. musicola, Cobb _ _ _ _ ^g
CHAPTER VI : THE RELATION OF INSECTS TO PLANT
DISEASES.
Insects as the Cause of Injuries and Diseases. Stigmonoses.
Insect Injuries as Openings for Infection. Insects as Carriers
of Infection. Susceptibility and Resistance to Insect Infes-
tations _____-_--_ 66
CHAPTER VII: NON-PARASITIC DISEASES - - . 69
CHAPTER VIII : ENTOMOGENOUS FUNGI - - - 73
SECTION II —PREVENTION AND CONTROL
CHAPTER IX: GENERAL CONSIDERATIONS.
Arable Crops and Parasitic Diseases. Sources of Infection.
Means of avoiding Damage. Resistance. Disease Evasion.
Elimination Methods. Spraying. Permanent Crops - - 81
CHAPTER X : FUNGICIDES AND THEIR APPLICATION.
The Copper Compounds.
Bordeaux Mixture -------- 88
Bordeaux Paste __-___-_ gi
Burgundy Mixture -------- 91
Copper Sulphate Wash --.__-_ 93
Cuprammonium Washes ------- 93
Sulphur Fungicides.
Sulphur ---------- 94
Lime-sulphur Solution --_-___ g^
Self-boiled Lime-sulphur ------- 96
Liver of Sulphur ___---__ 97
Ammonium Sulphide ------- 97
Iron Sulphide ________ 97
Potassium Permanganate --____ g7
Spraying.
Spray Injury --------- gS
Dusting _-__-_-___ gS
Disinfectants.
Corrosive Sublimate -____ — - gg
Formalin -__-____- 100
CONTENTS xiii
Page
Sulphuric Acid ________ loi
Copper Sulphate ________ loi
Bleaching Powder __--_--_ loi
Eau de Javel ---_----- loi
CHAPTER XI : WOOD-ROTS ; THE TREATMENT OF TREES.
Structure. Wood-rots. The Healing of Wounds. The Treat-
ment of Wounds. Antiseptics. Protective Paints - - 102
CHAPTER XII : THE CONTROL OF DAMPING-OFF - 107
CHAPTER XIII : PREVENTION OF FRUIT ROTS - - HO
CHAPTER XIV : PLANT DISEASE LEGISLATION.
Plant Importation. Internal Regulation - - - ~ 112
PART II
INTRODUCTION :
AGRICULTURAL CONDITIONS IN THE LESSER ANTILLES
SECTION I.-GENERAL DISEASES
CHAPTER XV: ROOT DISEASES,
Page
RoselUnia Root Diseases. Sclerotium Rots. Pomes Root Rot 126
CHAPTER XVI : STEM AND LEAF DISEASES.
Pink Disease. Thread Blights. Horsehair Blight. Algal Disease 148
SECTION II.— DISEASES OF PERMANENT CROPS
CHAPTER XVII : DISEASES OF CACAO.
Dieback. Diplodia Pod Rot. Phytophthora Pod Rot and
Canker. The Surinam Witch-broom Disease. Monilia Pod
Rot. Sphaeronema Black Spot and Bark Rot. Anthracnose.
"Male" Cacao. Woody Excrescences. {Root Diseases.
Algal Disease. Thread Blight. Horsehair Blight — See
General Diseases) - - - - - - - - 155
CHAPTER XVIII : DISEASES OF COCONUT.
The Red Ring Disease. Bud-rot in general. Bacterial Bud-rot.
Bud-rot caused by Phytophthora palmivora. Leaf-stalk Rot
caused by Phytophthora parasitica. Stem Bleeding Diseases.
Leaf-dwindling or Little-leaf Disease. Bitten-Ieaf Disease.
Dieback. Leaf Blight - - - - - - - 177
CHAPTER XIX : DISEASES OF LIME AND OTHER CITRUS
TREES.
Introduction -----__-__ 19
Limes.
Diplodia Dieback. Deficiency Dieback. {RoselUnia Root
Disease — see General Diseases). Red Root Disease. Collar
xiv
CONTENTS XV
Canker. Branch Galls. Anthracnose of Lime. Grey Blight. Page
Leaf Spots. {Seedling Diseases — see Damping ofE) - - 196
Citrus Trees in General.
Citrus Anthracnose. Gummosis. Foot-rot or Mal-di-goma.
Scaly Bark. Scab. Black Melanose. Citrus Canker. Diseases,
Injuries and Rots of Citrus Fruit - - - _ _ 210
CHAPTER XX : DISEASES OF COFFEE.
Viruela, American Leaf Disease. Brown Eye-spot. Zonal Leaf
Spot. Bark Disease. Nematode Diseases. Sclerotium
Disease of Liberian Coffee. {Rosellinia Root Disease. Thread
Blights. Algal Disease — see General Diseases) - - _ 225
CHAPTER XXI : DISEASES OF HEVEA.
The South American Leaf Disease — — — — — — 232
CHAPTER XXII : DISEASES OF MINOR FRUIT TREES.
Avocado.
Anthracnose --__--___ 235
Date Palm.
Leaf Disease. Fomes Stem-rot _____ 236
Grape- Vine.
Anthracnose. Powdery Mildew. Rust _ - _ _ 236
Mango.
Anthracnose. _______ 237
Nutmeg.
Canker. Perforated Leaves. {Red Rust. Rosellinia Root
Disease. Thread Blight — see General Diseases) _ - - 239
SECTION III.— DISEASES OF ARABLE CROPS
CHAPTER XXIII : DISEASES OF BANANA.
Banana Wilt, Panama Disease. Marasmius Root Disease and
Stem Rot. The Moko Disease. Eelworm Black-rot. Black
Spot of Leaves. Bonnygate Disease. Blackhead Disease.
Heart-leaf Disease -------- 241
CHAPTER XXIV : DISEASES OF CORN AND SORGHUMS.
Corn.
Brown Rust. Red Rust. Smut. Head Smut. Leaf Scorch.
Dry Rot. Root Disease. Minor Diseases _ _ _ 256
Sorghums.
Rust. Kernel Smut. Head Smut. _ _ _ 261
CHAPTER XXV : DISEASES OF COTTON.
Internal Boll Disease. Angular Spot. Black Arm. Bacterial
Boll Disease. Soft Rot of Bolls. Diplodia Boll Rot. Anthrac-
nose. Curly Leaf, Chibble Leaf. Loggerhead. Rust.
Red Leaf Blight. Yellow Leaf Blight. Macrosporium Leaf-
spot. Altemaria Leaf-spot. Leaf Mildew. Crinkled Dwarf,
Man Cotton, Mottled Hybrids - - - - - - zf^
XVI DISEASES OF CROP-PLANTS
CHAPTER XXVI : DISEASES OF SUGAR CANE.
Root Disease (General type). Root Disease (Acute type). Page
The Epidemic on the Bourbon Cane. Red Rot. Rind Fungus.
Pineapple Disease of Cuttings. Diplodia Rot. Wilt Disease.
Top Rot (Bud-rot). Gumming Disease (Cobb's Disease).
Smut. Schizophyllum Rot. The Iliau Disease. Cytospora
Rot. Leaf-sheath Rot. Red Spot of the Leaf-sheath. Ring
Spot. Eye Spot. Red Leaf-spot. Yellow Leaf-spot. Brown
Leaf-spot. Sereh. Mosaic Disease (Mottling Disease, Yellow
Stripe Disease). Chlorosis (Gall Patches, Moonshine). Fiji
Disease. Sclerospora Disease ---___ 289
CHAPTER XXVII : DISEASES OF ROOT CROPS.
Cultivated Aroids.
Dry Rot or Saltpetre. Pythium Rot _ _ _ _ 331
Arrowroot.
Burning Disease (Rosellinia). Cigar Disease _ _ _ 332
Cassava.
Wither-tip. Leaf Spots ------- 335
Onion.
Bacterial Soft-rot -____--- 335
Sweet Potato.
White Rust _-_---_-_ 336
Yam.
Wilt. Tuber Rot. Leaf Blotch - _ _ _ - 337
CHAPTER XXVIII : DISEASES OF LEGUMINOUS PLANTS.
Ground Nut.
Leaf Rust. Leaf Spot. Root Disease _ _ _ _ 338
Kidney Beans.
Anthracnose. Bacterial Blight. Stem Rot _ _ _ 340
Pigeon Pea.
Root and Stem Diseases. Rust _ _ _ - - 342
CHAPTER XXIX : DISEASES OF MINOR FRUIT PLANTS.
Pineapple.
Tangle Root. Root Disease, Wilt or Blight. Base Rot.
White Leaf-spot. Leaf-base Rot. Fruitlet Spot, Black Eye.
Black Spot. Thielaviopsis Soft Rot. Diplodia Core Rot.
Prevention of Fruit Rots __---_ 344
Papaw.
Black Leaf-spot. Stem and Fruit Rot _ _ _ _ 352
Tomato.
Blossom-end Rot. Leaf Mould. Septoria Leaf -spot. Bacterial
Wilt, Bubbly Fruit-rot ------- 353
CHAPTER XXX: DISEASES OF PLANTS NOT CLASSIFIED.
Agave.
Anthracnose -----.__ — _ 361
Rose.
Black Spot, Mildew. Other Diseases _ _ _ _ 361
LIST OF ILLUSTRATIONS
Mosaic Disease of Sugar-cane, coloured plate
Map of the Lesser Antilles
. Frontispiece
Page
. to face xi
Fig. No.
1 Fungus Hyphae in Cells
2 Phytophthora Faberi
3 Hyphse (Rhizoctonia) Basidia and Spores of Corticium
Vagum .....
Stigmatomycosis. Species A
,, ,, B (Eremothecium)
„ C . .
,, D (Nematospora)
Ascomycetes .....
Basidiomycetes ....
Types of Conidial Fructifications .
Love Vine on Solanum Sp.
Bird Vine {Struthanthus dichotrianthus) on Cacao Flowers
enlarged ......
Nematode Root Galls (Heterodera radicicola) .
Tylenchus musicola, Male ....
,, ,, Female
Coconut root with Nematodes
Coconut Parenchyma with Nematodes .
Egg of Aphelenchus cocophilus
Entomogenous Fungi ....
Cephalosporium Lecanii on Saissetia nigra
Cephalosporium Lecanii on Coccus Mangiferae .
Froghoppers {Tomaspis saccharina) killed by Metarrhiziu
Metarrhzium Anisoplioe Sorok . . . ,
Dusting with spores of Froghopper Fungus (Trinidad)
Equipment for mixing Bordeaux by gravity
Mixing Bordeaux by hand ...
Spraying Mango with Spray-motor Pump mounted on
2o-gallon oil drum ....
Charging Compressed Air Sprayer
Compressed Air Sprayer in action
Healing of pruning wounds (Mandarin Orange)
Effects of bad pruning (Cacao)
Rosellinia Pepo. Conidial Fructifications x 4^
Perithecia X 3* . '.
Smoky Mycelium on surface of bark X ij
,, „ Mycelium under bark, nat. size
,, bunodes, Conidial Fructifications x 4^
,, ,, Perithecia x 4
,, „ Mycelial Strands in Lime bark, X i^
Mycelial Strands in Limewood,
Transverse Section, x 4^
., ,, ,, Longitudinal Section, x i
„ (paraguayensis Starb ?) Perithecia x 3
Rosellinia Spp. ....
Root Disease of Cacao (R. Pepo).
Pink Disease of Cacao
Thread Blight of Cacao (Corticium Stevensii)
Algae Disease of Cacao, Active stage
„ , „ Later stage
xvii
4
21
27
29
29
30
30
37
39
41
55
57
I face 60
61
61
64
64
64
75
' face 76
,. 76
-, 76
79
80
80
90
96
103
103
) face 126
126
„ 128
„ 128
„ 128
„ 132
,. 132
,. 134
.. 134
„ 136
139
'face 146
148
.. 150
„ 152
-. 154
XVlll
LIST OF ILLUSTRATIONS
Fig.
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
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77
78
79
79A
80
81
82
83
84
85
86
87
88
89
90
91
No. Page
Diplodia Rot of Cacao Pod .... to face 158
Cacao Canker produced by inoculation . . . ,, 160
Cacao Pod Rot : Tip infection ....,, 160
Cacao Pod Rot, produced by inoculating stem . „ 162
Phytophthora Faberi , . . . . 163
Surinam Witch Broom Disease .... to face 168
A Typical Witch Broom . . . . . 169
Surinam Witch Broom Disease .... to face 170
Monilia Disease of Cacao (Ecuador) . . . ,,174
Sphgeronema Black Spot (Ecuador)
Colletotrichum Cradwickii, Flower Proliferation (Male
Cacao) Trinidad .....
Flower Proliferation (Male Cacao), Trinidad
Pods from Male Cacao Tree ....
Red Ring Disease : Fully infested tree .
Sections of Stem of tree in Fig. 00 at ift. gin. and 3ft. 6in.
Red Ring Disease, Medium Section of Infested Tree
Longitudinal Section of Segment of Stem
Transverse Section of Segment of Stem
„ ,, Section near upward termination of
Ring
„ Infestation of a young leaf
Effect of a Bud Rot epidemic, Trinidad
Coconut Bud Rot, outer leaves removed to show collapse
of heart leaves .....
Bud Rot, preceded by drooping of leaves (Tobago)
Phytophthora Bud Rot (Jamaica), central leaves alone
dead and broken
„ ,, advanced stage, central column
fallen out
Gumming of Coconut Stem
Little Leaf Disease of Coconut
Red Root Disease of Lime (Dominica)
Anthracnose of Lime
Scab on Lemon Shoot
Foot Rot on old seedling Orange Tree (a) Margin of
Rot indicated by chalk line .
(b) Treatment in progress
Canker on Grape Fruit
Canker on Leaves, showing pale zone around the spots
Canker on Young Twig
American Leaf Disease of Coffee
Brown Eye-spot of Coffee .
Anthracnose of Avocado
,, Mango
„ ,, Mango Fruit
,, Mango Leaves
Macroconidia of Fusarium Vasiniectum
Panama Disease, Early symptoms
Advanced stage
„ „ Longitudinal Section of a bulb and
base of the stem
, , Young Sucker infested from parent
plant through the neck
Moko Disease of Banana .
Tylenchus Similis ....
Cercospora Musarum
Bonnygate Disease of Banana
Sphsrostilbe Musarum, Conidia, Ascus and Ascospores
174
176
176
178
178
180
180
180
180
180
180
184
„ 186
„ 186
.1 190
>> 190
192
, 192
203
to face 204
204
204
,, 210
„ 210
212
212
212
„ 226
228
.. 236
236
236
.. 236
242
to face 242
242
.. 244
» 244
.. -48
249
, 251
to face 252
253
LIST OF ILLUSTRATIONS
XIX
Fig.
99
lOO
lOI
102
103
104
105
106
107
108
109
no
III
112
114
"5
116
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140
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142
143
144
145
146
147
148
149
150
No.
Corn Smut ....
Incidence of internal Boll Disease
Internal injuries to Cotton Bolls (St. Vincent, etc.)
Angular Leaf-spot of Cotton
Bacterial Boll Disease of Cotton
Anthracnose of Cotton Bolls
Curly-leaf Cotton, top view .
„ side view
Loggerhead Cotton, young plants
,, „ old plant
West Indian Leaf Mildew of Cotton
Marasmius Mycelium on Sugar Cane
Fructifications of Marasmius associated with acute Root
Disease of Sugar-cane (Trinidad)
Acute type of Root Disease : Infestation of young plant
cane with Marasmius (Barbados B 6032)
Acute type of Root Disease : Buds of young plant cane
infested with Marasmius
Fructification of Odontia Sp. on Sugar-cane
Odontia sacchari cystidium and basidium with single
spore .....
Himantia Stellifera, Mycelium on Sugar-cane
Himantia Stellifera ; Hypha, with stellate crystal and
swollen bodies .....
Red Rot of Sugar-cane ....
Colletotrichum falcatum . . . ,
Rind Fungus of Sugar-cane
Melanconium Sacchari, vertical section of Stroma.
Thielaviopsis paradoxa, Sporophores of Micro- and Macro-
conidia
in Sugar-cane cutting
in cells of Sugar-cane
Cephalosporium Sacchari
Ustilago Sacchari
Gnomonia Iliau, vertical section of Perithecium and Asci
Melanconium Iliau, Stromata and Conidia
Cytospora Sacchari, surface and sectional views of
Pycnidia, spore and sporophore
Cercospora vaginae .....
Ring-spot of Sugar-cane (Leptosphaeria Sacchari) .
Leptosphaeria Sacchari : Asci and spore
Helminthosporium Sacchari
Eriosphieria Sacchari : Asci and spore .
Cercospora Kopkei .....
Cercospora longipes ....
Mosaic Disease of Sugar-cane : Stem canker
Chlorosis of Sugar-cane ....
Vasculomyces Xanthosomae
Burning Disease of Arrowroot
Bacterial Rot of Onion ....
Anthracnose of Bean ....
Stem Rot of Papaw. ....
Base Rot of Pineapple cuttings .
Cross Section of Pineapple Fruit, showing two infections
of Black-eye
Thielaviopsis : Soft Rot of Pineapple
Blossom-end rot of Tomato
Septoria Leaf-spot of Tomato
Black Spot of Rose .
Rose Mildew ....
Page
to face 258
267
269
to face 270
.. 270
270
270
.. 272
.. 274
.. 274
.. 274
,. 288
„ 288
„ 288
„ 288
„ 288
291
to face 292
293
to face 298
299
to face 302
299
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315
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to face 318
319
320
321
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to face 326
332
to face 332
332
342
342
344
344
346
352
352
362
362
PART I
INTRODUCTION
THE NATURE AND CLASSIFICATION OF PLANT
DISEASES
Health is a state in which each organ performs its own function
and acts in harmony with every other, and disease in the broadest
sense of the word consists of any departure from this state.
A condition of disease proceeds from a derangement of the
function of any organ, and in plants this most frequently follows
upon derangement in structure.
Two classes of diseases are generally recognised as coming
within the scope of the mycologist or plant pathologist : parasitic
diseases caused by fungi or bacteria, and non-parasitic diseases,
in the sense of affections characterized by specific symptoms
and believed to result from some disturbance of function caused
otherwise than by a parasitic organism.
This limitation is one of convenience, not of logic. Insect
injuries to the living tissues react on the health or well-being
of the plant, and often produce serious derangements of function
resulting in general symptoms which in some cases have no
obvious relation to the wounds inflicted. These are as definitely
diseases as the affections produced by fungi, but are in practice
mostly left to the entomologist. It is a defect of this system that
the botanical aspects of insect injury often remain unstudied.
The term mycologist, it may be noted here, as applied to a
student of plant diseases, dates' from the time when the idea of
their causation was limited to fungus parasitism. It properly
applies to the student of fungi ; the general practitioner in
plant pathology is more largely concerned with the host, and
the range of his vision must extend at least to bacterial diseases,
virus diseases and those of non-parasitic origin.
A convenient approximation to a classification of plant
diseases, in the broader sense, which is not complete, but includes
the chief groups of diseases having more or less obvious specific
characters, may be made as follows : —
{a) Parasitic Diseases. — Caused by : —
1. Fungi.
2. Bacteria.
3. Infectious viruses. (Ultra-microscopic organisms ? )
I B
2 DISEASES OF CROP-PLANTS
4. Parasitic flowering plants.
5. Nematode worms.
6. Mites.
7. Insects.
(b) Non-parasitic Diseases.
Non-transferable, with specific S5niiptoms and fre-
quently with characteristic lesions.
As no strict definition can be given of disease there is no
fixed limit to such a list as this. Injuries, mostly direct, may be
produced by grazing animals, by hail, or by lightning, while
deficiencies in soil, unfavourable weather, an unsuitable climate,
and last, but not least, the unskilful or negligent cultivator,
may all he set down as disease-producing agencies, and as such
will be referred to upon occasion in subsequent discussions.
Naturally there exist at any given time recognised diseases
of which the cause is unknown or disputed. The safest rule in
such cases is to regard them tentatively as being of parasitic
origin if their mode of occurrence suggests that they are trans-
ferable.
The use of the term physiological disease is by common
consent undesirable, though there must be few pathologists
who have not found it convenient on occasion. It has repre-
sented a sort of Cave of Adullam to which diseases not known
to be of parasitic origin could be relegated, but is .objectionable
in form since all diseases properly considered involve disturb-
ances of physiological processes.
In relation to agricultural practice the most important
distinction among plant diseases is to be made between those
which are able to develop upon plants in normal condition and
those which occur to a serious extent only upon plants reduced
in vigour by unfavourable circumstances, as of soil, chmate,
insect infestation, or methods of cultivation. For those which
have this character in a marked degree the term debility diseases
is used in the following pages.
Section I— Causation
CHAPTER I
DISEASES CAUSED BY FUNGI
The Nature of Fungi.
Fungi are vegetable growths, that is, they are plants, but of
a relatively simple kind. They range in form from single
rounded cells or small cell-groups as in the yeasts, through
filamentous forms such as are familiar in the moulds common on
bread, cheese, or fruit, to elaborately shaped structures, with
tissues built up of filaments, more or less closely amalgamated,
of which mushrooms, toadstools, and the bracket-like outgrowths
which appear on dead trees are examples.
The nearest relationship of fungi is with another large group
of simple plants, the Algse, which similarly range from single-
celled or filamentous forms such as produce the green colour or
the green slimy growth in any body of exposed water to the
large and variously fashioned forms known as seaweeds.
There is an essential difference between Algae and Fungi
in the nature of the materials from which they derive their
nutrition. The Algse possess, in common with most forms of
vegetation, the green colouring matter chlorophyll, which
enables them to build up from inorganic materials — mineral
salts, carbon dioxide, and water — the organic compounds
required for construction and maintenance. Fungi, on the other
hand, have no chlorophyll and therefore no such ability, and can
live and grow only at the expense of material which has been
previously elaborated by some other plant or animal. It is the
mode of life arising from this necessity which gives to fungi their
great importance in human economy.
Transitional forms are, however, known between Algae and
Fungi in respect of the chlorophyll content of the former, and
some Algas may assume a parasitic habit.
There are fungi ready to take advantage of all classes of
vegetable or animal material if the conditions are favourable ;
they precede, accompany, or follow the even simpler organisms
known as bacteria, most of which are nourished in a similar way.
With them they are responsible for the degeneration of dead
vegetable or animal matter, which is called in general terms
decay, while the parasitic species have become adapted to feed
at the expense of living plants or animals.
3
DISEASES OF CROP-PLANTS
The Mycelium.
The way of life characteristic of the great majority of fungi
is for the invaded substance to be covered or interpenetrated
with an abundant development of slender filaments, which
absorb from it the food materials required. This structure is
the vegetative part of the fungus ; it is known collectively as
the mycelium, and its individual filaments as hyphae. In mass
the mycelium is frequently visible to the naked eye as a cobweb-
like, threadlike, fluffy or papery investment. Sometimes the
hyphae are combined into threads, as in the black and shining
strands of "horse-hair blight" and the whitish threads or strings
of thread blight. In certain species cord-like strands of mycelium
ramify through soU or vegetable mould, and such structures
in general have been named rhizomorphs, from their resemblance
in these cases to roots. In certain fungi
the mycelium collects into hard masses
of various shape and size, sclerotia, which
by their resistant nature serve to carry
the fungus through adverse conditions
or aid in its distribution.
If the abstraction of food materials
by the mycelium takes place from living
tissues or non-living but functioning
parts of a plant or animal the mode of
life constitutes parasitism ; if from ma-
terial no longer forming part of a living
structure, it is known as saprophytism.
The Place of Fungi in Nature.
P^S- 1 Fungi have a place in the economy
Fungus Hyphae in Cells of Nature far wider than that arising
from their part in the causation of plant
disease. Chiefly by their operation the great mass of vegetable
material formed each year in the shape of leaf and stem, flower
and fruit, is broken down and returned to the soil. Their
activity in this direction has a harmful side, from the point of
view of man, when it concerns such commodities as timber or
stored foods.
The rapid destruction of humus-forming materials in the
tropics as compared with temperate countries — resulting in the
absence of notable deposits of leaf-mould even from the forests —
appears to be connected with the suitability of tropical conditions
for the continuous activity of soil fungi. The only lasting
natural accumulations of vegetable material in the West Indies
known to the writer are in swampy situations, where the water,
by excluding air, prevents their operation. In the biological
processes of soils under forest growth or in orchard cultivation
it is believed that fungi replace to a considerable extent the
bacteria of arable land.
DISEASES CAUSED BY FUNGI 5
The Nature of Fungus Parasitism.
There are numerous gradations between the strictly sapro-
phytic and the strictly parasitic habit in fungi. Many species
which subsist typically on the dead or dying organs of plants
can advance some way towards the invasion of living tissue
if its resistance is weakened by age, poor nutrition, or insect
or mechanical injury. The debility diseases already referred to
are caused by the more advanced examples of this class, of which
the Diplodias afford the best illustration in the West Indies.
Certain fungi of this genus are abundant on recently dead her-
baceous stems, woody branches, and fallen fruits. Further,
they readily attack sugar-cane, cotton bolls, cacao pods, or
citrus fruits when these are over-ripe ; while in the same class
of material even a trifling injury at an earlier stage may afford
a foothold and permit of infestation. In the case of cotton it
would seem that under favourable circumstances the rot can
commence unaided. On lime and cacao branches, to mention
instances typical of many other trees, the Diplodias get a foothold
on a broken branch or dead twig and are only checked when they
reach a part situated in a line of normally vigorous growth.
Where conditions are adverse to the maintenance of vigour in
the tree they can do considerable damage.
From this class of weak parasites advance is made by in-
sensible steps to fungi which can establish themselves on plants
in normal or even supernormal vigour. It is somewhat vain to
attempt to classify the numerous grades of parasites ; they vary
in so many ways that each case must be taken on its merits ;
the variations will be best appreciated from the descriptions of
individual diseases given later. The most notable division is
that which marks off the extreme class of parasites. On the
one side are those fungi which, though capable of parasitism
and frequently typically parasitic, can also live on a greater or
lesser range of dead material, often of their own killing : on the
other side are the strict or obligate parasites, of which the most
advanced forms enter into a close relationship with the living
cells of the host and are dependent on the continuance of its life
processes for support. Of this nature are the rusts (Uredineae),
the smuts (Ustilagineae) and the powdery mildews (Erysiphaceae) .
Unlike nearly all fungi in the previous class, the obligate parasites
naturally cannot be grown in culture apart from their host.
Another distinction made, with more or less of value, is that
which marks off as wound parasites those fungi which cannot
attack living organs direct, being unable to penetrate the cuticle
of leaves or the cork of stems as the case may be, and only gaining
access when the protective layer is in some way broken.
Very important differences exist in regard to adaptation to
special hosts. There are at one end of the scale general parasites
like the Rosellinias later discussed, which under suitable condi-
tions can infest almost any plant which comes in their way.
6 DISEASES OF CROP-PLANTS
Others only attack plants of one order, genus, or species, and this
tendency reaches its extreme in parasites which are so restricted
in their range that invisible characters associated with a par-
ticular strain of the host species may partially or completely
prevent their development.
Reproduction and Infection.
The active feeding part of a fungus, its vegetative body, is
the mycelium already described, typically made up of an aggre-
gate of hyphae. A portion of mycelium may be detached, with
or without fragments of the material on which it is growing,
and serve to reproduce the fungus in new situations, as happens
with the root fungi of sugar-cane transported on cane cuttings
or trash, and in Rosellinia root diseases when fragments of in-
fested wood are washed by floods to lower levels. The common
Sclerotium diseases of legumes, sugar-cane, etc., are caused by
fungi in which no method of reproduction is known save the
characteristic hard, shot-like grains, formed of thick-walled
hyphas.
With very few exceptions, however, reproduction is further
provided for by the formation and release of spores — individual
cells or less frequently aggregates of 2, 3, or more cells — which
can remain, like seeds, dormant for a longer or shorter period and
then, under appropriate conditions, germinate and give rise to
a new mycelium. In a few of the more primitive fungi there are
sexual processes more or less preserved which resemble those of
algae, but in most members of the larger groups there is little
more than a reminiscence of sexual fusion, and in very many
no certain trace or no trace at all.
Classification, as in the flowering plants, depends mainly on
the form of the reproductive structures, and some detail as to
this will be given later : only the more general aspects of repro-
duction are at present in question.
The visible forms of the larger fungi, the mushroom-like,
bracket-like, and other shapes in multitudinous variety, are
structures erected by the usually obscure mycelium for the
production and discharge of spores. The provision of gills,
spines, tubes, pores, folds, convolutions, recesses, or chambers,
greatly increases the surface available for this purpose. A similar
end is served in the simpler forms by the outgrowth and branching
of the surface hyphge.
Broadly speaking, spores may be divided into {a) those
produced within a parent cell (an ascus or a sporangium), to which
the term spore is sometimes technically restricted (endogenous
formation), and (b) those which are budded off from more or less
free hyphae and are known as conidia or conidiospores, or from
h57phal terminations of special form (basidia) and distinguished
as basidiospores (exogenous formation). In some very simple
forms the whole mycelium divides up into conidia or gemmae.
DISEASES CAUSED BY FUNGI 7
Many fungi produce either ascospores or basidiospores in
their more elaborate fructifications, and also conidia in some more
direct and simple manner. In a vast number of species the
latter method is the only one yet recognised, in which case the
fungus is classed in a heterogeneous group known as the Fungi
Imperfecti, on the assumption that a more definite form of
fructification exists or has existed.
The longevity of spores is extremely variable in different species.
Many have been shown to be viable after months or years of
desiccation, others quickly die. Stahel has found, for example,
that the basidiospores of Marasmius perniciosus, the witch-
broom fungus of cacao, will not germinate after desiccation,
exposed in a thin layer, for an hour. In many species, of which
Colletotrichum falcatum, the cause of red-rot in sugar-cane, is
an example, special thick-walled resistant spores (chlamydospores)
are formed directly on the mycelium.
The dispersal of spores is effected most commonly by wind or
minor air-currents. They are frequently set free in vast numbers
visible like puffs of smoke, and their lightness makes it con-
ceivable that they may be borne considerable distances through
the air. They are carried by birds and insects, washed from one
plant to another by rains or transported by streams, and dis-
tributed by man, adhering to clothing, implements or articles
of merchandise. The chance of any individual spore finding an
appropriate resting place is very small, but this is offset by the
immensity of their numbers.
For germination to take place the appropriate conditions in
respect of moisture, heat and light must prevail, and these vary
with different parasites Humid conditions provided by duU
wet weather, by soil moisture, or by the transpiration of the host
itself, may suffice, or a definite drop or film of water may be
required. Heat is presumably never deficient in the tropics,
indeed there is reason to believe that the prevalent temperatures
are above the optimum for many species, a limiting factor
which may be held to account for the relative infrequency of
members of some of the parasitic orders largely developed in
temperate countries.
Infection of plants by a fungus parasite may take place by
the entry of mycelium nourished on some adjoining material,
such as the soil, the roots of another plant, or dead adhering
leaves or branches, or may be effected through the germination
of spores directly on the appropriate living organs. It implies
first the penetration of the outer defences of the plant : the cuticle
and cell-wall or the corky integument, as the case may be.
Advantage may be taken of wounds of various kinds for this
purpose, including as such the pathways to living tissue provided
by the attachment of dead twigs or branches. Entry may be
made through the stomata, or the fungus may be equipped to
force or to dissolve a passage, or to provide one by poisoning the
8 DISEASES OF CROP-PLANTS
surface tissue. Habits or powers of this nature in parasites are
usually specific. The germinating spore typically produces a
preliminary hypha, the germ-tube, by or from which the entry
is made.
Penetration being accomplished and the parasite brought
into relation with the living cells, the second line of defence, the
resistance of the protoplasm, is met with. Very little is under-
stood of the nature of this. The more or less mechanical process
of penetration may take place on a variety of plants, but only
when the host comes within the range of adaptation of the fungus,
which is often very restricted, can parasitism be established.
The subject is discussed further in the section relating to the host.
When the resistance of the protoplasm is overcome the
fungus proceeds to occupy the tissues. The mycelium may grow
into and through the cells of the infected tissue (intracellular
mycelium), or ramify between and about them (intercellular
mycelium) . In the latter event the cells are either killed and their
walls thus rendered permeable, or absorbing organs (haustoria)
are introduced. Certain fungi, e.g., the Fusariums causing wilt
disease, develop principally in the vessels (vascular parasites).
In a few cases, as in the powdery mildews (Erysiphacese) , the
mycelium remains external, sending only haustoria into the
superficial cells. The black blight or sooty mould fungi (Cap-
nodise) develop similarly on the surface of leaves or other organs,
but they have no powers of parasitism and do not live at tb
expense of the plant but of insect or flower secretions of tl
nature of '■ honey dew."
The Host : Resistance and Susceptibility,
In every case of parasitism there are two organisms to be
considered, the host and the parasite, and the reactions of the
plant attacked are at least as important as the nature of the
attack delivered. The growing recognition of this truth is re-
flected in the increasing fund of information regarding sus-
ceptibility, resistance, and immunity.
Effect of Agricultural Conditions on Resistance.
Some general power of resistance is conferred by the healthy
condition induced in a plant by growth in a congenial situation.
Each of the important crop plants of the West Indies is subject
to one or more fungoid affections which occur or are serious only
when temporary or permanent disabilities such as drought,
poverty of soil, and exposure have reduced the resistance normal
to the healthy plant. By avoiding unsuitable locations and by
the practice of good agriculture it is possible to maintain this
resistance at a high level, and by this means not only to escape
debility diseases but frequently largely to reduce the prevalence
of more definite parasites. To this effect is to be added the greater
ability of a well-grown plant to endure, to outstrip, or to recover
from the injuries inflicted.
DISEASES CAUSED BY FUNGI g
There are many other parasites, however, against which this
type of resistance fails. To them a better-developed plant may
mean merely an increase in available food material, while the
effect of manuring and good weather may be to develop a softer
type of growth which is the easier to infest. In the case of
Rosellinia disease of limes, the conditions which best suit the
tree best suit the fungus also, with the result that the finest
trees are most liable to attack and are most frequently killed.
Cacao canker and pod-rot are most prevalent in the well-enclosed
humid situations which favour vegetative growth, and in dealing
with diseases of this type a point has to be sought to which the
humidity can be reduced so as to check the fungus without unduly
hindering the development of the trees. In the growing of cotton
a similar situation in respect of boll diseases is met by planting
so as to take advantage of the wetter months for growth and of
the dry season for crop production.
Immunity by Evasion.
The case of cotton just referred to is an example of the way
in which diseases may be " resisted " by evasion. It has
frequently been found with short-term crops that the best avail-
able protection against a disease or pest lies in growing early
maturing varieties, or in planting to escape a season when a
particular disease is prevalent. The seasonal distribution of
diseases as experienced on early and late plantings of the same
crop is frequently quite marked. It may be determined according
to the particular case by the effects of climate on either the host
or the parasite.
The highly important effect on disease of the practice of
crop rotation may be put under this heading, since it mainly
depends on the temporary freedom from infestation to be secured
by planting disease-free material in uninfected soil.
Resistant Species or Varieties.
Resistance in the stricter sense of the word, and its complete
development immunity, depend on inherent properties of the
plants concerned, and may be transmitted to their descendants.
Such resistance may or may not be independent of variation in
the conditions under which the plants are grown ; sometimes
a degree of resistance which has proved constant under one set
of conditions is not developed when the circumstances are
changed.
The characters on which resistance depends are almost always
obscure. Attempts to connect its existence with special morpho-
logical features are seldom successful, though it may exist in
correlation with some difference of form. It appears to be in
most cases a property of the protoplasm. In the case of certain
obligate parasites resistance has been shown to arise from
10 DISEASES OF CROP-PLANTS
increased local susceptibility, which results in the death of the
cells at the point of infection and prevents the mycelium from
establishing the necessary connection with living tissues.
The production of resistant strains of cultivated plants has
usually been accompHshed by the propagation of chance-found
individuals showing the required powers. A frequent defect of
such varieties is that resistance may not happen to be associated
with a high standard in other desirable qualities. Some more
or less successful attempts have been made to secure the desired
combinations by crossing, a seductive but uncertain line of work
which has possibly had more advertisement than the stability
of the results yet warrants.
The most striking example of the use of resistant varieties in
West Indian agriculture is the replacement of the Bourbon cane
by seedlings resistant to red rot and less susceptible to root
disease, a measure which has preserved the sugar industry of
these islands.
Susceptibility to Exotic Diseases.
It seems most probable in the light of the available evidence
that the failure of the Bourbon cane in the West Indies arose
from the introduction of a fungus to which it had not previously
been exposed. In this manner have originated many of the
great epidemics of plant disease. Where host and parasite are
both native or have been long associated there is a natural
selection of resistant species or strains and the parasite is
frequently inconspicuous in amount and in its effects. Introduced
into a new country the parasite may find appropriate hosts quite
unable to withstand it. The chestnut bark disease, introduced
a few years ago into the United States on resistant species from
China, has made a clean sweep of the extensive forests of the
American chestnut in the Eastern States, and shows no real
abatement in its progress. Similar occurrences have been the
introduction of the mildews of the grape-vine and the gooseberry
from America to Europe, of the white pine blister rust in the
reverse direction, of citrus canker from Japan to Florida and
South Africa, and, according to the most probable theory, of the
introduction from Africa of the coffee leaf disease which nearly
exterminated the main industry of Ceylon.
A similar result may come from bringing exotic plants within
the range of native parasites, especially those occurring on related
species. The leaf blister mite of cotton in the West Indies, which
appears to be indigenous and is known to attack native Mal-
vaceous plants, caused first an epidemic and would now result
in an annual infestation of the Sea Island crop if a close season
were not enforced. The witch-broom disease of cacao in Surinam
comes from a wild Theobroma in the forests of that country.
The fungus causing pink disease of various cultivated trees and
shrubs, one of the serious troubles of the Malayan rubber planter.
DISEASES CAUSED BY FUNGI ii
is native on jungle plants in both the Eastern and the Western
tropics.
Local Factors affecting Prevalence.
An established disease may assume serious or epidemic pro-
portions under the influence of changes in local conditions. The
most general of these is the collection of plants previously-
scattered as individuals or small groups into large blocks of
cultivation, which happens when a new agricultural industry
arises, as in the case of rubber, or an old one is greatly developed,
as has been the case in recent times with cacao, limes, cotton
and coconuts in various parts of the West Indies. When a plant
is grown by the acre or the square mile in pure culture its parasites
are secure of their food in due season and the agencies of dispersal
have free play. The absence of serious diseases from scattered
plants is thus, as experience has regularly proved, no sort of
guarantee that wider cultivations will not suffer. A contributory
cause to the increase of diseases with the development of an
agricultural industry is the greater activity which arises in the
introduction of planting material from more or less distant
sources.
■ i A well-marked class of root diseases, due to fungus parasites
similar in the nature of their attack though often widely different
in taxonomic position, accompanies the establishment of crops
on newly cleared forest land. The decaying stumps and logs, and
the accumulations of humus to which they give rise, support
large numbers of fungi, mostly pure saprophytes but usually
including one or more species which are able to extend to the
roots and collar of the crop plant introduced. In arable cultiva-
tion, with few exceptions (see arrowroot), the cause and the
effect quickly disappear. With tree crops the original cause is
more slowly removed and the effect may later be continued for
a time by its victims, while in cacao orchards where conditions
approaching those of forest are maintained by the use of shade
trees, one such disease has shown itself in the West Indies to be
capable of permanent existence in favourable situations. The
tendency of such diseases in general is to decrease with time ;
sooner if appropriate measures are applied, later and after heavy
losses if they are not.
The reverse situation to this is seen in the increase of debility
diseases as soils decline in fertihty. A period in which they
are prevalent comes in the history of new industries and of the
exploitation of virgin soils before a tradition of manuring and
perhaps rotation is reluctantly acquired. The situation recurs
if agricultural practice declines for any reason. In the West
Indian sugar industry there has been evidence for many years
of a more or less general decline, slow but continuous, in fertility,
as shown by the shortening of ratooning periods and increasing
prevalence of root disease. This appears to be connected with
12 DISEASES OF CROP-PLANTS
less intensive cultivation resulting from the increase in size of
estates under the factory system, the reduction of humidity
by clearing away trees, the diminished supply and increasing
cost of the labour required for hand cultivation, and perhaps
above all to the treacherous facility with which the benefits of
organic manures can apparently be obtained by the use of
chemical substitutes.
The passing of estates into the hands of mercantile rather
than agricultural proprietors (a frequent consequence of agricul-
tural depression) with the break in traditions involved, has often
led to a visible decline in agricultural conditions and thus to
a corresponding increase in disease.
There is a further cause of debility disease very frequently
encountered : the planting of a crop in a situation which from
soil or climate is somewhat unsuited to it. If a district or an
island is successful with a given crop, attempts are always
made to extend it to marginal or supposedly similar locations.
Under the less suitable conditions production is smaller, and it
too frequently happens that the need for greater attention to
agricultural practice in order to offset the defective conditions
cannot or will not be understood or the expense cannot be
afforded. In such a case the soundest advice that can be given
with reference to the diseases which invariably occur is to plant
a crop more suited to the situation. It is advice which is fre-
quently required, seldom given, and never heeded until necessity
dictates it.
Returning to more definitely parasitic diseases, there are
various natural causes producing broad effects in the prevalence
of disease. The disastrous potato blight epidemic in Ireland
in 1845, the social consequences of which have had a marked
effect on British and American history, was brought on by a
period of dull and humid weather. Nearer at hand a week or
two of heavy rain in St. Vincent has more than once brought
about the destruction of the visible cotton crop by boll-rotting
fungi. An instance of the complexity of the factors sometimes
involved is afforded by the history of internal boll disease in the
same island, where the eradication of casual trees of two species,
by interfering with the breeding of an insect, has saved the later
pickings of cotton from a fungus disease which regularly destroyed
them.
The Principal Types of Fungus Diseases.
Every part of a plant is liable to attack by fungi, but each
individual parasite affects only certain parts or stages in a
certain way, and usually produces a disease of a constant and
recognisable type.
Leaf Diseases.
Fungus infections on leaves commonly give rise to localised
DISEASES CAUSED BY FUNGI 13
lesions in the form of spots, streaks or patches. The cause of
complete discoloration, wilting or shedding of leaves is more
frequently to be found in an infestation of twigs, branches,
stems, or roots, according to the distribution of the symptoms.
Leaves are most susceptible in the early and late stages of
their development. The tenderness of young leaves, including
the absence of cuticle, exposes them to infection by fungi which
are unable to attack mature leaves ; the anthracnose of limes
is an example of this. The fungus may later disappear, but the
effects of its presence remain in the shape of spots or distortions
which are difficult to account for if the early stages are not seen.
Senescent leaves show waning powers of resistance and are
frequently infested by parasites too weak to have much if any
effect until this condition is reached. Very many leaf spots are of
this nature. Influences which depress the vigour of the plant
may bring on the susceptible condition prematurely ; thus when
a coconut palm is infested with nematodes Diplodia attacks
the leaves at an earlier stage. Many secondary infestations are
brought about in a similar way.
Leaves are affected differently by parasites which establish
a relationship with the living cells, like the rusts and mildews,
as compared with those which live at the direct expense of the tis-
sues. In the former case the leaf though more or less discoloured
remains alive, it may be for its full period or even longer ; in the
latter case dead spots are quickly produced. A common type of
leaf spot consists of a dead central circle or streak, surrounded
by a discoloured margin of cells recently invaded or affected
by the secretions of the mycelium. Such an area may become
isolated from the surrounding tissue through the production
by the host of a ring of corky cells which prevents further ex-
tension, and if the dead tissue later drops out the type of injury
known as shot-holes is produced, in which the leaf is more or less
heavily perforated with roundish holes. In other types of
spotting the lines of the veins are followed, or the spot is bounded
by veins (angular spot) or quite irregular perforations may be
produced (Hevea leaf spot).
It is often difficult to distinguish between leaf spots caused
by fungi and those of insect origin, especially as the latter are
soon invaded by saprophytic fungi.
The infestation of leaves with fungi reduces their efficiency
to a proportionate degree and may lead to partial or entire
defoliation. This causes a severe check to growth, and, if re-
peated frequently, brings about the death of shoots or twigs
and sometimes of the whole plant.
Flower and Fruit Diseases.
The infestation of flowers is not common, but examples of it
are found in the blossom bUghts, due to Glceosporium spp., of
mango and of lime, in which buds, open flowers, and newly set
14 DISEASES OF CROP-PLANTS
fruits are rapidly infested and destroyed. The parasitism of
the smut fungi, which also destroy flowers, is of a special type
in which infection proceeds from within, and is described in
another section.
Fruits while yet unripe may be very quickly destroyed by the
Phytophthoras (soft-rot of cotton bolls, cacao pod-rot), may
be spotted or cankered by the anthracnose fungi and similar
parasites, may be scabbed or russeted by surface infections, may
develop cracks in consequence of the inability of infested spots
to keep pace in growth, and may be internally infected through
bug punctures while preserving an uninjured appearance.
As fruits ripen they lose, like senescent leaves, much of the
resistance which accompanies the condition of living activity.
A ripe fruit is a fruit that is finished with so far as the plant
is concerned, and if not protected soon becomes the natural
prey of various saprophytic fungi (see Chap. XIII). Some of the
ripe rots may commence prematurely before the fruit is picked ;
usually this happens in consequence of a wound or bruise.
Further examples of fruit affections will be found described
under diseases of Citrus spp.
The shedding of young fruits, of which cotton affords the
pre-eminent type, may be due to infection or insect injury, but
is frequently a result of the disturbance of physiological balance
by some change in external circumstances. It may also be an
adjustment of numbers to the amount of fruit which can be
brought to maturity, as seems to some extent to be the case
with coconuts. A similar adjustment in cacao occurs not by
shedding, but by the drying up of young fruits, and as these
invariably become infested with fungi the loss is often attributed
to fungus disease. That the inherent bearing capacities of
individual coconut and cacao trees vary widely does not affect
the fact of the adjustment, which is demonstrated by the pro-
portionate increase in the amount of fruit which matures when
cultivation and manuring are improved.
Stem Diseases.
Tip-wither, the dying of young green shoots, is commonly
due to fungi which also attack the young leaves, and may arise
from terminal infections or from the ringing of the young stem
by infections in the cortex. It may, of course, also be produced
by insect injuries or physical causes.
Dieback of twigs often follows on successive defoliations by
fungi or insects. It is frequently associated with infestation by
weak parasites such as Diplodia spp. or Colletotrichum glceo-
sporioides on trees rendered susceptible by exposure, drought,
or^ poor nutrition. These conditions are themselves capable of
originating dieback, and it is usually very difficult to form an
opinion as to how far the failure is due to or is aggravated by the
presence of the fungus.
DISEASES CAUSED BY FUNGI 15
The death of branches may be the result of direct infestation
with a parasite such as that of pink disease (see Chapter XVI)
or of an infection near the base by a fungus which has attacked
the bark or gained access from a broken branch or other wound.
The progressive dying of the topmost branches, Hke the dieback
of twigs, is less often due to local infections than to the ebbing
of the vitality of the tree as a result of drought, poor nutrition,
root disease, or old age.
With regard to the last-mentioned cause it should be always
remembered that the age and size to which a tree attains before
showing the signs of waning vigour are widely variable according
to situation. Trees grown in thin or poor soils, or in a situation
drier or more exposed than suits the particular species, reach their
natural limits in these respects in a period which is much shorter
than the normal one, while highly favourable conditions have
the opposite effect.
Bark rots are often of fungus origin, but may be bacterial,
or, as it appears in some cases, non-parasitic. Their effects are
frequently much increased by insects, as in the case of Citrus
trees which become infested with a beetle larva (Leptostylus).
Bark rot in the region of the collar is rather frequent, and is
often due to soil-inhabiting fungi or to others enabled to exist
in this situation by the moisture of the soil or the shelter of
weeds. The peculiar liability of Citrus trees to collar rot is
discussed elsewhere.
Cankers are usually local infestations of the bark in which the
persistence of the fungus and the reactions of the cambium give
rise to raised margins or irregular lumpy growth about the wound
produced. The application of the term is not clearly dis-
tinguished from that of bark rot, but the above would seem its
most appropriate use.
Gummosis is a S3^mptom of very variable origin, consisting
in the production of gum by the degeneration of cell walls, usually
in the neighbourhood of the cambium layer, i.e. just under the
bark as that term is commonly used. The gum may saturate the
overlying bark and solidify in a crust or in drops on its surface,
as in Diplodia disease of limes and collar rot of oranges. New
layers of wood or bark may enclose a site of gum formation in
the cambial zone, giving rise to gum pockets. Gummosis can
occur as a result of fungus infection, wounds, or chemical
stimulation.
Wood-rots, which are separately discussed in Chapter XI,
are as a rule the result of bark injuries of various kinds, the
breaking of branches, or unskilful pruning or trimming, which
expose the wood to infestation by fungi otherwise unable to gain
access. Their effect is ultimately to destroy the mechanical
support of the part affected.
Galls are abnormal outgrowths from leaves or from green or
woody^stems, and their tissues are soft or woody according to
i6 DISEASES OF CROP-PLANTS
their position. They are the result of some irritation of growing
tissue, the reaction often appearing very disproportionate to the
cause. The larger number of galls are due to insects, but there
are many which are of fungoid or bacterial origin. A woody
gall, once formed, has its own cambium layer, and consequently
continues to grow so long as this maintains connection with the
tree. Examples of fungus galls are afforded by the smut disease
of Indian corn, and Sphaeropsis disease of Citrus, while the
deformity produced in the so-called witch-broom disease of cacao
is more of the nature of a gall than of the structures after which
it was named. The crown gall disease, found on a wide range of
hosts, results from the presence in the tissues of Bacterium
tumefaciens.
Witches-brooms are structures similar in their origin to galls,
in which an abnormal number of crowded shoots is formed at
some point on a branch and appears as a tuft or mass of inter-
lacing twigs.
The affections of herbaceous stems may be surface spots akin
to leaf spots, invasions of the cortex alone, the vascular tissues
alone, or both together. Cortical invasion causes local rot or
canker, with yellowness of foliage or other sickly appearances as
general symptoms ; interference with the vascular bundles
produces progressive or sudden wilting.
The type of disease known as bud-rot is peculiar to monocoty-
ledons in which the terminal bud from which growth proceeds
is deeply enclosed in the bases of the older leaves. Being thus
supported it remains longer in a soft and plastic condition and
offers a large mass of highly susceptible material to any parasitic
organism which gains access to it. Bud-rots occur in coconut
and sugar-cane, but conclusions as to their primary nature should
be made with caution, as experience has shown that in both
plants the failure of some other part may deprive the bud of the
required nutrition and bring about its decay. The closeness of
its covering, by excluding air, probably accounts for the pre-
dominant part which putrefactive bacteria take in the process.
Root Diseases.
Diseases of the roots are in general the least satisfactory of
any to investigate and to combat. Experiment is difficult, and
observation cannot always be depended on to distinguish cause
from effect. The consequence is that knowledge regarding root
diseases is not usually definite, except where very pronounced
parasites are concerned.
The soil, with its accumulations of dead matter and its rela-
tively constant moisture, harbours many fungi, and among them
are found species possessing powers of parasitism in various
degrees.
Sclerotium Rolfsii destroys herbaceous plants ])y attacking
the stem and crown roots near the surface of the soil . The forest
DISEASES CAUSED BY FUNGI 17
root fungi (Rosellinia, Sphaerostilbe, and others) infest dead wood
or other vegetable material and either spread along the roots of
living trees or infest them at the collar from the soil. The
parasitic Fusariums obtain access by the roots to the vascular
system of the stem. These are all largely or entirely independent
of the vigour of the host, and their prevalence depends closely on
physical conditions.
The root fungi of sugar-cane (Marasmius, Odontia) live
normally on the withered leaves and other dead material of this
and other grasses, but by invading the roots contribute, to an
extent which is very variable and in any given case difficult to
define, to the results of adverse conditions in general. Under
certain special conditions Marasmius seems able to become a
more active parasite and completely infest the bases of the
shoots.
Notable Groups concerned in Fungus Diseases.
The Rusts {Uredinales).
The family Uredinales is made up of some two thousand
known species of obligate parasites on trees, shrubs, and her-
baceous plants (including ferns). Wherever vegetation of these
types exists, from the polar regions to the equator, some repre-
sentatives of the family are found. Conditions in the West
Indies are not so favourable to the development of rust-fungi
as are those of temperate regions. While recent research has
disproved the idea that the species are extremely few, it remains
true that with few exceptions they are scanty in their distribution
and inconspicuous in their development. A few species occur
on minor crop plants, but the damage done is usually quite
insignificant and affords a great contrast to the immense losses
frequently caused in temperate crops, especially in continental
areas. As regards tropical countries generally, there is an out-
standing exception to this position in the coffee leaf rust
(Hemileia), which is regarded as responsible for the ruin of the
coffee industry in Ceylon. That fungus is not known to occur
in the Western Hemisphere.
The rust fungi are strictly confined to growth on living
tissue, and the species are limited to individual or to related
species of host plants — the general tendency being to narrow
limitation in this respect — except that in many species the
character of heteroecism is exhibited In these cases the fungus
occurs in certain stages on one host, and is then capable of transfer
by spores to a second and sometimes a third host — commonly of
widely different botanical af&nities — and there goes through
other stages in an entirely different form. Only by infection
trials can the relationship of the forms on separate hosts be
determined.
Five distinct kinds of spores occur in the Uredinales, described
c
i8 DISEASES OF CROP-PLANTS
below in relation to the manner in which they are produced.
The Roman numerals are commonly used for reference. The
older terminology is given in brackets.
L JEcia. (secidia), which typically are cup-shaped structures
sunk in the substance of a leaf. The aeciospores (secidiospores)
are usually globular, or angular by compression, developed in
chains. They are capable of immediate germination and the
infection of a suitable host.
O. Pycnia (pycnidia), very small flask-shaped structures
usually, when present, associated with aecia, sometimes with
other forms, but never alone. The pycniospores are minute
unicellular bodies the function of which is either lost or has
escaped detection.
II. Uredinia (uredo-sori), circular or elongated pustules
from which arise stalked roundish unicellular urediniospores
(uredospores), often rough or spiny. These are usually produced
quickly and in abundance, and, as they germinate at once under
suitable conditions, serve for the rapid spread of the fungus.
Thick-walled resting forms of urediniospores which occur in some
species are distinguished as amphispores.
III. Telia (teleuto-sori). Mingled in the same sorus with
the urediniospores, succeeding them, or in separate sori, may
occur teliospores (teleutospores) which typically are stalked, one
to several-celled, thick-walled resting spores, but may diverge
widely from this form, even to the point of resembling secio-
spores. The real criterion of their nature is the manner of germina-
tion, in which each ceU produces a germ tube the tip of which
divides into a linear row of cells, typically four, each of these
giving rise to a short stalk on which one rounded thin-walled
spore is borne. This structure is regarded as a basidium, bearing
basidiospores, and the Uredinales are accordingly classed as
Basidiomycetes. The spores so produced germinate on suitable
hosts and produce a mycelium which develops spores of one of
the types described.
One species may possess in its full life-cycle all the forms
enumerated; or any or all of them, except the telial stage, may be
omitted. It is not, however, necessary that the stages be passed
through in sequence ; an indefinite number of generations, for
example, may arise from the successive production of uredinio-
spores. In the heteroecious species the aecia and pycnia are
found on one host, and the uredinia and telia, or telia alone, on
another.
The mycelium of the rust fungi is freely branched and septate,
usually intercellular, with haustoria penetrating the cells.
The commoner rusts on cultivated plants in these islands are
the following : Puccinia purpurea, Cke. sorghum rust, very
common and generally distributed on guinea corn, imphee, and
Sudan grass ; Puccinia Sorghi, Schw. {Maydis, Bereng.), brown
rust of Indian corn, rather conmion in some places, but not as a
DISEASES CAUSED BY FUNGI 19
rule doing appreciable damage ; Puccinia Cannes (Wint.) Henn.,
common on garden cannas ; Uredo Arachidis Lag., common on
ground-nut and sometimes destructive ; Cerotelium (Kuehneola)
Gossypii (Lag.) Arth., common and sometimes severe on some
perennial cottons, but not very often seen on Sea Island types.
The bright yellow Uredo form of Phakopsora vitis Syd. is generally
common on grape vines, and a brown Uredo on Dolichos ; the
pigeon pea is frequently seen in Trinidad with leaves rusted by
Uromyces Dolicholi, Arth.
Rust infestations may be kept down by spraying with
Bordeaux mixture (see ground-nuts) and resistant varieties have
been found in several important cases.
The Smuts : Ustilaginales.
The smut fungi are somewhat nearly related to the rusts,
though very different in their general habit. Like the rusts,
they are strictly parasitic, depending for nourishment on living
plants. They attack plants of widely different orders, perennials
as well as annuals ; their economic effects being most serious on
the cereal crops. They are few in number and of very small
importance in the West Indies.
Typically the host is infected in the seedling stage (the com
and sugar-cane smuts are exceptions to this) and the mycelium
keeps pace with the developing plant, growing sparingly, and
without producing any notable symptoms of disease, through the
softer tissues. The hyphae are hyaline, branched, sparingly
septate, mostly intercellular, with haustoria which enter the
cells. When the period of flowering is reached the fungus
develops rapidly in the anthers and ovaries and there produces
masses of spores (often black and powdery) which replace the
pollen and the seeds. In some cases swellings in which spores are
produced in a similar way are caused on stems or leaves.
The spores so formed (chlamydospores) are usually thick-
walled and very resistant, in some cases retaining their vitality
for many years. They germinate in water, producing a short
filament (pro-mycelium) on which elongated thin-walled basidio-
spores (distinguished in this family as sporidia) are formed.
These are capable of immediately infecting host-plants which are
in the susceptible stage. In nutritive solutions, as soil water or
moist manure, the sporidia give rise to a yeastlike form of growth
which may continue the life of the fungus until a suitable host
is encountered.
Two common types of infection occur in this family, the dis-
tinction of which is important in regard to control measures.
In certain species the chlamydospores, distributed, after they are
set free, in various possible ways but mostly by the wind, adhere
to healthy seeds and are planted with them, or are already
present in the soil in which planting takes place. They germinate
in the soil moisture, producing sporidia, and^these proceed to
20 DISEASES OF CROP-PLANTS
infect the host seedlings as they appear. Seedlings beyond a
certain size are no longer susceptible.
In other species the infection is through the flower. In this
type the chlamydospores are distributed at the time of flowering,
and arriving in healthy flowers germinate there and infect the
developing seed. The mycelium remains dormant through the
ripening process and until the seed in due course germinates,
when it grows up with the developing seedling as previously
described.
The corn smut (see Chapter XXIV) is different from either
of these types, being able to infect its host on any tender part
and at any age.
With smuts of the first type, control is easily obtained, where
uninfected ground is available for planting, by some form of
seed disinfection. Those of the second type cannot be dealt
with in this way, and seed from a clean source has to be sought.
With both types rotation of crops should be practised to
reduce infection from the soil, and the inclusion of diseased
plants in litter or manure should be carefully avoided. Nor is
it safe to feed smutted grain to animals, since a proportion of
the spores may pass uninjured through the alimentary canal
and be distributed with the manure.
The Powdery Mildews {Erysiphacece).
The Erysiphacese is a well-known family of Ascomycetes
whose members are strictly parasitic, growing as white webs
on the surface of leaves and tender stems, and deriving nourish-
ment from the underlying tissues by haustoria (in a few species
by short hyphae) which penetrate the cells.
The family has a wide range of hosts in temperate countries,
and species occurring on cereals, grape, hop, etc. give rise to
widespread and destructive epidemics. In the Tropics the
family is thinly represented and of small account.
The conidial fructifications, the only stage which appears to
be developed in these islands, consist in the great majority of
species of short upright hyphal branches bearing a chain of
hyaline oblong or rounded conidia. Forms of this type are
referred to the genus Oidium, in the absence of the perithecial
fructifications on which more definite classification is based.
The latter are subspherical bodies formed in the mycelium,
usually in autumn, bearing projecting bristle-like or elaborately
branched appendages, and containing one or more roundish asci
with 2 to 8 spores in each.
The Oidiums of the grape and rose are common in the West
Indies, and there are doubtless other species to which attention
has not yet been paid. The leaf mildew of cotton belongs to the
closely related genus Ovulariopsis.
The standard remedy for this class of disease is dusting with
powdered sulphur or spraying with a sulphur compound. Narrow
DISEASES CAUSED BY FUNGI
biologic specialisation in regard to host plants is sometimes
exhibited in this family, and resistant varieties are therefore
possible.
The Downy Mildews {Phytophthora).
Members of the genus Phytophthora are the most rapidly
destructive of plant parasites. One of them, P. infestans, caused
the potato famine of 1845 ^^ Ireland. P. faheri is the cause of
cacao canker and pod-rot. P. palmivora is the cause of a destruc-
tive bud-rot of palms in India, and has been recognised on coconut
in Jamaica. Another, or possibly more than one species causes in
the West Indies a boll
rot of cotton which is
very destructive in
wet weather.
The mycelium of
Phytophthora is hy-
aline, non- septate
when young, much
branched, and has a
characteristically
coarse and vigorous
appearance under the
microscope. Living
tissue is infested and
rapidly killed. The
common form of re-
production is by large
lemon-shaped conidia,
developed one by one
at the apices of coni-
diophores produced on
the surface of the medium. The massed conidia on the surface
of a cacao pod or cotton boll have a granular glistening appear-
ance ; the mycelium, in very moist conditions, may grow out
and form a white downy covering. The conidia typically ger-
minate by liberating a number of zoospores, which swim
actively in a surface film of water before settling down and
producing a germ tube. While in the mobile condition they are
splashed about by rain, blown about in drops of water by the
wind, and may conceivably be distributed by birds or insects.
In the absence of free water the conidia sometimes produce
germ-tubes direct.
In addition to the conidia, resistant chlamydospores and
oospores may be produced on the mycelium buried in the infested
tissues.
The occurrence and prevalence of Phytophthora diseases is
dependent on a high degree of humidity in the air about the
plants. Close planting, shade and shelter, the inducing of the
From a drawinglpy]
Fig. 2
[J. B. Rarer
Phytophthora Faberi
22 DISEASES OF CROP-PLANTS
growth of heavy foliage by nitrogenous manuring, all favour
their development. The cotton boll rot is frequently confined
to the branches close to the moist soil.
The potato blight is perhaps the most amenable of all impor-
tant diseases to Bordeaux or Burgundy sprays, and cacao pod-
rot has proved to be similarly controllable. These fungi have no
importance as saprophytes, as, although they can be artificially
grown in pure culture on vegetable media, in nature their exist-
ence on dead material is quickly inhibited by the growth of other
organisms. The removal of dead material is desirable however
on account of the resting spores it may contain.
The Anthracnoses.
The term anthracnose is not strictly specific, but is usually
and conveniently applied to diseases caused by fungi of the
genera Colletotrichum and Gloeosporium. These are conidial
forms, not strictly separable from each other, since the presence
or absence of dark-coloured setae (bristles), on which the division
is based, is not a constant character. Certain of the species have
been shown to be the conidial forms of Ascomycetes of the genera
Glomerella and Gnomonia, but the perithecia are rarely met
with in the active stages of disease.
Many species of Colletotrichum and Gloeosporium are pure
saprophytes, others have weak powers of parasitism which
enable them to attack senescent leaves or over-ripe fruit, and
many are virulent parasites capable of attacking perfectly healthy
tissue. All are able to exist as saprophytes on dead material.
The hyaline septate mycelium permeates the infested part
causing the death of the cells. The conidia are borne in acervuli
or pustules, for the production of which a matted or compacted
layer of h3^ha2 (the stroma) forms near the surface of the medium.
From this bed arise crowded short conidiophores, which bear
apical non-septate conidia, twice to several times as long as
broad. These are hyaline, but may appear coloured in mass.
The rupture of the overlying tissue exposes the acervulus or
fruiting layer, which continues to produce conidia. These
accumulate in little heaps, adhering in a gelatinous medium until
they are washed away by rain. The disease produced may be
one of leaves, flowers or fruit or any combination of these, or
it may extend to herbaceous stems or even woody twigs.
Characteristic anthracnoses in the West Indies are those of
mango {Gl. mangifercB )and of lime {Gl. limetticolum) . Both cause
small canker spots or tip-wither on the young shoots, dead patches
on the leaves, blight of the buds and open flowers, shedding of
newly set fruit, and spots or cankers on the fruit that remains.
Other local examples are the anthracnoses of Agaves, beans
(Phaseolus), cotton, papaw, and cassava. An example of a
ripe-rot Gloeosporium is everywhere familiar in the fungus
which causes spotting on ripe bananas {Gl. musarum). Col.
DISEASESiCAUSED BY FUNGI 23
falcatum produces the red rot of the sugar-cane stem and a red
stripe affection on the leaves and leaf-sheaths ; nearly allied
species occur on sorghum and other grasses. The citrus anth-
racnose {Col. glceosporioides), which under some conditions is
reported to produce on Citrus species in general effects similar
to those of the special anthracnose of the lime, is not known to be
of much account in the Lesser Antilles. The cotton anthracnose
Glomerella (Coll.) gossypii appears also to be responsible for much
less damage than is reported from it in the United States.
For the control of anthracnoses it is desirable, in the first
place, to secure clean seed. Adhering spores can be dealt with
by the methods elsewhere described, but in some cases the
cotyledons are infected while the seed is still on the parent
plant. Hot water treatment has been suggested in the case of
infected cotton-seed, but with what ultimate results is not
known.
For tree crops such as limes and mangoes the only remedy
is spraying, and this is the measure adopted for anthracnose in
general where the value of the crop warrants the outlay. The
control of the destructive epidemic of red rot on sugar-cane in
the West Indies was secured by the use of seedling varieties,
some of which have proved to be highly resistant, and others
apparently immune. Varieties of beans (Phaseolus) resistant to
the bean anthracnose fungus have been found, and the character
has been transferred to other types by crossing. Various strains
of this parasite have been shown to possess differential powers
of infesting particular varieties of the host.
On the other hand, many of the reputed species of
Gloeosporium and Colletotrichum have been shown by infections
from pure cultures to have a very wide range of possible hosts.
The Fusanums.
The genus Fusarium includes many purely saprophytic
species and a considerable number of vey destructive plant
parasites. The latter are tjrpically confined to one host plant,
frequently even to the narrower limits of particular varieties
or strains, but are capable of living indefinitely as saprophytes,
very often in the soil.
The species are very similar in morphological characters,
and for that reason, and because of the abundance of saprophytic
species likely to be confused with the parasites, have been
exceedingly dif&cult to identify. Of recent years the use of
standard culture media and of biometric methods in the com-
parison of species has removed much uncertainty and laid the
foundations of an exact knowledge which is much to be desired
with other genera.
The most important section of the genus is the group of
species, similar in biological and morphological characters,
which function as vascular parasites, and by blocking or destroy-
24 DISEASES OF CROP-PLANTS
ing the vessels produce wilt disease usually destructive of the
whole plant. The most notorious of these is the Panama disease
of bananas. Others attack cotton, okra, tomato, cow pea,
pigeon pea, etc., etc.
Other species of Fusarium cause rots of the lower stem
(foot-rot) of seedlings or young plants, as in cow pea, beans
(Phaseolus), and cotton in these islands.
Others again cause tuber rots (potato), fruit rots (tomato,
water melon, cacao), or stem cankers, while numerous mis-
cellaneous diseases have been described with which Fusarium
species have been associated.
The mycelium of Fusarium species is made up of septate,
variously branched, hyaline or light-coloured hyphse. It ramifies
through the vascular system (in the wilt diseases) or the tissues
in general (in the rots) and in some species develops at the surface
into a more or less compact layer forming the basis for the
production of conidia. The conidia are typically of two forms :
the microconidia, often called the Cephalosporium stage, which
are rounded in shape, produced one by one from the tip of a
short hyphal branch, and often adhering in roundish heads ;
and the macroconidia (Fusarium stage), which are elongated,
fusoid, usually more or less curved, and when mature generally
several-septate. The latter are produced on simple or branched
conidiophores, and accumulate in loose and or sometimes slimy
masses on the surface of the medium, when they may appear
either white or light-coloured, frequently with some shade of
pink. The microconidia are sometimes produced within the
infested tissues, and in Panama disease, according to Drost,
follow the course of the sap and form new centres of infection
in the vessels.
Chlamydospores, single or double, and occasionally in chains,
are capable of being formed by the mycelium of most of the
species. They are thick-walled and resistant.
Attempts to control the Fusarium wilts by treatment are
of no avail ; entry is commonly made from the soil, and the
injury is deep-seated. With annual crops relief from serious
infestation may be obtained by rotation, but, these fungi being
usually indefinitely persistent in the soil, a long period is some-
times necessary to reduce the fungus sufficiently for the sus-
ceptible crop to be repeated. In the Panama disease of bananas
infection is also conveyed in the suckers used for planting, so
that an uninfected source of planting material is required as
well as clean land. It does not appear that vigour in the
host, as affected by cultivation, manuring, soil and climate has
much influence in regard to wilt resistance.
The line on which the most notable successes in wilt control
have been developed is that of breeding resistant varieties or
strains, which is rendered possible by the narrow specialisation of
the fungi concerned. There are wilt-resistant cottons, cow peas,
DISEASES CAUSED BY FUNGI 25
cabbages, and melons, and some varieties of bananas are not
subject to attack by Panama disease.
The Diplodias.
Nomenclature in the genus Diplodia is particularly confused
as the result of attempts to divide up its species amongst smaller
genera : Lasiodiplodia, Botryodiplodia, Chatodiplodia, etc.,
based on characters that have proved to be unstable. The
present tendency is to revert to the use of Diplodia pure and
simple, and with this policy, in the present unsatisfactory con-
dition of knowledge concerning the delimitation of the species,
the writer heartily agrees.
There are many saprophytic Diplodias described, but the
species concerned in plant diseases are essentially weak para-
sites, which (as E. J. Butler has pointed out in relation to D.
TheohromcB), though capable of living as saprophytes, are more
often found on dying or recently dead plants than on old dead
plant tissue. It would appear that the advantage they secure
from their ability to infest weakly but still living tissue is neces-
sary for their success in competition with the pure saprophytes.
The limitations of the parasitic powers of a typical Diplodia are
well illustrated on lime trees. In young and vigorous trees the
infestation of the stub of a broken branch stops short at its base ;
in older weaker trees the fungus infests a proportional amount
of the wood and bark connected with the stub, giving rise to a
sharply defined dead segment, eventually running out to a
point, on the older branch or stem. In very weakly trees the
extension of this sort of damage gradually kiUs back the whole
tree.
Diplodias of the semi-parasitic type have a general and
abundant distribution in the West Indies, but how many species
are involved is not known. They have a range of feeding habit
wide enough to include fruit pulp and close-grained wood. The
diseases with which they are typically associated may be grouped
as follows :
[a) Diebacks of twigs and branches when these are in a
weakened condition, entry being made from dead tips, broken
branches, or other wounds productive of dead material on which
the fungus can get a footing. This type has attracted most
attention in the tropics on cacao, Citrus, and Hevea, but is
common on other trees of lesser importance.
[b) Root diseases of similar nature, occurring on plants
debilitated by age, poor or badly drained soil, or other causes.
These are met with occasionally on cacao, more frequently on
limes in the West Indies ; and on tea in the Eastern tropics.
[c) Rots of fruits, which very frequently start from wounds,
but do not always seem to be dependent on them, especially if
the fruit has been checked in its development. Examples are the
Diplodia rots of cacao pods, cotton bolls, citrus fruits and melons
26 DISEASES OF CROP-PLANTS
(d) Rots of' non-woody vegetative parts, as injured or
weakened stems of sugar-cane and the basal part of failing
coconut leaves.
{e) Rots of stored vegetables, such as sweet potato and
dasheen.
The mycelium of the common species of Diplodia is freely
septate, light to dark brown when mature, and imparts a slaty
or smoky colour to infested wood. The spores (pycnospores)
develop in small flask-shaped bodies (pycnidia) formed singly or
in groups, usually thickly set, below the surface of bark or rind
and later breaking through. The spores are oval, hyaline and
non-septate when young, dark and one-septate when mature.
They may be extruded and germinate in either condition, and
mistakes in identification are liable to arise from this habit.
The best form of control of the dieback diseases is cultural,
i.e., the maintenance of the trees in high vigour, and, with cacao
and limes, avoidance of undue exposure. Careful pruning of
failing branches, and prompt treatment of wounds have great
value in preserving oldish trees. In relation to fruit diseases the
fact mentioned above, that these fungi are only common on
dying or recently dead material, gives some force to the usual
recommendation regarding the destruction of diseased wood and
of such intestable material as cacao pods.
The Rhizoctonias.
The Rhizoctonias are soil-inhabiting fungi, distinguished by
the possession of mycelium and sclerotia of characteristic tj^pes.
One or more species are very abundant in temperate countries,
and several are reported to be prevalent in India. In the tropics
they have been very little noticed, but from occasional observa-
tions made it seems certain that at least one species is widely
distributed in the West Indies.
Rhizoctonia Crocorum (Pers.) DC. (violacea Tul.) is a fungus
occurring on numerous hosts but particularly on alfalfa. The
disease occurs in the fields in patches, in which the plants appear
yellow or chlorotic and eventually wilt rather suddenly. The
mycelium completely invests the root system with a weft of
hyphse which at first is pale buff to violaceous, later red-violet
and finally violet-brown. The sclerotia are flattened or rounded
bodies varying in diameter from a few millimetres to several
centimetres, deep violet-brown when mature and thickly clothed
with a persistent velvety felt.
No spore-producing stage of this fungus is known. It has
rarely been grown apart from the host and appears to be strictly
parasitic. It attacks developed plants and is not associated with
damping-off. It has not been recorded from the West Indies.
Rhizoctonia Solani Kuhn, is now known to be the mycelial
stage of Corticium vagum, B. et C. {Hypochmis Solani P. et D.).
It is a fungus of very general distribution in arable soils and
DISEASES CAUSED BY FUNGI
27
gives rise to root disease and to damping-off of cuttings and
seedlings of very many species of plants. According to G. L.
Peltier, epidemics are apparently due to a combination of factors,
such as the presence of a virulent strain of the fungus, a sus-
[From Duggar's Fungous Diseases of Plants
Fig. 3 HypHAE (Rhizoctonia) Basidia and Spores of Corticium Vagum
ceptible variety of plant, and optimum conditions of tempera-
ture and moisture for infection and development. Under
ordinary conditions most of the strains appear to be weak
parasites.
The hyphas freely penetrate soft tissues, and there is usually
more or less external mycelium, though never a copious invest-
ment as in the previous species. The description given by
B. M. Duggar is as follows :
28 DISEASESiOF CROP-PLANTS
" The externa] hyphge are somewhat coloured, usually yellowish brown,
and they are generally of two types. One type may be designated as
purely vegetative and another as constituting the external tufts or masses
when these occur. All hyphae are practically colourless when young,
vacuolate, more or less irregular, septate with the septa at intervals of
100-200 microns. The diameter of vegetative hyphae is 8-12 microns.
Branches arise, and when young these are inclined in the direction of growth
and are invariably somewhat constricted at the point of union with the main
hyphae. As the hyphae mature and become more deeply coloured they are
more imiform and rigid, the distances between cross walls are greater, the
constrictions where branches arise less marked, and the branches are
approximately at right angles to the main hyphae. On certain affected
plants a short tufted or mealy growth occurs and this is made up of hyphae
of very different characteristics. In the young condition threads are
profusely branched and lobed, sometimes botryoid, and they are ultimately
divided into short ovate cells, arranged in short chains, or elbowed, and
producing branches in a more or less dichotomous fashion. In culture the
denser masses give rise to sclerotia. With maturity these hyphae become
light brown in colour ; they break up into short hyphal lengths or single
cells, the indiA'iduals of which bear some resemblance to conidia.
"The sclerotia vary in size from those so minute as to be scarcely
visible to others which may be a centimetre or two in diameter. They are
generally more or less flattened, irregular, deep chestnut brown, and
generally smooth on the surface.','
The Corticium fructification is formed as a collar around the
base of living stems, or on objects lying on the soil, and consists
of a thin web or membrane, pale olive-buff to cream colour, on
which the scattered basidia are borne.
In the examples of damping-off which have come to the
writer's notice in these islands a mycelium with the characters of
Rhizoctonia has frequently been found, and it is probable that
this fungus is much more common than the classic parasite of
this affection in Europe, Pythium de Baryanum, if indeed the
latter occurs at all. A Rhizoctonia collar-rot of bean plants
{Phaseolus vulgaris) is not uncommon and the fungus has been
found on the roots of diseased sugar-cane seedlings. Whether
the collar or crown rot which is reported to prevent the cultiva-
tion of alfalfa is also due to Rhizoctonia cannot be said with
certainty, but specimens received from Montserrat suggest the
possibility. Other occasional root diseases of herbaceous plants
occur in which a sterile mycelium is concerned, and require to be
examined from this point of view.
The genus Marasmius.
The genus Marasmius contains several hundred known
species, most of which are woodland saprophytes on dead
vegetable matter. The fructification is of the toadstool form,
and is distinguished by the papery, not fleshy, consistency of its
substance, which when shrivelled by dr3/ing is capable of restora-
tion to something like its original form on being re-moistened.
Several species occur in the West Indies about the bases of the
shoots of grasses, including sugar-cane, and are well-known in
connection with root disease of that plant. Others occur on the
DISEASES CAUSED BY FUNGI
29
leaf-sheaths of banana, where they have also a connection with
disease, and the same or similar species are found on the "strainer"
of the coconut palm. These are at most rather weakly parasitic.
Fig. 4 Stigmatomycosis. Species A
A much more definite parasite is Marasmius perniciosus, Stahel,
the cause of the witch-broom disease of cacao in Surinam. The
black threads of the so-called horse-hair bhght are the mycelial
strands of a Marasmius, which is no longer regarded as more
than saprophytic in habit.
The Fungi infecting Bug Punctures [Stigmatomycosis).
A type of disease not previously recognised has in recent
30
DISEASES OF CROP-PLANTS
years been found to be wide-spread in the West Indies, in which
fruits and seeds are infected with fungi of what appears to be
a highly speciaUsed group by way of the punctures of plant-
feeding bugs (Heteroptera). The experiments which have been
Fig. 6 Stigmatomycosis. Species C
made indicate that the infection is actually conveyed by the
bug from plant to plant.
For disease of this kind the term stigmatomycosis is proposed,
a typical example being the internal boll disease of cotton
described in Chap. XXV. Two of the fungi concerned, or species
Fig. 7 Stigmatomycosis. Species D (Nematospora)
closel}?^ resembling them, had been previouslj^ met with ; one
of them, Eremothecium cymbalaria Borzi, corresponding with
the writer's Species B, was found in Italy in i888 in capsules
of Linaria cymhalaria and rediscovered in France in 1906-12
in fruits of Cachrys IcBvigata : the other ^ Nemaiospora Coryli
DISEASES CAUSED BY FUNGI 31
Peglion, resembling Species D of the writer, was obtained from
a diseased hazel nut in Italy in 1901, and appears to have been
seen in France in association with the previous species. Nemato-
spora Lycopersici Schneider, described in 1917 from fruits of
tomato supposed to have come from Cuba or Mexico appears
to differ only in the recorded measurements.
Four species have been met with in the West Indies, two of which remain
as yet unnamed. In species A, B, and C, the mycelium is hyaline, fine to
coarse, non-septate except in connection with reproduction, and branching
almost entirely by regular dichotomy. In cultures small bud-like pro-
jections may occur along the course of the hyphae. In Species D (Nema-
tospora) the thallus as developed in fruits and vigorous cultures is typically
yeast-like, and consists of a mixture of two forms : (i) toruloid cells and
cell-groups, the units of which are very variable in form but generally
elliptical or ovate, (2) much larger spherical cells, single or attached in
small groups. In old cultures or when growing in tap-water, a mycelium
is produced which is made up of long, sparingly branched and very sparingly
septate hyphs (Fig. 7).
The spores develop in sporangia which in the three typically hyphal
species are expansions of a terminal or intercalary section of a hypha.
In the fourth species they are formed from a single cell which separates
from the yeast-like thallus, by the lateral expansion of a hypha in the
hyphal form, or by direct outgrowth from a germinating spore. The
spores are set free in all the species by solution of the sporangium wall.
The spores of Species A are unicellular, falcate, measuring 18-21
microns : on the convex side they are provided with a thickened rib
which extends from about the middle to one end, where it projects in
a fine point. They are formed in large numbers in the sporangium, with-
out any appearance of regular arrangement (Fig. 4). The spores of
Species B are acicular, rather blunt at one end, sharp at the other, and
measure 13.5 microns. They are formed in two equal opposed conical
bundles, their broad ends interlocking (Fig. 5).
In Species C and D the spores are alike in form and occurrence. They
are spindle-shaped, produced at one end into a long thread-like appendage,
and have a slight projection on one side near the middle. They are formed
in two equal opposed bundles in eachsporangium, joinedby a thread formed
of their combined appendages. The number on each side varies from one to
ten or more. The two bundles are usually rather widely separated, but
have been seen to overlap and in extreme cases lie side by side. Occasion-
ally a single bundle is formed occupying the whole of a small sporangium.
The measurements obtained of the spore body, without the appendages,
have been 27-35 X 2 in C, 30-40 x 2-3 in D.
Germination in Species A is by an ordinary germ-tube generally
developed about the middle ; in B, C, and D it is preceded by the formation
of a spherical swelling, from which are produced one or two germ-tubes,
or in C and D several short beaded rows of cells which may set up yeast-
like budding. In the last three species the contents of the spore show,
from a division near the middle, a difference in refraction and in taking
up stains, but the existence of a septum cannot be detected.
These fungi have been found in no other situation than in
fruits and seeds, and in the writer's experience infections have
always been associated with bug punctures. A list of the plants
on which infections have been found is given below, and there
is no doubt that the number could readily be largely increased.
The effect on seeds is the production of brown or white
32 DISEASES OF CROP-PLANTS
sunken patches in the cotyledons, and may be trifling or severe.
In the tomato the pulp of immature fruit is blackened in spots
and the taste of ripe fruit spoilt ; in the case of the orange atten-
tion was drawn to the infection by the peculiarity of the flavour.
Species.
LEGUMINOS^. A. B. C. D.
Vigna catjang, V. unguiculata (Cow pea, Rounce-
val, Black-eye). -f 4-
Dolichos Lablab (Bonavist). +
Phaseolus lunatus, P. vulgaris (Lima, French bean) +
Canavalia gladiata (Sword bean), -j-
Crotalaria juncea (Sunn hemp). +
Crotalaria retusa. +
Tephrosia spp. -{■
Indigo/era spp. -f
Cassia spp. (herbaceous). -|-
MALVACE^.
Gossy^ww spp. (Annual and perennial cotton) + + + +
EUPHORBIACE^.
Ricinus communis (Castor oil). +
Jatropha urens. -j-
SOLANACE^.
Ly coper sicum esculentum (Tomato). + + +
Datura metel +
CUCURBITACE^.
Momordica Charantia (Coolie pepper). +
ASCLEPIADACE.,©.
Asclepias curassavica. -\-
RUTACE^.
Citrus sinensis (Sweet orange). +
The Sooty Moulds (Capnodios) causing Black Blight.
The Perisporiacec-E, which include the sooty moulds, are
Ascomycetes with superficial mycelium, occurring mostly on
leaves or fruits. The family is nearly related to the Erysiphaceas,
which it resembles in habit, but its members are at once dis-
tinguishable by their dark brown or black colour. They occur
as webs or closely investing films on a great variety of plants,
and the sooty moulds give rise to the condition usually called
" black blight " in these islands.
The further classification of these fungi is in a very confused
condition, and their nomenclature remains a matter of doubt
DISEASES CAUSED BY FUNGI 33
among the best authorities. Sooty moulds are not uncommon in
temperate countries and have there been usually placed in the
genus Capnodium, while those of the tropics were regarded as
species of Meliola until they were excluded by Gaillard in 1892
as non-typical of that genus. With this exclusion F. L. Stevens,
who has recently studied in detail the true Meliolas of Porto
Rico, entirely agrees, but he leaves open the question of their
correct position, save for the conclusion that in the main they
belong to the Capnodiae of Saccardo. Possibly the genus Fumago,
an older name than Capnodium, should be revived to contain
them.
The Meliolas proper appear as black blotches on leaves,
made up of a network of hyphae closely set with short branch-like
appendages, and bearing various reproductive bodies. They
appear to exhibit specialisation on particular plants or related
groups much in the same way as the Erysiphaceae and are probably
parasitic, though producing as a rule very little visible effect
on their hosts.
The sooty moulds, on the other hand, show no specialisation
and no sign of parasitism. They occur most frequently in
association with infestations of scale insects, aphides, white
flies, and the more sedentary species or stages of leaf-hoppers
(Fulgoridae, Membracidae, Jassidse), developing wherever the
sweet secretions of these insects (honey-dew) are sprayed by the
insect or spread by dew or rain, whether the situation be the
leaves and bark of the infested plant and its neighbours, or
even soil or stones. They will similarly appear and spread in
the nectariferous spray or washings from some profusely flowering
trees, and it is said that spraying a tree with a sugary solution
will produce a heavy infestation.
The injury to the plant due to prevalence of sooty moulds
arises from the film which covers the leaves, reducing the access
of light and obstructing the exchange of air and water vapour.
There is also some commercial loss from the disfigurement of
fruit. The degree of these effects naturally depends on the
amount and duration of the fungus, and this again on the con-
tinued prevalence of the insect which supplies its nourishment.
The effects of the insect, which is frequently obscure, are usually
mistaken for those of the fungus, which is prominent, and the
latter are consequently greatly exaggerated. Black blight,
in fact, is the great bugbear of the West Indian planter and
gardener, and it is to be regretted that some of the attention it
receives is not transferred to the many matters more deserving
of his solicitude.
It is not denied that long-continued infestation may have a
debilitating effect on a tree, and reduce its production of fruit,
but attention should be directed, not to the fungus, but to the
primary pest. The belief which exists that black blight is
contagious is true only in the very limited sense that plants
D
34 DISEASES OF CROP-PLANTS
closely adjacent to an infested tree often receive a share of the
secretions which give rise to it. Apprehensions of the general
spread of the affection from such a source — of the infestation of
cacao from a blackened mango tree, for instance — are baseless
as regards the fungus, and, where the plants are of unrelated
species, are usually so in regard to the insect as well.
Spraying with fungicides for the removal of black blight,
where, as is most commonly the case, this is due to scale insect
infestations, makes matters worse in the end by checking the
beneficial fungi which in favourable weather reduce the prevalence
of the insect. The remedy for black blight must be sought
from the entomologist.
The Classification of Fungi.
The current system of classification of fungi is in a very
imperfect condition in many of its sections. In its general lines
it is as nearly natural as the state of knowledge permits, but
in many of its subdivisions becomes frankly artificial. Judged
as a natural system it not infrequently brings together species
that are obviously not nearly related, and separates many that
are obviously akin. As an artificial system its weakness is
demonstrated by the facility with which half a dozen different
authorities can place the same fungus in as many or even a
larger number of genera.
The confusion which is so frequently apparent arises from the
lack of stable morphological features on which distinctions can
be based, on the difficulties of preservation in recognisable
condition, and not least on the irresponsible way in which new
species have been so often founded with a minimum of enquiry
on imperfect material imperfectly described.
Among the fiowerless plants three groups occur of which
the members are wholly destitute of chlorophyll and depend
(with a few exceptions among the bacteria) on parasitism or
saprophytism for a living. Each group contains parasites
which cause plant diseases.
Myxomycetes, Slime-fungi.
In the vegetative condition, during which they grow and
feed, these consist of naked protoplasm, either in small units or
in masses (plasmodia). In their final stage they come to rest
and produce spores, often in elaborate sporangia, from which
the vegetative form is again derived. The order Plasmodio-
phorales contains the only parasites of plants, the best known
being of the genus Plasmodiophora. The amoeba-like plasmodia
occupy soft tissues in the roots of certain plants, and give rise
to galls or distortions, as in the club-root disease of turnip,
cabbage, and other Cruciferae in temperate countries. Very
many saprophytic myxomycetes occur in the West Indies on
rotting wood and decaying vegetable matter generally.
DISEASES CAUSED BY FUNGI 35
SCHIZOMYCETES, BACTERIA.
Bacteria are exceedingly minute cells, enclosed in a cell
wall and frequently furnished externally with protoplasmic
threads which serve for locomotion. Some species form an
internal resistant body (endospore), which enables them to
withstand desiccation and in some cases exposure for a consider-
able period to the temperature of boiling water. Reproduction
is by division of the parent cell into halves, a method quite
distinct from the budding of the yeasts, which are true fungi.
Bacteria are classified into genera by form, and into species
mainly by their behaviour in cultures on various media. Their
part in the causation of plant disease is discussed in Chapter II.
Examples of bacterial diseases will be found under cotton,
tomato, and citrus.
EuMYCETES, Fungi.
The characteristics of fungi have already been described in
general terms. The vegetative body (thallus) of a fungus
typically consists of branching filaments (hyphse), growing at
the tip, free or combined into more or less definite structures,
but may consist of rounded cells free or in groups, multiplied
by budding, as is the rule in the yeasts, and is the case in certain
stages or conditions of normally filamentous fungi.
Classification is based on the nature and form of the repro-
ductive organs and spore-bearing structures. Where as is often
the case the same fungus has more than one method of spore
formation, the direct production of conidia from hyphae, free or
enclosed, is regarded as subsidiary to spore production by way
of asci or basidia, and any generic name based on the former is
superseded by the one appropriate to the latter. Thus Colletotri-
chum gossypii became Glomerella gossypii when the fungus was
found to possess an ascogenous fructification, though in this
as in many instances it is convenient to retain in common use
the name appropriate to the condition of the fungus as it is
ordinarily found.
Class I : Phycomycetes.
These are fungi which retain considerable resemblance to
certain groups of algge, from which they are thought to have been
derived, including a distinct provision of structures for sexual
reproduction. The latter have in some cases lost their function
and are often largely superseded by the formation of asexual
spores. The mycelium, which is commonly well-developed and
freely-branched, is without septa except in old hyphse or in
connection with reproductive structures. There are two sub-
classes :
I. Oomycetes.
Most of the species form oospores as a result of a perfectly
or imperfectly retained process of the fertilisation of an egg-cell
36 DISEASES OF CROP-PLANTS
by sperms produced in an antheridium. Asexual reproduction
is by sporangia, which release either a swarm of motile zoospores,
or conidia which germinate by the production of a germ-tube.
The genus Pythium contains the well-known fungus which
causes damping-off of seedHngs in temperate coimtries, but has
not been found conspicuous in the process in the West Indies.
Pythiacystis is a parasite of citrus trees. Albugo is a genus of
parasite's producing the white rusts, as that of sweet potato, in
which conidia are formed in sori under the epidermis of the host,
and oospoies occur embedded in the tissues. The family Perono-
sporaceae is of great significance in plant pathology ; its most
important genus, Phytophthora, is separately discussed on
pp. 21-2.
2. Zygomycetes.
Sexual reproduction in this group is by zygospores, formed
by the coniugation of two equal hyphal parts. Asexual repro-
duction, which predominates, is by sporangia, producing numer-
ous spores, or less commonly by conidia. Most of the species
are saprophytic, like the Mucors, which make a fluffy growth
on foods, some are weakly parasitic, as Rhizopus nigricans,
which causes soft rots of vegetables. Choanephora infundi-
lulifera grows on the flowers of cotton and hibiscus in the East
and West Indies. The Entomophthoraceaas are of importance
as parasites of insects, and include species of Empusa occurring
on the green scale and the sugar-cane froghopper.
Class II. Ascomycetes.
In the typical Ascomj^cetes the mycelium is freely septate
from the beginning of its development, there is little trace of a
sexual process, and the distinctive method of reproduction is the
formation of eight spores in a closed tube or sac — the ascus.
The asci are commonly produced side by side in a regular layer,
the hymenium, which may lie open on the fruiting body or on
the substratum, or be enclosed in various ways.
In addition to the ascospores conidia are usually formed by
the mycelium at some stage of its existence, and in many species
reproduction is generally effected in this way, the production of
ascospores taking place only under special and often rare con-
ditions.
There are a few groups of fungi of somewhat uncertain
affinities, of which the fructifications show more or less of an
approach to or analogy with ascus formation, which are placed
in the sub-classes Hemiascomycetes and Protoascomycetes.
The latter includes the yeasts, Saccharomycetaceae, which
typically have no hyphae, but consist of rounded cells reproduced
vegetatively by budding and in some circumstances forming
one to eight internal spores. Nematospora, found in cotton
bolls, etc., is at present placed in this family. The ascomycetes
DISEASES CAUSED BY FUNGI
37
proper (Euascomycetes) contain an enormous number of species,
a fair number of which are plant parasites.
The Erysiphaceae or powdery mildews (see p 20) and the
W'^.
Fig. 8 ASCOMYCETES
1. Peziza. 2. Balansia trinitensis. 3. Scleroderris.
4. Perithecia and Conidia of an Erysiphe. 5. Nectria
6. Xylaria. 7. Hypoxylon. 8. Pseudovalsa.
From Engler & Prantl. Nat. Pflanz.
Perisporiacese (which include the fungi of "black blight") have
mycelium which is superficial on the host plant, commonly on
leaves, and have the asci enclosed in more or less spherical
38 DISEASES OF CROP-PLANTS
fruiting bodies without special openings, the spores escaping by
the rupture or decay of the wall.
The largest number of important parasitic Ascomycetes are
included in the group known as the Pyrenomycetes, in which the
hymenium is enclosed in a spherical or flask-shaped fruiting body,
the perithecium, provided with an aperture through which the
spores escape. There are three orders of Pyrenomycetes.
1. Hypocreales.
The members of this order are broadly distinguished by the
light or bright colours of the perithecia, as distinguished from
those which are brown or black, and their softer consistency as
compared with the hard and often carbonaceous condition
common in the following orders. The Nectriae contain the
genus Nectria, species of which are very common as saprophytes
on dead bark in the West Indies and were at one time held
responsible for cacao canker. Some species are wound parasites,
but none with this character is known locally. Neocosmospora
occurs on the roots of leguminous herbs but is not usually re-
garded as a parasite. Ophionectria coccicola and Sphcsrostilbe
coccophila are of great importance as parasites of scale insects,
and species of Sphserostilbe cause serious root diseases under
certain conditions (see Limes and Banana).
2. Dothideales.
This relatively small order is characterised by the aggregation
of the mycelium into a black stroma in which the perithecia
occur as cavities. It is represented among the local parasites by
Phyllachora spp. on grasses and sedges.
3. Sphceriales.
This is an enormous order with perithecia mostly carbon-
aceous in consistency and black in colour. Rosellinia, Melano-
psammopsis, Leptosphseria, Gnomonia and Glomerella are ex-
amples to which reference is made in Part 2.
Class III. Basidiomycetes.
The typical Basidiomycetes are distinguished by the presence
in a hymenial layer of crowded sporophores of special type
(basidia) each terminating a hyphal branch, usually club-shaped,
and bearing on short stalks two or four unicellular spores
(basidiospores).
Parallel with the classification of the ascomycetes, there are
recognised in addition to the basidiomycetes proper (Eubasidii)
two somewhat anomalous sub-classes, the Hemibasidii and the
Protobasidii. The Hemibasidii contain two families only,
made up entirely of obligate plant parasites, the Ustilaginaceae
(see p 19) and the Tilletiaceae, both giving rise to smut diseases.
The mycelium produces the sooty masses of chlamydospores
direct, and these on germination produce a short filament on
DISEASES CAUSED BY FUNGI
39
which develop a small number of conidia (sporidia), regarded as
equivalent to basidiospores. In the Tilletiaceae the sporidia
Fig. 9 BASIDIOMYCETES
1. SCHIZOPHYLLUM CoMMUNE. 2. CyATHUS MiCROSPORUS.
3. Thelephora. 4. Craterellus. 5. Lycoperdon.
6. Lachnocladium. 7. Fomes. 8. Marasmius.
4, 5, 6, 8 after Nat. Pflanz
are elongated and frequently join by a small cross filament to
make pairs.
40 DISEASES OF CROP-PLANTS
The Protobasidii include the most important single order of
plant parasites, the Uredinales or rust fungi (see p. 17). The
relation to Basidiomycetes is shown in the germination of the
teliospores, formed direct from the mycelium, which in similar
fashion to the chlamydospores of the smuts typically produce in
germination a filament divided towards its free end into four
cells, each of which produces a sporidium, regarded as a basidio-
spore.
The Eubasidii contain, among other types, the mushrooms,
toadstools, bracket-fungi and other large forms which are the
most conspicuous and familiar of fungi. The number of plant
parasites among them is relatively small.
Some of the Hypochnacese form a thin webby covering on
living leaves, and have been held to be associated with thread
blights.
The Thelephoraceae form a thin, leathery, membranous, or
web-like covering on the substratum, often in extensive sheets
or patches. The genus Corticium contains the fungi of the West
Indian thread blights and of pink disease ; Odontia spp. are
associated with root disease of sugar-cane ; the Septobasidiums
are believed to live at the expense of scale insects (see Grey
Blight of Limes).
The Polyporaceae, many of which have bracket-like fructifica-
tions, take a large part in the destruction of dead wood, and in
some cases extend this mode of life to the wood of living trees if
they can obtain access through wounds, dead branches, or
debilitated parts. They are distinguished by the arrangement
of the hymenium as a lining to a system of pores, which may be
shallow, or deep and tubular. The fruiting bodies are often
large, and may be soft and fugitive or leathery to woody and
persistent.
The Agaricaceae are the gill-fungi, so called from the extension
of the hymenium over a series of thin flat plates of tissue, as in
the mushroom. Usually the fructifications consist of a roundish
cap (pileus), bearing the gills on its under side and supported by a
stalk (stipe), or attached by a part of its margin. Very few of
the Agaricaccce are parasites, but Marasmius contains injurious
species, one of which is the cause of a serious disease of cacao,
while others are associated with root diseases of sugar-cane and
banana. Schizophyllum is a common saprophyte which some-
times appears on failing sugar-cane.
Class IV. Fungi Imperfecti.
The exceedingly numerous groups and species of fungi
collected for the sake of reference in this class are not imperfect
in their powers of maintaining themselves, but have not been
found to possess any character which enables them to be fitted
into the classes already described. A very large number of
important plant parasites are found among them.
DISEASES CAUSED BY FUNGI
41
Fig. 10 TYPES OF CONIDIAL FRUCTIFICATIONS
1. Sphaeropsis. 2. Graphium (conid. stage of Rosellinia).
3. 4. Cercospora. 5. Aspergillus. 6. Penicillium.
'. Macrosporium. 8. Alternaria tenuis. 9. Oospora.
10. MucoR (Zygomycetes). 11. Monilia.
12. GlOEOSPORIUM LINDEMUTHIANUM. 13. FUSARIUM.
14. DiPLODIA.
After Nat. Pflam.
42 DISEASES OF CROP-PLANTS
Without doubt many or most of the Fungi Imperfecti have
affinities with ascomycetes or basidiomycetes, but it is probable
that in a large number of cases the " higher " form, of fructifica-
tion has fallen into disuse, and in others is produced only ex-
ceptionally.
Three orders are used in the current system of classification.
These are divided into families, and within the families a mainly
artificial method of grouping according to coloration and the
number of divisions in the spores is adopted.
1. Sphceropsidales.
This order consists of forms in which the conidia are produced
typically on short, closely-set conidiophores in enclosed spherical
or flask-shaped fruiting bodies (pycnidia) opening by a pore or
slit and resembling the perithecia of ascomycetes. In the
family Sphcerioidaceae, which contains those with black or dark
brown, often leathery or carbonaceous pycnidia, are a great
many kinds which occur on dead or failing leaves, fruits, and twigs,
and some which are definitely parasitic on similar material. To
this family belong Phyllosticta and Phoma, of which a very large
number of species have been described as occurring in leaf-spots
or in the cortical tissues of stems. Here also belong Diplodia,
which is elsewhere discussed, Phomopsis, Cytospora, Sphaeropsis,
and the large genus of leaf-spot fungi Septoria.
2. Melanconiales.
In this order of only one family, Melanconiacese, the layer of
short conidiophores is not enclosed in a regular pycnidium, but
occurs in groups or patches (acervuli) which are usually irregu-
larly enclosed at first, often only by the epidermis, and become
exposed sooner or later. It includes Gloeosporium and Colleto-
trichum, the fungi of the anthracnoses (see page 22), Melan-
conium (see sugar-cane), Septogloeum (ground nut), and Pes-
talozzia (coconut).
3. Moniliales (or Hyphomycetes).
This is an order of great diversity distinguished as a whole
only by the free-standing dissociated conidiophores or conidia-
forming hyphai. The forms of these range from hyphae which
divide up without much differentiation into conidia to special
conidiophores of peculiar structure or with elaborate branch-
systems. There are four families. The Moniliaceae (or Mucedi-
naceae) and the Dematiaceae contain the forms in which the
hyphae associated with reproduction are not more than loosely
aggregated ; the former contains the hyaline or brightly coloured
kinds, the latter those with either hyphae or conidia, or both,
dark coloured, mostly smoky brown or black. In the Moniliaceae
DISEASES CAUSED BY FUNGI 43
are included, to give a few examples, Oidium, Cephalosporium,
Aspergillus, Penicillium, Botrytis, Sporotrichum, Verticillium ;
in the Dematiaceae : Thielaviopsis, Cladosporium, Helmintho-
sporium, Alternaria, and Cercospora
The third family, Stilbaceae, consists of forms in which
several or many of the fertile hyphae are collected into a compact
bundle to form the conidiophore, the conidia being borne in a
head or tuft at the top or sometimes along the sides. Stilbella
(or Stilbum) which includes the conidial forms of Sphserostilbe,
Graphium, which similarly includes the conidial form of Rosel-
linia, Isaria, which has many species parasitic on insects, and
Gibellula, parasitic on spiders, are examples.
In the fourth family, Tuberculariaceae, the fertile hyphae,
together with an admixture of sterile hyphae, are gathered into a
rather compact body known as the sporodochium, which may be
globose, discoid, or form a somewhat irregular crust. Often
the hyphae are more or less radially arranged. The consistency
of the mass is often waxy or gelatinous, may be horny, or may be
merely made up of threads more or less embedded in mucilage.
The family is in many respects a vaguely defined one. Its most
important parasitic genus is Fusarium. iEgerita, the white-fly
fungus, is another example.
A further section of the Fungi Imperfecti, the Sterile Mycelia,
consists of those fungi for which not even a conidial form of
reproduction is known. The important sclerotium-forming
parasites Rhizoctonia and Sclerotium remain here. Himantia
stellifera, a mycelium on the roots of sugar-cane, will probably
be shown to belong to Odontia, a basidiomycete.
The genus Graphiola, parasitic on palms, of which G.
phcenicis occurs on probably every date-palm in the West
Indies, remains unclassified, though it produces spores. It
is so individual in form that no relationship to other fungi can
be established.
Systematists differ somewhat in regard to the classification
of fungi ; the following arrangement, which aims at simplicity,
is based on the system of Engler and Prantl. The positions both
of the groups of parasites mentioned in this chapter and of com-
mon saprophytes are indicated. It will be noticed that in this
scheme the Perisporiales are included in the group Pyrenomy-
cetes, which is characterised by having the asci enclosed in a
perithecium. In most recent classifications the Perisporiales are
treated separately, as has been done on p. 37.
Myxomycetes (slime-moulds).
ScHizoMYCETES (bacteria).
EuMYCETES (all other fungi).
Class I. Phy corny cetes.
{a) Oomycetes (downy mildews).
[h) Zygomycetes (moulds like Mucor).
44 DISEASES OF CROP PLANTS
Class II. Ascomycetes.
(a) Hemi- and Protoasci (yeasts, Exoascus, etc.)
(b) Plectasci (common blue, green and yellow moulds).
(c) Discomycetes (Sclerotinia and other cup fungi).
(d) Helvellineae.
(e) Pyrenomycetes.
1. Perisporiales (powdery mildews and sooty moulds).
2. Hypocreales (Nectria, etc.)
3. Dothideales.
4. Sphaeriales (most forms with black perfect fruiting
bodies).
Class III. Basidiomycetes.
{a) Hemi- and Protobasidii.
1. Ustilaginales (smuts).
2. Uredinales (rusts).
{b) Hymenomycetes (Marasmius, Corticium, gill and
pore fungi,
(c) Gasteromycetes (puff-balls, etc.)
Class IV. Fungi Imperjecti.
1. Sphaeropsidales (Diplodia, Phoma, etc.)
2. Melanconiales (anthracnose fungi, etc.)
3. Moniliales or Hyphomycetes (Fusarium, etc.)
CHAPTER II
DISEASES CAUSED BY BACTERIA
Bacteria are minute organisms, typically unicellular, but in
some species and under some conditions adhering in filaments
or massed in a colony in a gelatinous matrix. Some are so small
as to be close to the limit of visibility with the highest powers of
the microscope, and others probably exist which are beyond it.
The cells are enclosed by a thin membrane. Bacteria possess no
chlorophyll, and for the most part are either saprophytic or
parasitic, though there are some which are able to derive their
energy from inorganic compounds. In their mode of life they
consequently resemble fungi, but differ in their method of re-
production, which is by equal subdivision of the parent cell.
They are accordingly classed as Schizomycetes (fission fungi),
though there is no sound reason for regarding them as especially
related to fungi at all. Their possession of a cell wall is usually
held to justify their classification as plants. They are frequently
motile by means of protoplasmic threads (flagella). Some
species produce thick-walled resistant spores, but this is seldom
the case among the known parasites of plants.
The earliest definite discoveries of bacterial disease of plants
were made in the years round about 1880 by several independent
investigators. The destructive pear-blight of the United
States, worked out by T. J. Burrill, was the first to be clearly
established as bacterial in origin. For many years the attitude
of academic botanists, based on preconceived opinions as to the
ability of bacteria to enter or to lead a parasitic existence in
plant tissues, was hostile or sceptical, and, although overt opposi-
tion to the idea was silenced iDy the proofs brought forward in
1901 by Erwin F. Smith, it is only within the last few years that
the reluctance to accept it may be said to have died down in
Europe.
To the working pathologist it is now clearly evident that
bacterial diseases, though fewer in number and somewhat
restricted in range, stand in respect of specific parasitism and
capacity for injury on an equal footing with diseases of fungus
origin. There is indeed so close an analogy between the two
groups of causative organisms in the various degrees of parasitism,
in the nature of the attack and of infestation, and in the re-
actions of the host plant towards them, that it is unnecessary
to repeat at any length or much to qualify in respect of bacterial
45
46 DISEASES OF CROP-PLANTS
diseases the information on these subjects already given for
fungus diseases.
The bacteria which give rise to plant diseases form but a
minute fraction of the species known to exist, and with the ex-
ception of those involved in the rotting of ripe fruit, of storage
organs like the potato tuber, or of masses of soft tissue such as
the heart of coconut palms, they are parasites specific to the
particular plant or disease with which they are associated. A
single species may, however, as in the case of a fungus, give rise
to several forms of affection on the same plant (as in angular
spot, black arm, and boll disease of cotton) or produce the same
type of disease on a variety of plants (as in crown gall).
Apart from the parasites there is evidence of the existence
of specific epiphytic bacteria on the surface of leaves or other
organs. A good instance is afforded by the organism which
appears to be always present, in the West Indies as in the United
States, on the cotton plant, and takes some part in the infestation
of the contents of the boll when bug punctures permit its access.
One of the main objections to the possibility of bacterial
diseases was the supposed absence of any means of penetration,
and this was so far justified that the ability to penetrate cuticle
possessed by some fungi does not seem so far to have been
demonstrated in bacterial parasites. The latter can, however,
find their way through apparently uninjured cell- walls, for they
are often very evident inside, but it is not clearly known how
they do it. Some species invariably enter the plant by way of
wounds, so that insect injuries are frequently of great importance
in the incidence of bacterial diseases, others are capable of entry
by way of the stomata (angular leaf spot), the water pores of
leaves (black rot of cruciferae), the nectaries of flowers (blossom
blight of pears), and possibly the pistil (bacterial boll-disease).
The tissue of the living plant most liable to attack is the
parenchyma of storage organs or of young developing shoots,
and two types of infestation may be recognised. The tissue may
be destroyed by the action of secretions which dissolve the middle
lamella (the material which cements together the walls of the
separate cells) or the bacteria may enter into relations with the
cells which induce abnormal growth and proliferation, resulting
in the formation of galls or swellings. The latter type may be
compared with the most advanced form of fungus parasitism,
in which the parasite derives its food from cells which continue
their existence.
The most generally destructive type of bacterial disease is
caused by the vascular parasites, of which Bacterium solanace-
arum on the tomato is an example met with in the West Indies.
In this type, although there is some amount of invasion of the
parenchyma, the principal seat of infestation is the vascular
bundles, the conducting elements of which are affected and often
blocked by the parasite.
DISEASES CAUSED BY BACTERIA 47
With regard to the phenomena of resistance and suscepti-
bility, and to the measures available for prevention and control,
no general distinctions need be made between bacterial and
fungus diseases. The methods of distribution are to a large
extent the same, except that transport of the dried organisms
by the wind is not apparent. Insects undoubtedly have very
considerable importance in this respect.
CHAPTER III
DISEASES CAUSED BY INFECTIVE VIRUSES
The diseases to be discussed in this chapter, which include
several of great economic importance, have been usually relegated
to the undefined category of " physiological " affections. Though
knowledge regarding them is very uneven, it would now seem
both possible and desirable to bring them together as a class,
distinguished by the possession of an infective principle which
can be transferred to a healthy plant and reproduce the disease,
but which within the range of our present technique gives no
evidence of association with an invading organism. It would
further appear that a sub-division of this class may be made by
distinguishing between affections in which the juice of the
diseased plant is generally and directly infective, as in the true
mosaic diseases, and those in which this property is absent and
infection is restricted to special means. In some of the latter
class transmission depends on contiguity in a manner which is
still obscure, in others it is regularly effected by insects. Usually
it may be effected artificially by budding or grafting where these
are possible, provided that organic union of stock and scion
takes place.
The word virus used in connection with these diseases must
not be taken as more than a convenient expression of the idea of
an infective principle of unknown nature, recognised only by the
effects it produces. The validity of such a conception may be
more readily conceded if it is remembered that our knowledge
of enzymes, useful as it has proved, is of the same nature.
The virus diseases are typically systemic, affecting all parts
of the plant. They have apparent analogues in animal and human
pathology, some highly infectious, others restricted in their
transmission.
In their external symptoms these diseases are liable to be
closely simulated, on the same or on other plants, by appearances
produced in some cases by the action of external factors, as in
the dwarfing of shoots and mottling and crumpling of leaves
induced in cotton and ground-nuts by out-of-season planting,
in other cases by genetic factors, as in certain hybrids and loss-
mutations of cotton in which similar characters appear, in still
others by insects, as in froghopper blight of sugar-cane, which
closely reproduces many of the symptoms of sereh.
48
DISEASES CAUSED BY INFECTIVE VIRUSES 49
The investigation of virus diseases has in consequence been
greatly confused by dependence on symptoms as evidence of
identity. This has resulted in wide disparities between the
conclusions of investigators dealing with externally similar but
separate diseases. There is the further difficulty that wide
differences in resistance and response on the part of the affected
plants exist in relation to these as to bacterial and fungus
diseases.
The Mosaic Diseases.
The name mosaic disease, which has reference to irregularities
of distribution of the green colour of the leaves, has come to be
especially associated with a well-marked class of diseases which
have in common, in addition to this symptom, an infectious
virus, contained in the expressed juice of most or all parts of
the plant, capable by inoculation or (except in sugar-cane mosaic)
by contact with a wounded surface of setting up the disease in a
healthy plant of the same species. The name has been applied
to other types of disease, as, for example, the mosaic disease of
cotton, which is a deficiency disease, but will probably be re-
stricted in future to the class under discussion.
The best-known mosaic diseases are those affecting tobacco,
tomato, potato, and some other Solanaceous plants ; there are
also similar diseases of bean, cucumber, lettuce and spinach.
Recently the mosaic disease of sugar-cane has assumed great
importance in the West Indies. It is not a typical member of
this class, not having been found to be transmissible by contact
or ordinary wounding, and only with great difficulty by artificial
inoculation.
There is close correspondence in the general characteristics
of these diseases as developed on their appropriate hosts, indi-
cating a generic relationship between them. Such differences as
occur are of a minor and specific nature.
The following summary of symptoms given by Allard for
tobacco mosaic includes the characters of the whole class.
1. Partial or complete chlorosis (reduction of chlorophyll).
2. Curling of the leaves.
3. Dwarfing or distortion of the leaves.
4. Blistered or " savoyed " appearance of the leaves.
5. Mottling of the leaves with different shades of green.
6. Dwarfing of the entire plant.
7. Dwarfing and distortion of the blossoms.
8. Blotched and bleached corollas,
g. Mosaic sucker growths.
10. Death of tissues.
These symptoms may be present in part only or on part of
the plant only, and may be lightly or severely developed. In-
fection appears to be in all cases quite general, so that removal
of the distorted parts of partially affected plants is no remedy.
£
50 DISEASES OF CROP-PLANTS
Apparent recovery may take place only in plants not fully
susceptible. It is not usual for death to result directly from
this type of disease, though more or less necrosis of tissue fre-
quently occurs. In spinach blight this feature is unusually
marked, and, contrary to the general rule, its development
continues until the plant is killed.
The conditions governing infection by contact and the
question of the distribution and characters ot the virus have been
closely investigated by Allard, working on tobacco mosaic. The
virus permeates all parts of the plant, including the corollas of
the flowers, the hairs on the leaves, and the roots. It is still
contained in juice expressed through a Chamberland orBerkefeld
filter. Plant material dried and ground retained its virulence
i| years. The virus preserved in ether, toluene, and glycerine,
was infective 4 months later, as was juice which had been allowed
to undergo natural fermentation. It is highly resistant to the
action of powerful chemicals, withstanding nitric and hydro-
chloric acid added to the virus solution until the concentration
reaches i per cent., and phosphoric, citric and acetic acids up to
2 to 5 per cent. Carbolic acid had no effect in a 2 per cent,
solution and the much more powerful germicides cresol, creolin and
phenoco showed no apparent increase over carbolic acid in their
effect on the virus. Chloral hydrate (10 per cent.) and naphtha-
lene, camphor and thymol in excess had no appreciable effects.
Mercuric chloride i in 100 affected the virus but little, and
quinine bisulphate i in 25 had no appreciable effect after 19
days' treatment. Alcohol destroyed the virus in concentrations
above 50-55 per cent., copper sulphate showed itself rather
toxic, while 4 per cent, formaldehyde had a rapid action and was
found suitable for sterilisation purposes. The virus is destroyed
by temperatures above 80° C.
A dilution of the virus to one part in 1000 in water was as
effective in producing infection as the original virus ; at one in
10,000 some attenuation was shown, and one case occurred even
from a dilution of one in 1,000,000. The inoculations for these
tests were made with only so much of the diluted virus as could be
introduced by the prick of a needle.
The incubation period of the mosaic diseases is variable. In
tobacco the shortest time observed has been 6-7 days, and
usually 12-15 days are required. The length of the period
depends on the kind and age of the plant, and on such external
factors as accelerate or retard growth.
Infection can be artificially effected by inoculation as above,
and by the touching or handling of healthy plants after diseased
ones. Spraying the virus on to sound leaves does not readily
give rise to infections, but injury to the leaf hairs such as arises
from handling is sufficient to open the way for infection to take
place.
It is characteristic of^this type of disease that the infective
DISEASES CAUSED BY INFECTIVE VIRUSES 51
principle does not reside in the soil. Even buried diseased
material does not as a rule produce many infections, and such
cases as do occur are believed to depend on root injuries which
give access to the virus. Steam sterilisation is effective in remov-
ing any risk of infection from diseased material retained in the
soU.
The transmission of mosaic diseases in the field or green-
house has been repeatedly shown to be effected by aphides
(" plant lice " or " green fly.") Most of the experiments have
only gone so far as the transfer of the insects from diseased to
healthy plants, with others from healthy plants as controls, but
in spinach blight McClintock and Smith have demonstrated that
the offspring of virus-bearing aphides, as far as the fourth
generation, may transmit the disease without themselves having
had access to infected plants.
The distribution of mosaic diseases suggests the probability of
the occurrence, at least occasionally, of transmission by seed. In
several cases, as in tobacco and spinach, the results of attempts
to demonstrate this have been uniformly negative ; in others, as
in the mosaic diseases of bean (Phaseolus) and cucumber, the
occurrence of seed transmission has been established.
The viruses of mosaic diseases exhibit specific limitations
similar to those of organic parasites. The infectivity of a virus
ma}' be confined to one host or it may produce disease, of varying
severity, in related species. The disparities in the results ob-
tained by different groups of investigators even suggest the
occurrence of different " strains " of virus in the same host
species ; in fact, it may be said that in respect of susceptibility,
resistance and immunity the mosaic diseases exhibit the same
phenomena as do those originating from fungoid or bacterial
parasites.
Virus Diseases with Restricted Transmission.
Under this heading it is proposed to include for comparison
the following diseases : curly-top of beet, leaf-roll of potato,
sereh disease of sugar-cane, peach yellows, and peach rosette.
These have all shown themselves to be capable of transmission
to healthy plants, while affording no evidence of causation by
any visible parasite. This leads to the assumption, as in the
first group, that an infectious virus is concerned in their pro-
duction, but in distinction from these it has not been found pos-
sible to produce infection by artificial means, if budding and
grafting, which involve organic union, be excluded from this
category.
The affections mentioned have great economic importance,
and include some of the most widely discussed plant diseases,
but further experimental information, especially regarding
sereh disease, is required before a really definite comparison xan
be made.
52 DISEASES OF CROP-PLANTS
These affections resemble the typical mosaic diseases in that
they are systemic, affecting the whole plant, that recovery is
unknown, and that they find expression in general effects on the
foliage and in the eventual dwarfing of the shoots. Their
symptoms, however, are not nearly so uniform.
In curly-top, leaf-roll, and sereh a degeneration (necrosis)
of the phloem strands in the vascular bundles has been claimed
to be characteristic, and has been variously regarded as the
cause or the consequence of the visible changes in the organs
of the affected plant.
In the curly-top of beet the veins of the leaves are distorted,
and the leaf blade is either curled inwards, or retracted and
puffed out between the veins. The foliage is dull-coloured and
brittle, eventually turning yellow from below and dying. Dense
masses of rootlets are thrown out. None of the symptoms is in
itself specific, but the effect on the veins is regarded as the most
definitely characteristic. Where the stage of stem production
is reached the development is feeble and results in dwarfed
and stunted shoots which may or may not survive to the point of
producing seed. Plants may be infected and killed in the seedling
stage.
In potato leaf-roll growth is slow, the shoots are dwarfed by
the reduction of the internodes, the leaves are discoloured
yellow and are rigid and brittle, the edges of the leaflets are
rolled inwards. In a later phase the leaf tissue dies locally,
producing black or brown spots. The symptoms may appear
from the outset of growth, or develop only at the top of the shoots.
In sereh disease of sugar-cane there is great variation in the
development of symptoms. Typically there is a shortening
of the internodes, which in the leafy top of the shoot results in
a characteristic fan-like arrangement of the leaves ; the leaf-
blades are short and narrow and may die in an irregular manner ;
the stems are crowded with aerial roots ; the buds develop pre-
maturely, until in the later stages the stool is reduced to a dense
mass of grass-like shoots. Two or three years are occupied in
reaching the extreme condition. The vascular bundles of
diseased canes, especially in the rootstock and at the nodes,
are reddened by the deposit of gum in their elements.
The most characteristic symptoms of peach yellows are
prematurely ripe, red-spotted fruits, and premature unfolding
of the leaf buds into slender pale shoots, or into branched,broom-
like growths. In successive seasons the trees become gradually
stunted and enfeebled, dying in 3 to 5 years.
Peach rosette is a rapidly developed affection in which all
the leaf buds grow into compact tufts or rosettes, containing,
though seldom more than two or three incles long, usually
several hundred small leaves. A tree thus attacked dies during
the following autumn or winter.
As already stated, symptoms of the types described above may
DISEASES CAUSED BY INFECTIVE VIRUSES 53
be produced by the operation of external factors, and diseases
of similar type exist which appear to differ only in the absence
of known transmissibility. This being the case, most importance
must be attached to the facts regarding transmission. In the
diseases under discussion the occurrence of epidemics and the
infection of plants derived from material originating outside
their range afford sufficient evidence of the general fact, but
except in the case of curly-top little is known of the manner in
which transmission takes place.
It is now well established with regard to curly-top of beet
that it is readily and regularly transmitted from diseased to
healthy plants by the small Jassid leaf-hopper Eutettix tenella,
and recent experiments have indicated that for this to take
place a period of at least 24 hours' " incubation " of the virus
in the body of the insect is necessary. Leaf-hoppers from
healthy beets or from other plants do not produce infection.
All attempts at transmission by inoculation or contact have failed.
The disease is continued by cuttings, but not, so far as experience
has gone, by seed ; it can be transferred by grafting.
The remaining diseases are transferred from infested to neigh-
bouring healthy plants by some means as yet unknown. All
have shown themselves capable of rapid extension from a small
initially infected area. Potato leaf-roll and the peach diseases
are transferable by grafting. Leaf-roll and sereh are continued
by cuttings. Peach yellows is definitely stated not to be contracted
from soil in which diseased plants are grown, and no statement
has been seen that this can occur in curly-top. The infection
of leaf-roll has been regarded as persisting in the soil for two
or more years, but confirmatory evidence of this is lacking
and survival may admittedly have been due to deeply buried
tubers. Good evidence is produced, however, that transmission
takes place through the soil from one growing plant to another.
Whether soil infection occurs in sereh does not appear to be
known, and the manner of its occurrence would appear to be
equally capable of explanation by a theory of insect transmission.
CHAPTER IV
PHANEROGAMIC PARASITES
In point of species the mistletoes form the only numerous group
of parasitic flowering plants in these islands, but in abundance the
first place is easily held by the love-vine, which occurs in great
masses in some stretches of uncultivated country, and is fre-
quently highly conspicuous on roadside trees and shrubs. In
neither case does the prevalence of these plants make more than a
narrow contact with plant industries.
Love Vine : Vermicelle.
The plant commonly known by these names is Cuscuta
Americana L. of the order Convolvulaceae, which is common
in all the principal islands save Barbados, where it is distributed
rather scantily. It is found as a rule on waste land, on wayside
shrubs and trees, and on hedges or scattered bushes among
gardens or small holdings. The writer has never seen it attacking
arable cultivations of any description. It causes annoyance
from its infestation and disfigurement of hedges, especially of
hibiscus ; and occasionally gets into cultivated trees and bushes.
The plant is without leaves, and forms when well developed
large confused masses of slender flexible stems, bright golden
yellow in colour. It is completely parasitic. Where it comes in
contact with twigs in a suitable condition it twines around them
and becomes firmly attached by sucker-like haustoria which
make connection with the tissues and put the parasite into com-
munication with the food supplies of the host. Flowers in small
clusters are freely produced and give rise to numerous seeds.
The plant is an annual one, and sooner or later turns brown and
dries up. The seeds, which are tenacious of life, germinate in the
soil and produce a filament, the free end of which circles round in
search of a suitable stem, and, when this is found, coils round it
and loses its connection with the ground. The plant is also
freely reproduced from fragments of its stems when these find
themselves in an appropriate situation, and transference of this
kind is sometimes due to malicious persons or to children.
In several islands the destruction of love- vine is enjoined
by ordinance on owners or occupiers of land, but eradication is
troublesome to achieve, and the regulations in consequence are
difhcult to enforce. The common method is to cut down or
54
PHANEROGAMIC fPARASITES
55
prune back the affected plants, burying the material removed or
burning it when it is dry. Small pieces left on living twigs will
produce a fresh growth, and if seeds have been allowed to ripen
and fall they may give rise to new infestations after a long or
Drawn by]
Fig. 11 Love Vine on Solanum Sp.
[W. Buihn
shorter time. Destruction before seeding is therefore very
desirable.
For the control of similar parasites on arable crops in Europe
and America most reliance is placed on securing clean seed, but
56 DISEASES OF CROP-PLANTS
the use of spray fluids is also described by various authors. It
would seem that treatment of this kind might be more widely
adopted in the West Indies.
The substance most recommended is copper sulphate in 2.5 per
cent, solution, and 0.5 per cent, sulphuric acid is also mentioned.
Cooper's Cattle Dip, ij oz. to i gallon of water has been used with
success in Trinidad on hibiscus. Trials might be made with
strong lime-sulphur sprays. No treatment can be expected to
destroy the parasite without also destroying leaves and green
shoots, but strengths can be found at which the older twigs
will not be seriously affected.
Love-vine is itself subject to parasitism by more than one
species of fungus which occasionally destroy it, but this kind of
control, being dependent on special conditions, is not likely
to be much extended by artilcial means.
The crop plant on which love-vine gives most trouble is
perhaps the lime, which is liable to become heavily infested.
Cacao and cassava are not uncommonly attacked, and the
parasite has been reported on sugar-cane. The favourite host
of all is the common hibiscus, but there appear to be few shrubs
or trees which are not subject to infestation.
Love-vine is a plant which forces itself on the attention, and
as it frequents the borders of highways its presence is noted
even by the unobservant. Consequently ^t becomes, like black
blight, an object of more public concern than its relative im-
portance as a pest can be considered to justify.
Cassytha.
A parasite so closely resembling love-vine in habit that it
is not usually distinguished from it, though belonging to a different
natural order, is Cassytha filiformis L. (Lauracege), which is well
distributed but not nearly so common as Culbuta. It may be
distinguished in the vegetative condition by its more wiry, very
finely striated stems, and its duller colouring of green or greenish
yellow. It is more restricted to waste places and bush land.
Bird Vine : Mistletoe.
A very considerable number of species of mistletoe
(Loranthace£e) are distributed through the West Indies on native
and introduced trees. Though they are by no means confined,
in most cases, to individual or even related hosts, there is evidence
of considerable selectivity in their distribution.
Unlike the love-vine, the mistletoes are fully supplied with
green leaves, but their seeds germinate, not on the ground,
but on the branches of trees, and their root system becomes
embedded in the wood of the host and draws from it the supply
of water and mineral salts which ordinary plants obtain from
the soil. In addition to this drain on the sap the heavy masses
of leaves occupy space and light to the exclusion of part of the
PHANEROGAMIC PARASITES
57
Drawn by] [W. Buthn
Fig. 12
Bird Vine {Slruthanthus dichotrianthus) on Cacao.
58 DISEASES OF CROP-PLANTS
foliage of the infested tree. The combined effect is to produce a
sickly ill-nourished condition in the host and frequently the death
of the terminal parts of infested branches.
By analogy with the European species it is assumed that the
distribution of the local mistletoes mainly depends on birds,
which feed upon the berries and leave the sticky undigested
seeds adhering to the bark of trees subsequently visited. The
primary root produced in germination penetrates the cortex
as far as the wood, where it spreads out into an ultimately woody
organ of attachment. The wood of the host is not actually
penetrated, but the base of the organ becomes more deeply
buried every year by the addition of the annual rings of wood
to the branch, with which the parasite keeps pace by step-like
or finger-like extensions.
When its first appearance is neglected, bird-vine may become
a serious pest of cacao and of limes, to mention the two most
important of its hosts. The species Dendropcmon caribceus
Krug. et Urb. is very abundant in semi-neglected lime fields in
Dominica. Another species is reported to be common on avocado
in the Virgin Islands.
Of the numerous species occurring in Trinidad, by far the
worst pest of economic plants is Struthanthus dichotrianthiis
Eichl, which attacks cacao, mango, orange and many other trees.
It is peculiar in the possession of aerial roots which run along the
branches of the host and rapidly extend the infestation.
The only remedy for mistletoe is cutting out and pruning,
carried out in the course of systematic inspections. The cleaning-
up of infested trees throughout a district is desirable, and legisla-
tive action to secure this has been taken in Trinidad and St. Lucia.
An insect, not identified, has been reported to destroy the
berries of the Dominica species above mentioned.
Alectra : Cane Killer.
In 1892 C. A. Barber described the occurrence of a Scrophu-
lariaceous parasite reported to be destructive on sugar-cane
ratoons in Dominica. The species was Alectra brasiliensis,
Benth., which is recorded as also occurring in Guadeloupe, St.
Vincent and Trinidad. The plant is erect, unbranched, one to
three feet high, with opposite, toothed green leaves, about an
inch long, arranged closely on the stem. The flowers occur in the
axils of the leaves, are yellow and inconspicuous, and may be
recognised by the curved stigma which appears like a broad
green band across the throat of the corolla. Immense numbers
of minute rod-shaped seeds are produced in the capsules.
The parasitism is partial only, consisting of the attachment
of the roots (which are orange coloured) to those of sugar-cane or
other plants. It is only effective on old ratoons, and apparently
does not occur where tillage is adequate. Nothing has been heard
of its existence as a pest in recent years.
CHAPTER V
DISEASES DUE TO NEMATODES (EELWORMS)
Nematodes are worms of relatively simple structure, typically
slender and cylindrical in shape, as indicated in the popular
names eelworm and threadworm, although in certain stages
some species may become swollen into gourd-like or other forms.
Nematodes are hatched from eggs, usually outside the body
of the female, have a larval stage in which the sex organs are
not developed and a sexually mature stage in which there is
usually more or less difference in size or form between the male
and the female. It must be noted, in view of the unfortunate
American usage of the word worm for caterpillar, which has
gained considerable currency in the West Indies, that nematodes
are not stages in the development of insects or of anything else :
they are and remain worms.
As is the case with the fungi, the order contains a large
number of species which live in soil or water or on decaying
vegetable or animal remains, and others which are parasitic to
various degrees. While the parasitic fungi mostly attack plants
and are comparatively rare on animals, the greater number of
nematode parasites are found in animals, and a much smaller
number are known as the cause of injury or disease in plants.
In animals of all descriptions particular nematodes inhabit
the blood, the muscles, the stomach and intestines, the lungs,
the heart and other organs, and among them are such notorious
organisms as Ascaris, Strongylus, Trichina, Ankylostoma and
Filaria.
Nematode diseases of plants have lacked adequate attention
owing to their position in a no-man's-land between the entomo-
logist and the mycologist, but recognition of their variety and
importance is growing. One species which has become well
known is Heterodera radictcola, which infests and damages the
roots of more than 500 known host plants. It is more usual,
however, for a given species to be restricted to a few hosts or
to one only. Where several are attacked a certain amount of
biologic speciahsation has in some cases been observed, some
difficulty being apparent in the accommodation of a strain bred
on one plant to a host of another species.
In recent years the investigations of N. A. Cobb have brought
to light the fact that predaceous nematodes exist, some of which
feed on other nematode species injurious to agriculture. This
59
6o DISEASES OF CROP-PLANTS
information suggests possibilities in regard to the existence or
establishment of control on lines familiar in the case of insect
pests.
The nematodes parasitic in plants are typically minute, one
millimetre (^V inch) in length or less, and unless massed in
large numbers are invisible to the naked eye. The head is
provided with a pore-like mouth through which, in many species,
can be protruded a needle-like organ, the oral spear, with which
punctm-es may be made in the plant tissues to liberate the juices
or allow of the passage of the worm. The degree of development
of the spear is to some extent an index of the mode of Hfe of the
species. Behind the mouth parts is a tubular oesophagus with
a bulbous expansion, and salivary glands which, like the spear,
may be well developed or rudimentary. It is probable that
the latter provide in some species an excretion toxic to the tissues
attacked, as in some sucking insects. There is next a straight
gut terminating in an anal orifice near the tail end of the worm.
The method of feeding, in conformity with the structure described,
is by the ingestion of fluids only.
In most of the nematode diseases of plants which have been
made known the worms have been endoparasites, existing for
the greater part of their active life within the tissues of the in-
fested plant. Other species are known to live in the soil and
puncture the epidermis of young roots without entering : the
effect of which, from the nature of the case, is difficult to estimate.
Recently two species, referred to below, have been described
which are fully parasitic, but are ectoparasites, ascending the
outside of stems and lodging in buds, under leaf-sheaths, or in
similar situations, puncturing the tissues and feeding from
without.
The plant-feeding nematodes have a transparent skin, the
external layer of which is a highly impenetrable cuticle. The eggs
are similarly enclosed. Considering their fragile appearance,
both worms and eggs are surprisingly resistant to solutions of
chemical poisons. They are somewhat more susceptible to
gases, and the best results obtained in the treatment of infested
soils by chemical agents are probably those obtained with
carbon bisulphide or ammonia. The usefulness of lime in this
connection is disputed, but in any case only heavy applications
can be expected to have any notable effect.
The resistance of the worms to dryness varies very consider-
ably with the species. It is not uncommon for a period of
dormancy in a dried-out condition to form part of the normal
life history, and this has been shown to be capable in some species
of extending to six months and more with little mortality or
loss of vigour. The larvae of the wheat nematode, which assume
a resting condition in altered grains of wheat, have been recorded
as resuming their activity after being dried for 27 years. Where
a condition of dryness does not come in to cause cessation of
^■^^
DISEASES DUE TO NEMATODES (EELWORMS) 6i
activity the indications are that the worms cannot exist more
than a few months without food.
For active hfe nematodes in general are dependent on the
presence of moisture or a high degree of humidity. The rice
nematode has been found able to move over a water-free surface
in a saturated atmosphere, and to travel on the surface of the
Fig. 1.
After N. A. Cobb
Fig. 14 Tylenchus musicola. Fig. 15 Tylenchus musicola,
Male Female
living plant at humidities below go but not below 75 per cent,
of saturation. The same species can survive about a month of
total immersion in water, but according to Cobb many soil-
inhabiting and plant-infesting nematodes succumb after a sub-
mergence of a few days.
The quality of resistance to nematode attack is shown in
certain cases by species or strains of plants related to those
infested. Thus Liberian coffee is reported to be definitely re-
sistant to nematode troubles experienced in Java and the French
62 DISEASES OF CROP-PLANTS
Antilles, and the iron cow pea is a variety adopted for its im-
munity from the attacks of Heterodera radicicola, which are ruinous
to other varieties.
The life histories of plant-infesting nematodes and the nature
of the diseases they produce may best be illustrated by the
following summarized description of typical examples.
Heterodera radicicola, Greef.
This is the best known of the nematode enemies of plants,
owing to its cosmopolitan distribution and its great range of
hosts. It is abundant in both temperate and tropical countries,
but in those with a severe winter it is mainly or entirely a green-
house pest. It infests very many common vegetable crops,
various trees, and a large variety of weeds and plants growing
wild. As regards crops more specially tropical it acts as a serious
check to the cultivation of the Chinese banana in Egypt, has
been credited with great destructiveness to coffee (concerning
which see remarks under diseases of that crop) and has been
recorded from Hawaii and Java on young and soft roots of sugar-
cane. In the British Antilles its status is mainly that of a garden
pest.
The effect of Heterodera attack on roots is the production of
galls, which differ in size and shape according to the nature of
the infested plant. On the lower stem and collar of trees the
infested bark becomes swollen and irregular, often presenting
a rough and scaly appearance on the outside for a foot or two above
the soil.
The eggs are laid in the galls or swellings, and hatch in situ,
or in the soil if they are liberated by the death and decay of the
galls. From the soil the tiny larvae, which possess an oral
spear, force their way into tender roots and proceed to develop.
The body at first swells out to the shape of a bolster, from which
state the male worms later return to the ordinary slender eel-
worm form, which they develop inside the old skin as in a cocoon.
The female continues to broaden out until it reaches the form
of a gourd, broad and round at the posterior end and tapering
off towards the head ; its length is about one millimetre and its
breadth more than half of this. The time occupied from entering
the root to reaching maturity is about 6 weeks. As the develop-
ment of the ovaries proceeds the female is reduced to a lifeless
bag fully occupied by the 300-500 eggs. The males meanwhile
perish after copulation.
The principal effect of infestation on the plant appears to be
due to root interference arising from malformation and from the
premature death of infected roots. The plant attacked is liable
to become stunted and sickly, or even to die outright. The
secondary effects may be equally or more important, since access
to other parasites, especially fungi, is afforded directly by the
galls and indirectly by the debility induced.
DISEASES DUE TO NEMATODES (EELWORMS) 63
Satisfactory control of Heterodera is obtainable where the
soil can be heat-sterilised, as in small plots and seed beds. Control
by chemical agents has not been found to be feasible on any
adequate scale. The system for which the best results have been
claimed is that of trap crops, in which highly susceptible plants
are thickly sown in infested ground to attract the larvae and
pulled or ploughed out before these can reach maturity. Starving
out the worms by a complete fallow or the planting of an immune
crop is another well-recommended measure. Carbon bisulphide
has its advocates in dealing with permanent crops but it is
difficult to conceive of more than palliative effects resulting from
this treatment of an organism so well protected during much of
its life history by the tissues of the host. The worm has been
shown to have little power of resistance to desiccation and heat,
which perhaps accounts for its absence as a serious pest of arable
land in these islands.
Tylenchus devastatrix, Kuhn.
The eelworms of the genus Tylenchus, which includes many
serious plant pests, retain the normal worm-like form in both
sexes throughout the life cycle.
T. devastatrix affords an example of a species which infests
tissues above ground and affects from within the development
of the organs of the plant. It causes dwarfing, crumpling and
other deformation of leaves and abortion of ears in rye and oats,
ring disease of the bulbs and yellow leaf-spots on hyacinths, a
seedling disease of onions resulting in abnormal forms, bud
proliferation and reduction in leaf size in clover, misshapen spots
on potato tubers, and the drying up of flowers in the teasel.
The worms penetrate seedlings or young plants from the soil
and ascend to various locations in the stem by way of the
parenchyma. Here they breed, and are returned in the course
of time to the soil when the plants die down.
Tylenchus tritici.
The wheat eelworm similarly enters into the seedlings, ascends
the stem (where its presence gives rise to stunting and the curling
of leaves), and eventually bores into the ovaries, which develop
into gall-like bodies resembling somewhat the normal grains
among which they occur. The eggs are laid in this situation,
and each gall when ripe contains some 500-600 larvae, which after
hatching enter upon a resting stage. It was from this condition
that the frequently quoted example of revival after 27 years of
desiccation occurred. The worms resume activity when the
gall, in company with the grain, is resown, and make their way
through the soil to the seedlings of the new generation.
Tylenchus angustus, Butler,
The rice eelworm is one of the examples referred to above of
ectoparasitic nematodes. It is the cause of the destructive
64
DISEASES OF CROP-PLANTS
" ufra " disease of rice in India, and appears to be confined to that
crop.
The worms exist in a free-swimming condition in the water
which covers the fields, and ascend the outside of the plants when
the humidity of the air permits. They infest mainly the leaf-buds
at the tops of the shoots, the upper nodes of the stem beneath the
leaf sheaths, and the developing ears. In no case do the worms
enter the tissues, but puncture the epidermis and absorb the
juices so obtained. There is no evidence of toxic action. Severe
infestations lead to the ruin of the crops.
X5
After N. A. Cobb
, \X200
.x.750^
AJter N A Cobb After N. A. Cobb
Fig. 16 Coconut Root Tig 17 Coconut Parfn- Fig. 18 Egg of Aphe-
wiTH Nematodes chyma with Nematodes lenchus Cocophilus
Reproduction proceeds vigorously on the growing plant
and eggs and larvae in all stages are found mingled with the
adults in the leaf-buds and around the young ear. With the
drying out of the ripened plant the worms coil up in the upper-
most nodes and within the glumes of the ear, and become dormant.
In this condition they are returned to the soil, and await con-
ditions suitable for the infection of the succeeding crop.
Tylenchus ribes, Taylor.
This species infests the buds of black currant bushes in
England, ascending the stems on the outside during wet weather
in the same way as the rice eelworm. Reproduction takes place
within the bud and the worms collect there in visible cottony
masses containing many hundreds. The colonies dry out in this
situation, and the worms revive and migrate when moist con-
ditions are restored. Seventy per cent, of the worms have been
DISEASES DUE TO NEMATODES (EEL WORMS) 65
found to survive 6 months' desiccation, returning to normal
activity after 1-2 hours' immersion in water.
Aphelenchus cocophilus, Cobb.
An extended account of the disease caused by the coco-nut
eel-worm is given in the section on diseases of that crop. The
worms occupy and breed in the ground tissue of a continuous
zone in the stem of the palm, and of the stalks and bases of the
leaves. They also spread outwards from the base of the stem
in the cortex of the roots. The life history has not yet been
worked out, but present indications are that the worms may
ascend the outside of the stem and enter the leaf base from below;
i.e., on its outer side, and near its attachment to the stem.
Tylenchus similis, Cobb, and T. musicola, Cobb.
These species cause diseases of banana in the West Indies
which are elsewhere described. They enter the roots and root-
stock, causing slight swellings and cracks in the roots, and
blackening the tissues of the pockets they occupy in the infested
organs. T. similis also attacks sugar-cane roots, causing and
occupying blackened dead patches and cavities in them, but
producingfno swelling.
CHAPTER VI
THE RELATION OF INSECTS TO PLANT DISEASES
The injurious relationships of insects to plants may be roughly-
divided as follows :
1. Direct injuries of the nature of wounds (traumatisms).
2. Systemic or organic disturbances originated by sucking
insects (stigmonoses).
3. The exposure of internal tissues to infection.
4. Transference of infection.
Insects as the Cause of Injuries and Diseases.
The principal of the relationships specified above is that which
in practice is the concern of the economic entomologist, and
consists in the production of injuries mainly direct and of the
nature of wounds. Thus the plant or any of its parts may be
destroyed by biting or boring insects or its juices be drawn upon
and its tissues perhaps poisoned by sucking species.
Whatever the nature of the injury it must have a reaction on
the condition of the plant, and as regards this there is in many
cases no essential difference between the results produced by
insects and by fungoid parasites. The reduction of leaves,
flowers, or fruit, the girdling of a stem, the destruction of roots,
have much the same results when caused by one or the other.
From this point of view insects as well as mites and nematodes
are on the same footing as fungi and bacteria in the causation
of plant diseases. The main difference is that with insects the
injury and the manner of its production are usually direct and
obvious.
Stigmonoses.
There are exceptions, however, especially where sucking
insects are concerned. The attacks of plant-feeding Rhynchota
— hemipterous bugs, aphides, scale insects, leaf hoppers and the
like — to which may be added thrips and red spider, often result
in more or less general symptoms not obviously related to the
direct injury inflicted. To this type of affection the term
stigmonose has been applied. No better example could be quoted
than the froghopper blight of sugar-cane, or the effects of thrips
on cacao and sweet potato. The starved and chlorotic condition
induced in certain Solanaceous and other plants by lace-wing
bug infestations is another familiar example.
66
THE RELATION OF INSECTS TO PLANT DISEASES 67
In some such affections there is reason to believe that besides
the drain on the juices of the plant there is a poisoning effect
from a toxin injected by the insect into the tissues, the immediate
action of which is to overcome the resistance of the protoplasm
lining the cell wall and thus allow abstraction of its liquid con-
tents.
Insect Injuries as Openings for Infection.
The protective function of surfaces of cuticle or cork and the
dependence of many fungus parasites on wounds to secure an
entrance have been described. Most commonly such injuries
originate from the attacks of insects. Not only do fungi gain
access by the grosser wounds, but it has been repeatedly shown
that the infection of leaves or fruits may depend on injuries of
this nature not appreciable to the naked eye, and the presence
or absence of an unconsidered insect may in this way determine
the prevalence of a fungoid or bacterial disease.
Insects as Carriers of Infection.
In addition to their function in opening up ways for infection
evidence is accumulating that in plant as in animal diseases
insects frequently transfer the actual means of infection from
plant to plant. The covering of the insect body is usually very
suitable for the carriage of fungus spores or bacteria, and next
to wind insects are probably the most effective agents in distri-
buting these, and are capable of extending the range of a disease
over the long distances sometimes covered in insect migration.
It is not necessary that the insects themselves should be of in-
jurious kinds ; they may visit the affected plant to feed from its
flowers or be attracted by and perhaps bred from the tissues
soured or decayed as a result of disease.
The fungi causing the internal disease of cotton bolls are
transferred from the fruits or seeds of numerous wild and cul-
tivated plants to the bolls, and from one boll to another, by
plant-feeding bugs and by no other known means. The form
in which the fungus concerned is carried has not been detected,
but there is good reason to believe that the infected material
is introduced into the boll or other fruit at the time of feeding.
In the mosaic diseases infection commonly occurs through the
agency of aphides, and in one such disease, spinach blight, the
virus is handed on to succeeding generations of the insects, as
shown by the power of these to produce infection. The virus
of curly-top of beets is transferred by a Jassid bug, and it has
been recently demonstrated that a period of incubation of about
24 hours in the body of the insect is necessary to the process.
The difference between these diseases and the stigmonoses should
be noted : in the latter the affection is caused by the insect
itself.
68 DISEASES OF CROP-PLANTS
Susceptibility and Resistance to Insect Infestations.
It has been frequently remarked that there is a reverse
relation as well as the direct one between the development of
infestations of certain insects and the state of health of the plant
infested. Plants growing under wholly favourable conditions
may exhibit a resistance — or in unfavourable conditions a
susceptibility — which determines the extent to which the insect
concerned can establish itself upon them. Such a relation
exists in several of the stigmonoses. A. F. Woods, who originated
the term, remarks that aphides are specially fond of plants which
have made a poor starved growth, and increase rapidly upon
them. The cacao thrips, with a whole country-side of cacao
apparently open for it to feed upon, infests certain patches year
after year, only occasionally and temporarily extending to the
surrounding fields. Usually it can be shown that the infested
areas are on poorer or less well-drained soil, are insufficiently
sheltered, or perhaps have been depressed by some more tem-
porary disability.
It is the general experience with the froghopper blight of sugar-
cane in Trinidad that it is in fields in poor tilth, in ratoons on
red clay soils, which quickly lose the effects of cultivation, and
in local patches subject to some similar defect, that the insect
finds the most suitable conditions for rapid multiplication.
Infestations of the bug Helopeltis which causes the mosquito
blight of tea are reported to depend similarly on cultural con-
ditions, being especially liable to follow upon water-logging of
the soil. Bugs of this species in captivity are said to have
proved unable to maintain themselves on twigs supplied from
resistant bushes, but to recover and thrive on material from
plants in a susceptible condition. Scale insects frequently show
greatly increased powers of infestation on plants in an unthrifty
condition or growing in situations in some respect unfavourable.
Thus young lime trees grown with a shelter crop, the system under
which they thrive best, have remained practically free from scale
when plants of similar age in the open have had to be frequently
sprayed to keep them alive.
CHAPTER VII
NON-PARASITIC DISEASES
The term non-parasitic is now coming into use for the class of
diseases often called " physiological." Plant pathology has
developed almost entirely on a basis of parasitology, and the
newer expression is perhaps the most appropriate for what in
practice has been not so much a natural group as a limbo where
all affections not shown to have parasites as their inciting cause
were detained. It is not possible to have a strictly definitive
name for such an assembly, and there is in fact no strict dividing
line between parasitic and non-parasitic diseases.
A cacao tree growing in a light soil and subjected to undue
exposure suffers repeated losses of its leafy twigs from dieback,
and eventually, if the adverse conditions continue, dwindles and
dies with all the evidences of inanition. The immediate inciting
cause of the dieback would appear to be the want of balance
between loss of water from the broad thin leaves and its absorp-
tion by the roots. If now, as frequently happens, the failing
twigs and branches are attacked by Diplodia, the essential
causation is not changed, since the fungus cannot attack healthy
twigs, but its presence may increase the severity of the original
disease. In a similar way, the production of disease even by
obligate parasites may depend on some effect of the physical
environment upon the host.
It is convenient to include with parasitic diseases those
border-line cases in which a parasite typically appears, though
logically, having reference to their actual inciting causes, such
affections as root-disease of sugar-cane and dieback of limes
and cacao should probably be classed as non-parasitic.
This policy leaves in the group to be considered at least one
apparently definite class — affections directly produced by im-
perfect adjustment between the plant and its physical environ-
ment, or, in other words, by unfavourable climatic and soil
conditions. These affections show gradations from simple
injuries which would hardly be called diseases to systemic
disturbances affecting the whole plant. It must be confessed
that of the more complex of these types we have very little
comprehension, and cases frequently occur in which diseases
believed to be of this nature are shown to be of parasitic origin.
There is also the difficulty already mentioned in connection
69
70 DISEASES OF CROP-PLANTS
with the virus diseases, that a similar set of symptoms, as for
example in the conditions known as gummosis and chlorosis,
may arise from different causes, some of which may be parasites
and others physical factors.
The systematic discussion of atmospheric and soil conditions,
and the effects their variations produce on plants, is more appro-
priate to a treatise on plant ecology than to a handbook of plant
diseases. A few of the more definite examples occurring locally
will be mentioned as illustrations, and for fuller information as to
these the reader is referred to Part II. The sections on cotton
and citrus diseases, particularly, may be consulted in this con-
nection.
The red leaf blight of cotton and chlorosis of sugar-cane
illustrate the kind of effect that may follow from soil defects.
Growing plants of Sea Island cotton may at any stage begin to
show a rusty discoloration of the foliage, and the leaves so
affected fail prematurely. The symptoms may be slight or
severe and in the latter case the development of the plant is
permanently checked. The condition is at the present time very
prevalent in St. Vincent, where it has been shown by S. C.
Harland to be a result of potash deficiency. A yellow leaf
blight somewhat similar in its nature sometimes occurs, which
renders the plants very susceptible to the attacks of certain
fungus leaf-spots.
Chlorosis of sugar-cane occurs in Antigua, Barbados, and other
islands with limestone soils, and persists on a particular spot
year after year. According to H. A. Tempany, this is due in the
first instance to the interaction of salt subsoil water with lime-
stone, producing sodium carbonate, and from recent research
in Porto Rico, the effect would appear to be produced by a
further reaction which prevents the plant from obtaining iron,
an element necessary for the development of chlorophyll.
The leaf-curl of Sea Island cotton which occasionally appears
on a large scale in these islands is in some way intimately con-
nected with dull rainy weather and is especially liable to occur
in shaded and sheltered situations. The internodes grow out
to abnormal length, the leaves are strongly crimped at their
margins, and the flower buds are shed while very small. The
condition may come on at any stage, and passes off entirely with
a change of weather.
The loggerhead disease of the same plant occurs sporadically
in St. Kitts and Nevis, and in July, 1914, appeared on a large
scale. The plant assumes the form of a low dense bush with
crowded foliage. The leaves are mottled with dark and light
green areas, and the veins often develop imperfectly, causing
internal puckering of the leaf blade. The flower buds mostly
fail as in curly-leaf, while those surviving develop with a crumpled
corolla. There is no evidence of infectiousness, and distribution
may either be scattered or nearly general throughout a field.
NON-PARASITIC DISEASES 71
The diseased condition may supervene upon healthy growth, or
vice versa, or the plant may be uniformly affected throughout
its vegetative phase. The disease is not so clearly dependent on
weather conditions as curly-leaf, but the only severe recorded
epidemics of the two affections occurred together in St. Kitts,
in 1914. Previous to that time loggerhead was only known to
occur in plants raised out of the ordinary season, and since 1914
its presence has not been reported. Both affections are indepen-
dent of the source of seed.
To some extent in St. Kitts, Nevis, and much more distinctly,
according to specimens and accounts received, in St. Croix, the
leaf-curl disease exhibits also the character of the tomosis
described by O. F. Cook as occurring on cotton seedlings and
young plants in the United States, namely, the fraying of the
edges of the leaves and their perforation with small irregular
holes.
The blossom-end rot of tomatoes is an example of a well-
marked specific disease which develops in close relation with
soil and water factors, is not infectious, and has in its inception no
recognisable connection with any parasite. It is marked by the
appearance of a water-soaked spot, which soon turns black, near
the blossom end of the fruit when the latter is about one-half or
two-thirds grown ; the spot increases rapidly in size and finally
spreads deep into the tissue of the fruit ; the affected tissue
becoming black, hard, and leathery, and the fruit much flattened.
The affection is not uncommon in the West Indies.
The disease may be induced or increased in vigorously growing
plants by irregular watering or excessive transpiration, by raising
the soil temperature, and by the use of organic and certain
chemical manures. In the writer's experience it has been com-
pletely arrested by lightly shading the plants, protecting the soil
with trash, and regular moderate watering. C. Brooks, from a
very thorough study of the disease, arrived at the conclusion that
it is probably due to the development of harmful humic and
ammonium compounds and an accompanying decrease in the
supply of nitrates.
A good deal of importance is attached in the study of some
other non-parasitic diseases, particularly certain obscure affec-
tions of Citrus species, to the relation of the plants to the nitrates
and ammonium compounds in the soil. The subject is a complex
one, involving the activities and interactions under given con-
ditions of the son bacteria, and the results obtained, though
highly suggestive, are hardly definite or general enough for concise
statement. It is a matter of frequent experience, however, of
which local examples could be given, that in particular circum-
stances the application of rich nitrogenous manures has been
followed by serious damage to citrus trees, and this might be
accounted for, in relation to the soil flora, on the lines of the
theory regarding blossom-end rot quoted above.
72 DISEASES OF CROP PLANTS
A marked example of the action of the physical environment
on the life and vigour of a perennial plant is afforded by the life
history of lime trees in t'ae drier islands, described in another
section. When they sur\ive the initial critical period of two or
three years the trees become established and flourish for about
ten years, after which their vigour declines and they gradually
die back, the process being accelerated by, but not dependent
on, the scale insect and fungus infestations to which their les-
sened resistance renders them liable. No agricultural measures
have availed to arrest this decline, which by comparison of
locahties is judged to be the effect of irregularity of conditions,
especially of the reduction of atmospheric humidity in the dry
season.
Another notable case relates to orange trees in Dominica,
which on their own roots, but not when grafted on sour orange
stocks, are invariably limited to a relatively short period of healthy
maturity by the oncoming of gummosis of the collar.
Conditions simulating the effects of disease may be produced
by genetic changes, perhaps to be explained as loss mutations,
taking place even in what are believed to be pure lines. Ex-
amples of this are the " man cotton " and " crinkled-dwarf "
rogues in Sea Island cotton described by Harland. In the latter
case the plant is severely dwarfed, bears yellowish-green, crinkled
and ragged leaves much smaller than the normal, and is rendered
nearly sterile by shedding. The " man cotton " differs from
normal in the smallness of the vegetative parts, with complete
male and nearly complete female sterility. Typical]}'' it is a tall
lanky plant with very short branches. Both types of rogues,
when crossed with normal Sea Island, give a normal first genera-
tion. The crinkled dwarf type is known to exhibit simple
Mendelian segregation in subsequent generations.
An example of what may perhaps be called genetic incompati-
bility, producing similar results, is afforded by crosses between
Sea Island and some local perennial cottons. Part or the whole
of the first generation consists of plants with mottled leaves,
partly suppressed internodes, and premature cork formation
on stems and petioles. The writer's attention has on several
occasions been directed to this condition as a new disease.
CHAPTER VIII
ENTOMOGENOUS FUNGI
The occurrence of fungus parasites attacking insects has long
been known in certain cases, but in recent years this knowledge
has been considerably increased, and, as regards tropical forms, is
capable of very much greater extension. The rich insect fauna
and moist conditions of Trinidad, for example, yield in suitable
situations and on various types of hosts a large variety of forms,
most of which appear to be as yet undescribed and are certainly
unstudied. The scale insects (Coccidae) of that island, though
numerous in species, are relatively unknov^n as pests, a position
mainly due, it would appear, to their inability to attain to large
numbers without suffering from an epidemic of fungus disease.
The remaining islands are more or less subject to annual
or occasional outbreaks of scale, and the control of these by
fungi has been the subject of considerable attention, mainly as
a result of what might fairly be called the white-fly fungus boom in
Florida a few years ago.
Our knowledge is yet far from exhaustive, even in the limited
field of scale-insect parasitism, either as to the fungi concerned
or their relative distribution and efficiency, but certain general
principles have by now emerged.
While these are not different from what might have been
intelligently anticipated, they are valuable as being the results
of experience and observation. As such they may be worth
stating in view of the impossible hopes which still linger here
and there with regard to the artificial distribution of these fungi.
Speaking first of all quite broadly, it may be said that the
efficiency of the fungi is proportional to the humidity of the air
amongst the plants on which their hosts occur. Their relative
abundance in the islands of the Windward and Leeward groups
follows pretty closely the amount of rainfall usual to each island,
though the distribution of rain is so local that the wetter islands
each have areas in which the fungi are of little use, and the driest
island has sheltered moist situations in which they are effective.
Still speaking broadly, it may be said that in Dominica, in normal
years, the control is as efficient as natural agencies can well
produce. Conditions are generally such that fungus parasites
keep in check established colonies of scale and follow so closely
on new infestations that the effects of the insects are negligible.
In most parts of St. Lucia, and in the most humid districts of
73
74 DISEASES OF CROP-PLANTS
Grenada and probably of St. Vincent and Montserrat, the control
approaches to that in Dominica. In the drier districts of these
islands the fungi, while present, are not so effective, or are more
restricted to a seasonal activity. With reasonable accuracy it
might be said that excepting the shield-scale fungus and the
Aspergillus on sugar-cane mealy-bugs, the limits of possible
cacao cultivation are the limits of the effectiveness of scale-
destroying fungi.
Consideration must, however, be given to the fact that the
matter is not quite so simple as has been assumed for the purposes
of general statement above. The rapidity of effective repro-
duction by the insects has to be taken into account as well as
the powers of attack of the fungus parasite, and in this the
condition of the plant is usually, perhaps always, the determining
factor.
Even in the favoured districts of Dominica and St. Lucia,
plants transferred from the nursery to the field may become
badly infested until they get estabhshed, and require, or would
be the better for spraying. In connection with older trees,
the influence of other factors stands out where fungus control is
less complete, as in cases observed in Grenada, where particular
mango trees were heavily infested, though the shield-scale fungus
was present in some abundance, while others near by were quite
clean.
Resuming the consideration of the effects of humidity on the
fungi in question, their distribution in Barbados is instructive.
The rainfall is low, and the island in general is open and wind-
swept. With the two exceptions noted, the fungi in question
are absent save in deep gullies, and in closely sheltered spots
among the hills where the rainfall is greatest. A similar, though
perhaps less extreme, situation seems to prevail in Antigua,
St. Kitts, and Nevis — islands of relatively low rainfall.
Considering next the fungi mentioned above as exceptions
whose distribution is not so strictly limited, the case of one of
them, Aspergillus fiavus, the sugar-cane mealy-bug parasite,
is readily accounted for. It is not less dependent on moisture,
but finds it accompanying its hosts in the enclosure formed by
the base of the leaf-sheath around the nodes of the cane, a
situation in which water collects and lingers.
The shield-scale fungus is more remarkable. Even in Bar-
bados it persists through the driest season with little shelter,
and becomes very active with somewhat small encouragement
in the wetter months. The only relevant difference known to
the writer between its spores and those of the other scale fungi
is that they are enclosed in a drop of mucilage. Whether this
or some constitutional factor accounts for its hardiness would
be difficult to determine.
Having considered the influence of weather conditions on the
natural occurrence of scale-destroying fungi, there remains to be
ENTOMOGENOUS FUNGI
75
Fig. 19
ENTOMOGENOUS FuNGI
1 HiRsuTELLA ENTOMOPHiLA on a beetle (Colaspis sp.).
2 Cephalosporium Lecanii on Coccus sp.
3 Metarrhizium Anisopliae on a Homopteron, probably a Jassid.
4 Gibellula arachnophila on a spider.
5 Aschersonia turbinata on a scale insect.
6 Gibellula sp.
7 CoRDYCEPS sp. on larva of Rhinoceros Beetle (Strategus).
8 Cordyceps Lloydii on an ant (Campanotus sp.).
9 IsARiA Sphingum ou a moth.
76 DISEASES OF CROP-PLANTS
discussed the question as to how far their efficiency can be in-
creased by artificial distribution. As a consequence of the
success, since shown to be due to very special circumstances, of
the introduction of Australian lady-bird beetles into California
to prey on the cottony cushion scale, a very large amount of
attention has been given to the question of natural enemies of
injurious insects. The results have been usually disappointing,
but the rules of the game are now pretty clear. The rapid
increase of a pest which may take place when it is newly intro-
duced into a country where natural enemies adapted to destroy
it are few or absent may be checked in some cases by bringing
in such enemies from countries where they exist. To use a
relevant illustration : should some kind of introduced white
fly become troublesome in the West Indies, it would be worth
while to introduce the fungi which are parasitic on certain
species in Florida, in the hope of their exercising a check on the
insect in these islands. Were the introduction successful,
the control might be more efficient or less so than it is in Florida,
according to the extent to which the conditions favour the fungi.
It would not be expected to be complete. Once given a start,
however, the fungus parasites might establish themselves and
constitute permanent factors modifying the activity of the
insect.
When such a factor is already well-established in the country,
and it is desired to increase its effect, the element of artificiality
becomes much greater. The fungus (or other parasite) already
has its place, its settled lines of action, so to speak. By cultivat-
ing it, by distributing it, it may be increased for the time, but
when such means are no longer used it will settle back to much
its old position, which is determined by natural conditions, and
changes only in response to their fluctuations. This has been
and is the situation with regard to most work on the fungus
control of insects.
The method has an insidious appeal, in that it seems to
promise to the planter a self-acting remedy, but experience shows
that, as might have been expected, results are not to be obtained
without an adequate amount of trouble.
The use of fungi does afford an alternative to the employment
of poisonous sprays, but save that in the one case a living organism
is used, one method is as artificial as the other, and it depends
entirely on the particular circumstances of the case which is
more efficient or less troublesome. That is a question for the
entomologist and for the planter. In Florida, in connection with
white fly control, each method has its advocates. In Trinidad,
periodic dusting with fungus spores has been tried against
froghopper on sugar-cane, spraying with insecticides in this case
being out of the question.
In one respect the use of fungi has a disadvantage as compared
with the use of poisons. The latter may be applied with advan-
Fig. 21 Cephalosporium Lecanii on Coccus Mangiferae
Photo by]
Froghoppers (Tomaspis saccharina)
Killed by Metarrhizium
ENTOMOGENOUS FUNGI ^j
tage at the beginning of an attack, the former do not become
very effective until their hosts are plentiful. Like all fatal
parasites, they tend to cut their own throats. The more
thoroughly they kill off their hosts, the less material they have
to live upon. When the chances of infection have been thus
reduced, the surviving insects, or colonists from another area,
start a fresh wave of insect infestation, which in time is again
overtaken by the rising numbers of the parasite.
This succession is perhaps most easily seen in the West
Indies in the action of the shield-scale fungus working on more or
less isolated trees. In lime plantations, it takes place in sections
of the cultivations rather than on individual trees.
Several observers agree that, in the case of white fly control
in Florida, the parasites under favourable conditions, and without
artificial aid, become effective about every third year.
It was noted above that in certain localities conditions are
so favourable to the spread of scale fungi that the matter may be
left to itself. What can be done in the districts not so favoured ?
Unless there is at some period of the year a season when the fungi,
if they are present, can be observed to become active, it is not
worth while to trouble about them. If they are not present at
all, it is probable, since they are so well distributed through the
islands, that the conditions are not favourable. Where they
have a period of valuable activity in the wet season, followed
by comparative scarcity in the dry, something may be done. It
will be found that, while they disappear from view in exposed
situations, they continue a visible existence in damp and sheltered
places. The spores of most species do not appear to be very
resistant, and it is probable that it is from such places that the
spread begins when conditions become favourable. By dis-
tributing the material thus available, or by taking material
from the earliest plants to become infested, it is possible, and has
often been definitely claimed, that the progress of the fungus
may be considerably hastened ; especially is this the case with
isolated trees.
In Trinidad, a series of steam-heated cabinets has been
used for turning out the spores of the froghopper fungus Metar-
rhizium anisoplioe on a large scale, and these have been dusted
on the cane fields. The amount of fungus control in treated
as compared with untreated fields has not been demonstrably
increased and the practice is in abeyance or abandoned as being
too uncertain or indefinite in its results.
Various methods are available in making use of natural
material. Branches from the fungus-infested tree may be
tied among the leaves of the tree to be infected. Leaves bearing
the fungus may be dipped in water and rubbed on the leaves
bearing the scales, or pinned in contact with them. The material
may be stirred up in water to wash off the spores, or ground up
and mixed with water in the case of leaves.
7% DISEASES OF CROP-PLANTS
This mixture may be sprinkled on to the trees with a brush,
or the twigs bent down and dipped into it, or it may be appHed
with a syringe or other type of sprayer after being strained
through a wire sieve. Cloth strainers keep back the spores.
Brass or copper sprayers may be used if they are clean and the
material is not allowed to stand long in the tank.
Comparative trials of these methods have not been made
in the case of the West Indian species. Morrill and Back favour
the spore-spraying method for distributing the Aschersonias
concerned with white-fly control in Florida.
The condition of the material used would appear to have
more influence on successful introduction than the method of its
application. It should be used as fresh as possible, and, in the
active spore-producing stage, recognizable on examination
with a pocket lens. In the case of the shield-scale fungus, a
powdery glistening appearance denotes the presence of the spores.
In the red-headed and white-headed fungi, the loose tufts of ripe
spores can be easily made out.
When a branch or twig is tied into a tree, the immersion of
the cut end in water contained in a narrow-necked bottle is an
obvious benefit. The occurrence of the fungi on insects attack-
ing potted plants suggests that these might in some cases be
used with advantage.
The caution must be repeated that all these measures depend
entirely for their success on the conditions being right for the
development of the fungus. In dry weather they are useless.
Moreover, experiments have repeatedly shown that, once a fungus
has been given a general start, attempts to increase its efficiency
by further spore-spraying have little visible effect.
Caution should also be observed in making use of infested
material lest scale insects not already present on the plants be
introduced. Leaves of grape-fruit examined by the writer, on
which the scales were weU infested with three species of fungi,
nevertheless had numbers of the young of the mussel scale
crawling over them more than a week after they were picked
and dried.
In the writer's judgment much more hope of effective inter-
ference lies in the direction of increasing humidity by the pro-
vision of windbreaks and other forms of shelter. With cacao,
and more easily with cotton, this may be carried too far, since
diseases of pods and bolls will also be encouraged, but with
citrus trees, which are the most liable of common crops to suffer
from scale infestation, shelter can scarcely be overdone save in
districts so wet that it would not be needed. With orchard
crops shelter takes two forms : a temporary crop in which the
young trees are nursed, and permanent windbreaks and hedges
enclosing or dividing up the fields.
Young coconut trees in dry situations may be sheltered with
great advantage in the reduction of scale attacks.
ENTOMOGENOUS FUNGI
79
The common parasites attacking Coccidae in these islands
are the following :
Sphccrostilbe coccophila
Ophionectria coccicola
Cephalosporium lecanii
Aschersonia spp.
Myriangium Duricei
Septohasidium spp.
The first four attack individual insects
the red-headed fungus,
the white-headed fungus,
the shield-scale fungus.
the black fungus.
Myriangium and
Drawn by] U- ^- ^orer
Fig. 23 Metarrhizium anisoplioe Sorok
Septohasidium extend over whole colonies, spreading over the
surface of leaves or enveloping leaves or branches.
An undescribed Empusa species appears to be fairly common
on Coccus viridis, the common green scale, developing within the
body so that its victims merely look dry and brown. Aspergillus
flavus is very common on Pseudococcus calceolarice, the sugar-
cane mealy bug ; VerticilUum heterocladum has been found on
citrus white fly in a Barbados gully.
Aphides are frequently found covered with a Hormodendron
sp. which has been experimentally found to be able to kill Coccus
viridis.
Thrips on cacao in St. Vincent are subject to epidemics of
8o DISEASES OF CROP PLANTS
Sporotrichum globuliferum, and a temporary high death-rate
has been produced by spraying the spores of this easily cul-
tivated fungus on thrips-infested trees in Grenada.
Of the plant-feeding bugs, Dysdercus spp. the cotton stainers
and Nezara viridula, the green bug, have been seen to be subject
to epidemics of Sporotrichum globuliferum and Isaria Pattersonii,
respectively.
The froghopper in Trinidad is attacked every year by
Metarrhizium anisoplics and an Empusa sp., which destroy
large numbers of adults when conditions are favourable.
An epidemic outbreak of disease caused by Botrytis rileyi,
on caterpillars [Thermesia gemmatalis ?) infesting a field of cow-
peas in St. Kitts, was so sudden in its effects that the stiffened
bodies clinging to the leaves produced the appearance of a
sprinkling of snow.
The sugar-cane moth borer Diatroea is subject in the larval
stage to the attacks of Isaria Barbcri.
Gibellula arachnophila is fairly common on leaf-inhabiting
spiders.
Besides these established examples there are many indi-
vidual records of species of which the distribution is unknown.
There can be no doubt that fungus parasites play a considerable
part in controlling the numbers and distribution of insects, a
control which in many cases is the less conspicuous because of its
efficiency. Its importance in regard to scale insects is strikingly
demonstrated by the infestations which develop on citrus trees
after spraying with Bordeaux mixture, which suppresses the
fungi without affecting the insects.
»i
i'-M
P'lolo by] [J. fi. Rore
Fig. 24
Dusting with Spores of Froghopper Fungus (Trinidad)
Fig. 25
Equipment for mixing Bordeaux by Gravity
Section II — Prevention and Control
CHAPTER IX
GENERAL CONSIDERATIONS
Arable Crops and Parasitic Diseases.
A field is to be planted with a certain crop, which is subject
to specific diseases. The grower requires to know what can be
done to prevent their appearance, and, should they appear, how
the resulting damage may be kept to the minimum. For every
crop and for every disease of it the answers to these questions
must vary, but the general principles of prevention and control
are nevertheless capable of useful discussion.
Sources of Infection.
For a parasitic disease to develop in a crop a primary infection
is necessary, and the sources from which infection may arise will
first be considered.
I. The parasite may be in the soil, continuing its existence in
the soil itself, or on the dead or living remains of the previous
crop. To remedy this condition the process of starving out the
parasite by denying to it its appropriate host is commonly
adopted. This may take the form of resting the field for a period,
but usually the result is more efficiently and more economically
attained by planting a different crop. In the practice of crop
rotation from the point of view of eliminating diseases the
principles to be kept in mind are (a) that the successive crop-
plants should not be susceptible to attack by the same parasites,
and (b) that a crop needing deep or frequent cultivation will
produce the quickest results. Even where no alternative crop is
available much may be done in the desired direction by early
removal of the remains of the previous crop and thorough culti-
vation before replanting. Many attempts have been made
to find some direct means of soil sterilization by chemical or
.physical agencies. It may be stated with confidence that no
method involving the use of chemicals or heat is known which
*is at the same time cheap enough and effective enough to be
applied to areas larger than those of seed or nursery beds or the
sites of a few infested plants or trees.
The practice of flooding fields where it is practicable may be
very useful.
8i G
82 DISEASES OF CROP-PLANTS
2. The parasite may be introduced with the planting material.
In crops which are grown from seed a parasitic fungus or
bacterium may be introduced with the seed, either adhering ex-
ternally or, if it is from an infested plant, as an internal infection.
The simplest means of avoiding this is to plant, when possible,
seed from a perfectly healthy source ; where this cannot be
guaranteed, some form of seed sterilization, the methods of which
are discussed on a later page, may be adopted. With some crops
the trouble this involves is exceedingly well repaid. The same
considerations apply with even greater force in the case of crops
planted from cuttings. It is often desirable to supply these
from nurseries, which may be established in favourable situations,
perhaps at a safe distance from the main crop, and can be given
special attention to keep them healthy.
3. Infection may come from standing crops or allied plants.
It is commonly the case in the tropics that an older crop may
be still standing when fields of the same plant are being estab-
lished. In this event the conditions are favourable to the
passage of spores from old to new, blown by wind or washed by
rain, carried by birds and insects, by labourers and by imple-
ments passing from one field to another. For this, if the co-
existence of crops of different ages is a necessity, as is the case
with sugar-cane, there is little remedy. Where the necessity
does not exist a voluntary or compulsory adjustment of the,
time of planting may be of the greatest value, as is demonstrated
in those islands where a close season for cotton has been adopted,
involving the destruction by burning or burying of all old plants
by a certain date some time before planting is allowed. There
is much that could be done of a related nature by the control of
plants, wild or escaped, growing by the sides of roads and traces
and on waste land. Frequently these plants serve to carry
over the pests and diseases of allied crop-plants. With some
initial trouble and a little care from time to time such situations
may be covered with a growth of some harmless and self-main-
taining plant, which may moreover serve a useful purpose as
fodder, mulch or firewood.
An instance of the kind of action described is afforded by the
very effective control of the insect principally involved in the
transmission of internal boll disease of cotton, which has been
achieved in St. Vincent by eradicating two useless Malvaceous
trees on which the insect bred between cotton seasons.
4. Infection may be introduced with manure.
With most fungi, some of the smuts being exceptions, well
rotted pen manure may be considered safe. This cannot be said
for vegetable mulches when these are derived from plants which
are identical with or related to that composing the crop. Sugar-
cane trash, for example, may carry many of the diseases to
which that plant is liable, and the material from infested fields
should not be applied until it has been well rotted down in a
GENERAL CONSIDERATIONS 83
pen. In a few cases such material should be promptly burnt,
but as the loss of organic matter is to be avoided in arable agricul-
ture under tropical conditions burning should only be resorted to
when specifically advised.
Means of avoiding damage.
As the incidence of many diseases is unpreventable by the
means so far discussed, and in many more is usually unpre-
vented, consideration passes to the means which may be adopted
for control.
Resistance.
Of these the most effective and the least troublesome, because
it is automatic, is that which lies in the resistance of the plant
to infection or to serious infestation. This, it has been pointed
out in a previous chapter, may be due to some hereditary quality
or character, or it may depend on the condition of the plant as
determined by its environment. Some degree of this acquired
resistance can be had in all crops and under all conditions by
giving the best care possible under the circumstances, but its
extent is limited by the possibilities of soil, situation, and climate,
which may not be favourable to the best development of the
plant. This kind of resistance is effective against most types of
disease caused by saprophytes with weak powers of parasitism,
such as root disease of sugar-cane and the Diplodia diebacks,
and also against the numerous class of special parasites which
principally develop on senescent leaves but advance the time of
their attack on weakened plants. The West Indian leaf-mildew
of cotton is a good example of this, as are several of the true
rusts and many leaf -spot parasites. A vigorous plant moreover,
even when not resistant, may keep pace with the development
of the disease and produce a crop in spite of it.
It happens in some cases, though they are naturally difficult
to recognise, that a dangerous degree of susceptibility in otherwise
weU-cultivated plants may arise from the shortage of some par-
ticular substance in the soil. There are several well-marked
instances on record where this condition has arisen through
a shortage of lime. In others it has been shown that resistance
could be increased by the application in the particular case of
potash or of phosphates.
It must be fully recognised that there are many diseases
against which the type of resistance just discussed is not effective,
in which in fact the parasite may flourish all the better for the
abundance or succulence of its food supply, or in which, as in the
Rosellinia diseases, the conditions which favour the most perfect
development of the host are those which also best suit the
parasite.
Inherent (hereditary) resistance, on the other hand, may be
effective against infection or infestation with the most virulent
84 DISEASES OF CROP-PLANTS
of parasites, but as a rule, where it is not due simply to inherent
vigour, it is narrowly restricted in range. Sugar-cane resistant
to red rot, or bananas resistant to Panama disease, for example,
are not necessarily protected against any other sugar-cane or
banana disease. Nor is resistance of this type necessarily
constant under varying environmental conditions, so that varieties
perfectly resistant in one locality have sometimes proved sus-
ceptible in another, and resistance which operates for the ordinary
prevalence of a disease may fail against the mass attack of an
epidemic. The utilisation of resistant varieties is also subject
to the disability that their produce may be unsatisfactory in
some commercial quality, as in the bananas resistant to Panama
disease, or may cover only one section of the demand for a
graded article such as cotton.
It will be seen that dependence on acquired resistance, or
resort to inherently resistant varieties, is subject to many limita-
tions. The possibilities of these methods should, however,
be always kept in mind, and growers may usefully call attention
to apparently resistant plants when they occur ; the critical
testing of this quality is a technical matter best handled by the
pathologist.
Disease Evasion.
There are various ways in which disease may, under appro-
priate circumstances, be evaded. Land may be used which is
virgin to the crop, or remote from the same or related crops.
Of more general application is the timing of a crop to avoid the
seasonal prevalence of diseases ; thus cotton, which is highly
susceptible to damage from boll diseases in wet weather, may
be planted so as to bring the fruiting period within the edge of
the dry season. With short-term crops such as Indian corn
and many garden vegetables, the first plantings often escape
pests and diseases to which later ones are liable.
Elimination Methods.
The simplest kind of treatment for the control of a disease
which has gained access to a cultivation consists in the removal
of diseased plants or their affected parts : whole plants may be
pulled up or cut down and burnt or buried, diseased leaves and
fruit may be collected from the plants or from the ground,
diseased twigs or branches pruned away, affected bark or cankers
cut from stems ; while root diseases may be restricted by the
isolation of infected trees. Measures such as these are frequently
the only ones it is possible or profitable to adopt, and they are
commonly desirable as supplementary measures where spraying
or other treatment is applied. The effectiveness of elimination
in the control of disease varies widely. In the case of coconut
bud-rot thorough campaigns against diseased trees have been
reported very successful, and results may be similarly good in
GENERAL CONSIDERATIONS 85
most slowly developed affections, but when spores are produced
quickly and in vast numbers, as they are in so many diseases,
the effects are not as a rule satisfactory unless the plants are
also sprayed.
Spraying.
The object of most spraying with fungicides is not, as seems
to be commonly thought, the cure of disease, but the prevention
of further infection. Fungoid and bacterial parasites, with the
exception of the powdery mildews and a few others of similar
habit, enter more or less deeply into the tissues and cannot be
reached by a spray which does not at the same time destroy the
affected part itself. The action of the copper sprays, of which
Bordeaux mixture is the type, is to prevent the successful
germination of spores lying on the surface ; this they do most
effectively at the time of germination, so that protection is
best afforded by an enduring film in which the toxic ingredient,
the copper, which is at the same time the substance injurious
to the plant itself, is rendered soluble by slow degrees. For
the destruction of developed mycelium where it can be reached,
as in the mildews, the sulphur sprays are found more effective.
From the nature of the process spraying is most applicable
to affections of leaves, flowers, and fruit, especially when the
disease is of a seasonal and more or less epidemic nature.
Permanent Crops.
Considerations different in some respects from those applying
to arable crops arise in reference to permanent crops, in which
the same trees occupy the same ground for many years. The
protection given by rotation of crops is not available, and the
death of individual plants is much more serious, since they may
take years to replace. The adoption of more careful and expen-
sive treatment may be commercially sound, for not only the
immediate crop, but the future history of the trees has to
be considered.
In the West Indies there are all stages of tree cultivation
from the simple afforestation of land with limes, coconuts, or
cacao — further attention being confined to cutlassing the under-
growth and removal of the fruit — to careful orchard methods in
which the cultivation of the soil, the use and sequence of the
different types of manure, and the pruning of the trees receive
their due consideration. In the former system, as in forestry
proper, individual treatment for disease is seldom regarded as
practicable ; dead branches may be removed at intervals,
dead trees are replaced by " supplies," and that is about all,
except where the persistent march of a root disease may compel
attention. In the more intensive system reached on some cacao
estates the recommendation of sanitary measures and of the
use of sprays is not considered unreasonable. No doubt as
86 DISEASES OF CROP-PLANTS
time goes on and beneficial results are demonstrated this
disposition will increasingly prevail.
For leaf, flower and fruit diseases of trees, the most general
remedies are (i) wide spacing, to secure access of air and sun ;
(2) efficient drainage, to reduce humidity in rainy periods, and
(3) spraying. Where the first measure conflicts, as it may, with
the requirements of good vegetative development, a compromise
has to be made. In the case of debility diseases due to the
effects of exposure close planting may have distinct advantages.
Any trees which harbour pests and diseases liable to be
transmitted to the crop should be as far as possible eliminated
from the neighbourhood.
Pruning for twig and branch diseases, and trenching for root
diseases are dealt with in later sections.
There are two pieces of general counsel, of which the sound-
ness is beyond dispute, which may be offered in the present
connection to planters of permanent crops. The first is that
thought should be given to the possible incidence of disease in
laying out fields, so that, by wide and regular spacing, the pro-
vision of a regular drainage system, and the separation of blocks
by passable roads, accessibility and some degree of isolation may
be secured.
The other is that arrangements should be made for steadily
replacing, section by section, trees which have passed their
prime. In the modern commercial orchard of temperate countries
only trees in full vigour are tolerated, and the considerations
are the same, if their recognition is at present less, in the West
Indies. One great advantage is the resistance offered by such
trees to pests and diseases and their greater recuperative powers
when injured.
CHAPTER X
FUNGICIDES AND THEIR APPLICATION.
Apart from experiment stations the spraying of plants, whether
for insecticidal or fungicidal purposes, has come little into practice
in the West Indies, and less for the latter than the former purpose.
This can be attributed to the usually unintensive methods
of raising crops, to the lack of adaptation of available methods
to the needs experienced, to the general unsuitability for spraying
operations of the type of labour employed, to the difficulties of
maintaining the apparatus, especially rubber parts, in order,
and not least to a reluctance to face the trouble and supervision
involved. Even the simple operation of dusting cotton for
caterpillar attacks is often badly organised and carried out,
and the most general use of a fungicide, that of Bordeaux mixture
for treating cane cuttings, is made in a routine fashion which is
frequently inefficient. Improvement in these matters depends
on agricultural education, and on the ability of agricultural
officers to demonstrate the advantages to be secured.
The use of fungicides may be (i) protective against infection,
as in the spraying of leaves and fruits, the dipping of sugar-cane
cuttings, and the coating of exposed wood ; (2) remedial, as
when a fungus growing on the leaves of a plant is directly at-
tacked ; (3) disinfectant, when fungi, bacteria, or their spores
existing on seeds or other planting material, on a growing plant,
or in the soil are destroyed to prevent infection.
By far the most important function, in the extent of its
application, is the protective one, and the most common mis-
apprehension regarding fungicides is that spraying designed for
this purpose is of value as a remedial treatment, which very
frequently is not the case.
Bordeaux mixture is the standard protective fungicide,
but it has little remedial value. For the destruction of a fungus
on living organs, which naturally can as a rule only be atten-j.pted
when the mycelium is superficial, the sulphur fungicides are
in general most effective. For disinfection various poisons are
available for different purposes, such as mercuric chloride,
formalin and copper sulphate.
It is necessary to bear in mind that the pint of the United
States and Barbados is 16.6 fluid ounces as compared with the
20 ounces of Imperial measure, so that to get comparable results
in using American formulse with British measures five-sixths
87
88 DISEASES OF CROP-PLANTS
of the indicated number of gallons should be used. The
formulae in this book are given in Imperial measures.
The Copper Compounds.
The fungicidal action of the copper compounds is principally
of value in preventing the successful germination of spores, and
for this purpose they have to be adapted for use on the tender
surfaces of leaves and growing fruits. In order to have avail-
able over as long a period as possible an amount of soluble copper-
salt sufficient to kill the fungus without damaging the almost
equally susceptible tissues of the plant, they are applied in
compounds which only slowly set free the actual fungicide.
Bordeaux mixture, which consists of copper salts formed by the
interaction of copper sulphate and lime, is for general purposes
the most convenient and effective.
Bordeaux Mixture.
The popular way of indicating the strength of Bordeaux
mixture is by formula, thus 6-4-50 is recognised to mean the
mixture obtained by the use of 61b. of copper sulphate, 41b. of
stone lime, and 50 gallons of water.
For purposes of scientific discussion the strength is best
expressed as the percentage of copper sulphate used in prepara-
tion, and the Imperial gallon of water having a weight of lolb.*,
the use of ilb. of copper sulphate to each 10 gallons of water
makes a i per cent, mixture, which is about the standard strength
required for spraying and dipping. A 2 per cent, mixture is some-
times used, and 0.5 per cent, may be found to be strong enough for
a particular purpose or may have to be adopted to avoid injury.
These strengths are obtained by doubling or halving the pro-
portion of copper sulphate and lime in the i per cent, mixture,
taking therefore alb. or |lb. of copper sulphate for each 10
gallons of water.
The amount of lime used, provided that it is of good quality,
admits of considerable latitude. According to S. U. Pickering,
the requirement to decompose the whole of the copper sulphate
is one-fifth of its weight of pure lime. In ordinary practice a
considerable excess of lime is used, commonly a weight equal to
that of the copper sulphate, and the mixture is in consequence
strongly alkaline. This gives a wide margin to cover defects
in the lime, and has advantages, but it should be noticed that in an
emergency, or where lime is scarce and dear, half this quantity
of good lime can be used without changing the nature of the
resulting mixture. Attempts are sometimes made to use air-
slaked lime, but this is quite unsafe and produces a worthless
mixture. Where stone lime is unprocurable soda should be used
instead. (See Burgundy mixture).
* The American gallon of water weighs 8.345 lb.
M-- ,
•mmmt^
*:
Photo
li,-. 26
Mixing Bordeaux by Hamj
[/. B. Rorcr
^^
Photo b\]
Spraying Mango wit
/ / A ;</
■RAMOTOK I'l'MP MOINIED ON _MI-GaLLON OIL URLM
FUNGICIDES AND THEIR APPLICATION 89
Physical properties.
The physical properties of Bordeaux mixture are all- important,
since on the nature of the precipitate depends the adhesion of
the spray. In the freshly made mixture this should be fiocculent
or gelatmous in appearance, light blue in colour, settling very
slowly when allowed to stand. In a few hours the sediment
becomes more crystalline, settles quickly after shaking, and has
lost its power of adhesion when dried. This is the reason for the
customary warning to use only the freshly made mixture. The
flocculence of the precipitate can however be preserved to
some extent by adding skimmed or whole milk at the rate of
J pint (5 oz.) to the gallon, which also greatly increases adhesion.
The nature of the precipitate originally formed depends to a
large extent on the method of preparation. The mixing of
strong solutions of copper sulphate and lime gives a very heavy
sediment, and should be avoided. The ordinary way is to
dissolve the copper in one half of the water, mix the milk of
lime with the other half, and then run the two together. This
gives a satisfactory result, is normally the most convenient,
and is least likely to go wrong in unskilled hands. It is quite
possible to get good results, and under some circumstances with
more convenience, by mixing one constituent with about five-
sixths of the water and adding the other with the rest. The
best result of all, according to O. Butler, is obtained from the
addition of weak copper solution to strong milk of lime. Should
strong milk of lime be added to weak copper or strong copper to
weak lime, it is essential that the liquid should be thoroughly
agitated during mixing.
Solutions of copper sulphate must not be brought into contact
with metal vessels. For milk of lime this does not matter. The
prepared mixture can be contained in galvanised receptacles
provided it does not stand more than a few hours. When only
one wooden tub is available it may be used to contain the copper
sulphate and most of the water, the lime being added from a
bucket with vigorous agitation, or if the method of strong copper
to weak lime is adopted (which is less desirable) it is only neces-
sary to have a bucket made of wood, in which to dissolve the
copper, pouring the solution into the middle of the large re-
ceptacle containing the lime and water and stirring vigorously.
The most satisfactory addition to Bordeaux mixture for
increasing its adhesion, valuable where sprayed foliage is exposed
to tropical rain, is resin soap. This is prepared by boiling
2 lb. of resin with i lb. of washing soda in i gallon of water
until the mixture is clear, and adding this quantity to 25-30
gallons of Bordeaux as found necessary for the purpose in hand.
This also improves the wetting and covering power of the mixture
on such leaves as resist in these respects the ordinary type of
wash. It is possible that a weak Bordeaux with this addition
may be found to be as effective as a strong one without it.
90
DISEASES OF CROP-PLANTS
For insecticidal purposes lead arsenate paste 1-2 ounces to the
gallon (or powder ^-i ounce), or Paris green i ounce to 7 gallons
(added to the milk of lime before mixing), as stomach poisons,
or Black-leaf 40 (nicotine sulphate) i ounce to 4-6 gallons or i
teaspoonful to the gallon, as a contact poison, are frequently
added to Bordeaux mixture.
G. E. Sanders and A. Kelsall have worked out a method of
combining white arsenic, which is relatively cheap, with Bordeaux
mixture. One pound of superfine white arsenic (which should
pass through a screen of 250 meshes to the inch), mixed with one
pound of dry hydrated lime, is first added to each 10 gallons of
the water used in preparing the usual 10 per cent, stock solution
of copper sulphate. The latter, thoroughly stirred before dilu-
tion, is then used in the ordinary way.
Preparation.
The standard method for making the usual alkaline Bordeaux
mixture will now be described. The proportions are indicated
in the following table : —
Bluestone
Stone lime
Water
Class
Per cent.
lb.
lb.
gallons.
Strong
2
10
5-10
50
Ordinary
I
5
5
5^^
Mild
0.8
4
4
50
Weak
0.5
2^
2i
50
Paste
I
2
li-2
It is usual to have two tubs each holding 25 gallons and one
barrel holding 50 gallons. The copper sulphate is dissolved in
25 gallons of water in one tub. When left standing it dissolves
best if it is suspended near the surface tied up in a piece of
sacking. The lime is slaked in a bucket by adding water a little
at a time and at intervals of 5-10 minutes until it heats, swells
up, and crumbles, after which water can be added freely. Time
and trouble can be saved with some limes by using hot water.
The resulting milk of lime is turned into the second tub and made
up to 25 gallons. The contents of the tubs are now run together
at the same rate into the barrel, or baled out bucket for bucket.
The mixture is then ready for use, and the more quickly it is
used the better. If required, the preservatives, adhesives or
insecticides previously described can now be added. In filling
sprayers the mixture should always be passed through a strainer
of fine copper or brass gauze.
When Bordeaux mixture is needed from time to time, it is
convenient to prepare stock solutions of each material. These
are usually of 10 per cent, strength, i lb. of copper sulphate to
each gallon of water in the one case, and i lb. of lime to the
FUNGICIDES AND THEIR APPLICATION 91
gallon, previously slaked as directed with part of the water,
in the other. To make i per cent, mixture an equal quantity
of each is separately diluted by the addition of four times as
much water, and the two lots then mixed as before, so that to
make 50 gallons of mixture, 5 gallons of copper stock solution
is mixed with 20 gallons of water, and 5 gallons of lime stock
solution with the like amount. The stock solutions may, if
desired, be made up at double the strength given above. They
keep well if covered, but if evaporation has taken place should
be made up with water to the previous level.
Where water at the required level is available, it is an advan-
tage to have the stock solutions and dilution barrels on a raised
platform, so that the solutions can be run by gravity into the
mixing barrel, and if this is also raised the mixture can be again
run into the sprayers.
Testing.
When the indicated amount of good lime has been used the
mixture cannot contain unaltered copper sulphate, but to guard
against mistakes, and when the quality of the lime is uncertain,
it is advisable to test the mixture. There are three ways of
doing this : —
(i) A fresh solution of potassium ferrocyanide is prepared by
dissolving a crystal of this substance in a few spoonfuls of water.
A piece of white blotting paper is moistened with the Bordeaux,
or a little is poured into a white plate, and a drop or two of the
testing solution added. A brown discoloration indicates that
more milk of lime is required.
(2) A piece of blue litmus paper is dipped into the mixture.
If it turns red, add more milk of lime.
(3) A bright iron nail (avoid steel) is allowed to stand for
some minutes in a little of the mixture. If a black deposit or a
coating of metallic copper is formed on the nail, add more milk
of lime.
Bordeaux Paste.
The thick creamy compound known as Bordeaux paste is
used for application to bark or exposed wood, or for any similar
purpose in which a temporary paint protective against fungus
infection is required. It is prepared by dissolving i lb. of copper
sulphate in i gallon of water (or using i gallon of 10 per cent,
stock solution) and separately slaking 2 lb. of lime in \ gallon of
water. When the latter is cooled the two are stirred together.
The addition of milk (i pint or more to the gallon) or sugar
(4 ounces to the gallon) greatly improves its keeping qualities.
Burgundy Mixture.
Preparation.
A mixture which closely resembles Bordeaux and is regarded
as interchangeable with it in use is prepared in a similar way
92
DISEASES OF CROP-PLANTS
but with ordinary washing soda (crystallized sodium carbonate)
substituted for the lime, using i| times the quantity. The
formulae for various strengths are given in the following table : —
Bluestone
Soda
Water
Class
Per cent.
lb.
lb.
gallons.
Strong ...
2
8
10
40
Ordinary...
I
4
5
40
Mild
0.8
4
5
50
Weak ...
0-5
2
2.
40
Very weak
0.4
2
2^
50
Paste
2
2|
2
The 0.8 strength has been found by S. C. Harland to be the
maximum that can be applied to Sea Island cotton without severe
injury, and, according to E. J. Butler, the 0.4 mixture is popular in
some parts of France and Germany for spraying vines. The
addition of milk, ^ pint (5 oz.) to the gallon preserves and in-
creases the adhesion of the precipitate, or resin soap can be
added to increase adhesion as with Bordeaux.
Burgundy mixture can be prepared by the ordinary Bordeaux
method of mixing equal amounts of the total water, one contain-
ing the copper and the other the soda. It may also be con-
veniently prepared by dissolving the copper in seven-eighths of
the water, and using the remaining one-eighth (as many pints
as there are to be gallons of mixture) to dissolve the soda. The
latter solution may be made in a bucket or a kerosene tin, the
copper solution being then stirred vigorously and the soda
solution steadily poured into it. The colder the solutions, the
better the precipitate. The mixture should be used fresh unless
a preservative has been added. Burgundy paste is prepared in a
similar way to Bordeaux paste, and should if possible have
milk added.
Stock solution of copper can be used as for making Bordeaux,
but it is better to keep the soda in solid form.
Testing.
There is less need to test Burgundy than Bordeaux, provided
the soda is of good quality and the weighing accurate. The
soda should be crystalline, without much white powder.
The test is best made with litmus paper, adding more soda if
blue litmus is turned red, and more copper solution if the turning
of red litmus blue shows that the mixture is more than mildly
alkaline. The results of the ferrocyanide test and the iron
test do not afford an index to the safety of Burgundy mixture.
Advantages.
Where spraying is done on a large scale and lime can be
FUNGICIDES AND THEIR APPLICATION 93
obtained, there is no particular advantage in substituting Bur-
gundy for Bordeaux mixture, and the latter will usually be the
cheaper. But where good lime is not available the use of soda is
necessary, and when only a small quantity of mixture is required
at a time it is much more convenient. Weighed quantities to
make i gallon (i^oz. copper sulphate and 2oz. soda) can be kept
in separate small bottles and made up as required with very
little trouble.
Burgundy mixture is less likely than Bordeaux to contain
particles which will clog the nozzles of sprayers.
Copper Sulphate Wash.
At a strength of 31b. to 40 gallons of water copper sulphate
is sometimes used as a bark wash or disinfectant. It cannot be
applied to leaves or to tender tissues of any description.
Cuprammonium Washes.
The Cuprammonium washes, made by dissolving copper or
one of its compounds in ammonia water have no advantages,
and some disadvantages, as compared with Bordeaux mixture
for the purpose for which the latter is ordinarily used, i.e., pre-
vention of the germination of spores. They have, however,
retained an application to cases (1) where the visibility of the
deposit left by Bordeaux spraying is objected to as unsightly ;
(2) where direct effect on a developed fungus is required. The
results of a comparison are summed up by O. Butler to the effect
that the cuprammonium washes are more effective than Bordeaux
mixture when large amounts of soluble copper are required to
give protection, less effective when small amounts suffice. These
fungicides have the practical disadvantage in hot climates
that strong ammonia is difficult to keep and dangerous to handle.
It should be diluted with 5 times its volume of water as soon as
it is obtained.
The original eau celeste (cuprammonium sulphate) is made by
adding ammonia to a solution of copper sulphate. Modified
eau celeste (cuprammonium hydrate) may be made by adding
ammonia to Burgundy mixture until the precipitate is dissolved.
The method of preparing cuprammonium hydrate usually
recommended is by dissolving commercial copper carbonate in
ammonia according to the formula : —
Copper carbonate ... ... 5 ounces.
Strong ammonia 3 pints.
Water 40 gallons.
The ammonia is diluted with i gallon of water, and the copper
carbonate, previously made into a paste with a little water, is
94 DISEASES OF CROP-PLANTS
slowly mixed in, stirring all the time. The remaining water is
then added. The solution does not keep well, and should be
used fresh.
According to O. Butler the amount of ammonia indicated is
insufficient, and should be increased to i| pints to each ounce of
copper carbonate, a stable stock solution being prepared accord-
ing to the formula : —
Copper carbonate ... ... i ounce.
Strong ammonia ... ... i| pints.
Water ... ... ... i pint.
adding water at the rate of 3 gallons to each pint of stock solution
when required for use.
Probably the method which will be found to be in many
respects the most convenient is that of dissolving the precipitate
of Burgundy mixture by adding sufficient ammonia. According
to O. Butler the product compares very favourably in efficiency
and cheapness with the ammoniacal solution of copper carbonate
made according to the ordinary formula.
Sulphur Fungicides.
Sulphur.
Finely divided sulphur is a valuable fungicide for use when
action on exposed mycelium is required, as in the case of the
powdery mildews (Erysiphaceae). It is applied direct in the
form of dust, or is deposited from liquid sprays.
The dry material has two commercial forms, powdered
sulphur or sulphur flour, obtained by grinding, and sublimated
sulphur or " flowers of sulphur " deposited from sulphur vapour.
The most satisfactory and economical is that which is most
finely divided, and both forms vary considerably in this respect.
Examination under a lens or sifting through fine gauze enables
comparisons of quality to be made.
The fungicidal action of sulphur depends on fumes of uncertain
composition which are given off under the influence of the sun's
heat. Its effects are therefore obtained to the fullest extent in
warm dry weather. Rain and high winds should as far as
possible be avoided, and where this cannot be done, lime-sulphur
solution should be used. The applications are most effective in
the early stages of development of the fungus. The mixture
of an equal quantity of powdered lime with the sulphur, often
recommended, was found by F. M. Blodget to neutralize to a very
considerable extent its fungicidal action.
Lime-Sulphur Solution.
Lime-sulphur solution may be used to replace sulphur where
an adhesive spray is preferred to a powder. It has a special
FUNGICIDES AND THEIR APPLICATION 95
use as a substitute for Bordeaux mixture where the effect ofthe
latter in protecting scale-insects is a disadvantage (often the case
with citrus trees), and in concentrated form it has a powerful
and sometimes very valuable elfect in keeping the bark of trees
clear of moss and other surface growths.
Stock solution is prepared according to the following formula :
Best stone lime (about 95 per cent.) 50 lb.
Flowers or flour of sulphur . . 100 lb.
Water 40 gall.
An iron vessel must be used for the boiling, and must be of
sufficient size to contain the large amount of froth formed in
the early stages. About half the water is first made hot, the
lime added, then the sulphur, with vigorous stirring until both
are well mixed, and then the rest of the water. The fire should
not be allowed to get fierce, and splashes on hands or clothing
must be avoided. Either about 3 gallons of extra water should
be added at the beginning, or the water lost in boiling should be
replaced from time to time, preferably by hot water, so that the
level when boiling is complete is near 42 gallons hot or 40 cold,
as shown by a measuring stick or mark previously prepared.
The volume at boiling point may be taken as 1.05 of the volume
cold. Actual boiling should continue for 45 minutes ; long
boiling reduces the amount of sulphur in solution. A steam
jet may be used for boiling, in which case no extra water is
required. The sediment should be strained out before storage,
using 30-50 mesh brass or tinned iron (not copper) gauze.
If the available boiler is small, half the water may be kept
back until boiling is complete, but the resulting solution is not
quite so good. If preferred the lime and sulphur may be placed
in the boiler and well mixed with half the water before heating,
and the remaining half added a little later.
The stock solution will keep well if not exposed to air. It
may be stored, after being well mixed to secure even distribution
of the sludge, in full stone jars or bottles kept closed down, or in
barrels if covered with a layer of mineral oil (kerosene or pre-
ferably lubricating oil).
For use as a spray the solution thus prepared, after straining,
needs dilution at the rate of about 20 gallons of water to i gallon
for resistant foliage, 30-40 to i for more tender leaves. To
avoid the danger of scorching foliage, the strength may be
controlled by means of a hydrometer, the diluted spray being
used at strengths of approximately i|° Baume for resistant
and 1° or 0.7° Baume for more tender foliage. The correct
strength for the work in hand must be determined by experiment.
The new Baume scale is used in the following dilution table,
but hydrometers graduated on the old scale may be used viithout
affecting the results to a significant extent.
96
DISEASES OF CROP-PLANTS
Stock S
OLUTION
Gallons of
Watef
TO Each Gallon
of Stock.
Spec.
B°
Sp. Gr. I. on
Sp.
Gr. I
007
Sp.
Gr. I . oo.s
gravity
Baum6 1.5"
Baume 1
.0°
Baume 0.7°
1. 210
^\
18
29
41
I.215
26
19
30
42
1.225
27
20
31
44
1.236
28
21
33
46
1.246
29
22
34
48
1-257
30
23
36
51
1.268
31
24
37
53
1.279
32
25
39
55
1.290
33
26
41
57
1.301
34
27
42
59
1-313
35
28
44
62
For use as a bark wash the stock solution is diluted at the
rate of i gallon to 7-10 gallons of water. Arsenate of lead may
be added to lime-sulphur sprays, 1-2 oz. of paste or |-i oz. of
powder to the gallon, and increases the fungicidal value as well
as acting as a poison for biting insects. Paris green cannot
be used with lime-sulphur.
Lime-sulphur solution has a strong caustic action, and
contact with skin or clothes should be avoided. Copper vessels
should on no account be used in its preparation or application.
This includes copper spraying machines, which suffer rapid
deterioration. Tinned or galvanised receptacles may be em-
ployed. The spraying outfit should be well washed out after
use.
Self-Boiled Lime-Sulphur.
This is a rather weak fungicide which is safer than Bordeaux
or boiled lime-sulphur on tender leaves or flowers. In its pre-
paration the slaking of the lime is used to supply the necessary
heat. About 10 lb. of best stone lime is placed in a barrel ;
an amount of water nearly sufficient to cover the lime is added,
and, as slaking begins, 8 lb. of sulphur, carefully freed from
lumps, is well stirred in ; the barrel is then covered to keep in
the heat and the mixture allowed to stand with occasional
stirring for not more than 10-15 minutes. When the action is
judged to be complete more water is added and well stirred,
and the quantity of wash brought up to 40 gallons. The mixture
should have the coarser particles of lime strained out, but the
finer sediment should be well worked through the strainer. As
the sediment settles quickly good agitation is needed in the
sprayer. The mixture can be used as a basis for an iron sulphide
spray (see below).
Since different limes vary greatly in their heat of slaking
exact instructions cannot be given for the time of cooking.
•.^-p^Nj
S^^BHi
Fig. 29
Compressed Air Sprayer in Action
FUNGICIDES AND THEIR APPLICATION 97
Some lime is so sluggish that it is necessary to use hot water
to start the process and difficult to raise enough heat to complete
it, while with hot limes care has to be taken to stop the cooking
by adding cold water to prevent the sulphur going into solution .
Liver of Sulphur.
Liver of Sulphur has been used in gardening practice for
spraying against mildews and in cases where the deposits left on
fruit or foliage by other fungicides have to be avoided. The
name in its stricter sense is applied to potassium sulphide, but
the commercial substance is a mixture of several compounds.
Its fungicidal value depends on its content of sulphides, which
varies greatly in different samples. Exposed to air its colour
turns from liver brown to greenish yellow, and eventually to
grey, and its value is lost. It should be stored in well-sealed
tins or bottles, which must be kept full. It is applied at a
strength of i oz. to 2-3 gallons of water, but owing to the un-
certainty of its composition, and its tendency to burn when used
at really fungicidal strengths, its use is not to be recommended.
A mmonium Sulphide.
This compound has recently been brought forward as a
much more efficient and less injurious fungicide for use against
mildews than liver of sulphur. Its preparation can only be
undertaken in a chemical laboratory, so that the purchase of
commercial stock solution is necessary. Except for ripening
fruit or ornamental plants it has no advantage over lime-sulphur.
Iron Sulphide.
This is recommended as a good fungicide non-injurious to
fruit. A self-boiled lime-sulphur mixture (see above) is prepared
with 10 lb. of lime and 10 lb. of sulphur to 34 gallons of water,
and 3 lb. of iron sulphate (copperas) dissolved in 6 gallons of
water is added. This gives a black spray.
Potassium Permanganate.
A solution of this substance in water is effective against
surface mycelium and some spores. It is convenient for use in
small quantities on garden plants. J. J. Taubenhaus found that
sweet peas could be sprayed with solutions up to 3 per cent,
without injury, and that 0.5 per cent, strength was effective
against anthracnose. To prepare a i per cent, solution dissolve
I ounce in 5 pints of water ; 2 ounces of soap may be added if
found advantageous to increase the wetting properties. Potassium
permanganate may be added to Bordeaux mixture with a view
to increasing the effect against exposed mycelium.
Spraying.
Successful and economical spraying depends on the applica-
tion of the fluid in such a manner that after drying a thin and
even deposit is left on the leaves or other parts of the plant.
This can only be secured by a fine mist-like discharge carefully
98 DISEASES OF CROP-PLANTS
distributed, and this in turn depends on the form of nozzle and
the amount of pressure apphed.
Spraying machines range from power outfits with petrol
engines, through various forms of hand-operated tank or barrel
pumps, to knapsack types and hand-syringes. In the choice of
a type the nature of the ground to be worked over and the
spacing and height of the plants are first considerations. There
are few local crops to which power or traction sprayers can be
adapted. The most practical is usually some kind of wheeled
tank with a hand-pump attached, and, according to the power
of the pump, one or two 50-feet lengths of hose. Additional
wheeled tanks of convenient form may be used to keep the
sprayers supplied with fluid.
Knapsack sprayers of the ordinary type require in the tropics
more than ordinary endurance on the part of the carrier to
maintain a satisfactory pressure. The compressed-air type,
consisting of tanks charged by a separate pump, is the best for
use in situations out of the reach of wheeled tanks, and may be
used by climbers for sprajdng coconut trees. The Abol type
of syringe is very useful for small spraying jobs in gardens.
The mist-like discharge required for good spraying will not
carry far, consequently the nozzles have to be raised to nearly
the height of trees to be sprayed by means of bamboo rods,
which can be used up to about 10 feet in length.
The aim in spraying should be thoroughly to cover the plant,
while keeping the drip from the leaves to a minimum. The
quality of the work can only be judged after the spray is dry,
and expertness is only to be acquired by practice.
Spray injury.
Damage to foliage from Bordeaux mixture is notoriously
erratic. It is agreed that the scorching effect which may develop
from properly prepared mixture depends a good deal on the
weather, being often increased by dull humid conditions. The
susceptibility is greater when the leaves are young and when
they are approaching senescence, and may be much increased
by insect injuries which in themselves are little noticeable.
Different varieties of the same plant species often show great
differences in susceptibility. There is little information on the
subject with regard to tropical plants and conditions. On
cotton the effect of too strong an application is the shedding of
the incipient flower budo
Lime sulphur injury is less dependent on external conditions,
and may be met by reducing strength or in extreme cases by the
use of the self-boiled wash.
Both copper and sulphur sprays often show a distinct tonic
effect on vegetation apart from their fungicidal action.
Dusting.
The application of fungicides in the form of dust has been
FUNGICIDES AND THEIR APPLICATION 99
practically confined to sulphur, to mixtures of sulphur and lime,
and of sulphur with 10 per cent, of dry arsenate of lead. Recently
dry Bordeaux powders have been placed on the market and
are coming into favour, but it is difficult to foresee a future
for them in places liable to tropical showers. There are various
types of dusting machine to be obtained, the best of which,
short of power or traction outfits, are certain types with hand-
operated fans. For garden purposes sulphur may be sifted out
from a cotton bag.
Disinfectants.
The principal use of disinfectants in plant pathology is for
the treatment of seeds ; they also have an application to certain
other forms of planting material, to the stems and branches of
trees, to wounds and, to a very limited extent, to the soil.
Disinfection of Seed.
In a number of diseases the disinfection of seed affords the
best opportunity of eliminating a parasitic organism or of reducing
its prevalence in the resulting crop. It is of particular value
with new introductions, and in planting short-term crops after
rotation or a close season.
As a general rule disinfection only extends to spores or other
material adhering externally, but in the case of certain smuts
it is possible to destroy internal mycelium bj^ means of heat.
There is no universally applicable method of seed dis-
infection. There are such marked differences in the suscepti-
bility to poisons of seeds on the one hand and parasites on the
other that a treatment which is successful in one case may
entirely fail in another. There are spores, for example, which
can germinate in strong Bordeaux mixture and others which
can withstand immersion for a considerable time in i per cent,
copper sulphate solution. Difficulties are experienced in securing
the wetting of certain kinds of seeds, as for example cotton seed,
which is difficult to free from the air entangled in the fuzz.
Corrosive Sublimate.
Mercuric chloride, or corrosive sublimate, has a wide range of
effectiveness. It is used in solution in water, most usually at a
strength of one part in a thousand (i oz. to 6 J gallons). It has
been used on a considerable scale for soaking cotton seed, but the
method is subject to the disability mentioned above unless the
seed has been delinted with sulphuric acid or wetted with alcohol,
measures only applicable on a small scale. The effects of
mercuric chloride are liable to be neutralized by the presence of
organic substances of an albuminous nature, and it is inad-
visable to use the same lot of solution more than once. It is
desirable to wash seeds after treatment.
The solution is often recommended for use as a disinfectant
on newly exposed wood of trees, preparatory to a more per-
manent coating of a preservative paint.
TOO DISEASES OF CROP-PLANTS
Mercuric chloride must always be treated with the caution
due to an exceedingly powerful poison. Metal vessels should
not be used to contain the solution.
Formalin.
Formalin is the name given to a solution of formaldehyde gas
in water, containing, when at full strength, 40 per cent, of the
gas. It gives off a vapour which irritates the eyes and throat.
Both the solution and the gas are very powerful disinfectants,
and are only limited in application by the susceptibility of some
seeds and other treated material to injury. They afford the most
generally satisfactory means of disinfection.
In the disinfection of seeds formalin is used at strengths of
2 to 0.25 per cent, of the commercial liquid, according to the time
of exposure. For prolonged treatment, as of potatoes for scab,
or grain for smut, strengths of 0.4 to 0.25 per cent, are used, the
thoroughly wetted material being covered up for about two
hours. In using the 2 per cent, solution seed is soaked for 15
to 45 minutes according to susceptibility. In some cases better
results are obtained by long exposure to weak solutions, in others
the reverse. A i per cent, solution of formalin contains i oz. of
formalin in 5 pints, and from this the other strengths can easily
be calculated.
Certain kinds of seeds cannot be safely treated with liquids,
and others are very difficult to wet thoroughly. These diffi-
culties can be overcome by the use of formaldehyde vapour,
which is in general less injurious and more effective than the
liquid treatment. An apparatus has recently been described
which is suitable for use where there is regular need for disin-
fection of a large variety of seeds. It consists of an arrangement
for passing steam, mixed with atomized formalin, into a gal-
vanized iron can with perforated shelves. 10-20 ounces of for-
malin per 1,000 cubic feet, allowed to act for 2 hours, is about the
average quantity required.
The method of generating the gas commonly used is that of
pouring formalin over potassium permanganate crystals at the
rate of i imperial pint to 9 ounces. The crystals should be placed
at the bottom of a large bucket (for quantities up to 3 pints) and
this placed on a tray of some kind, as the mixture foams
vigorously. The fumigatorium should be ready for immediate
closing when the formalin has been added, or an arrangement
should be made for the release of the liquid after closing.
The above method is also applicable to the fumigation of fruit
previous to storage, and has been used with success for pineapples.
For the disinfection of the soil of seed beds, formalin of 0.5
per cent, strength, at the rate of about three quarters of a gallon
to the square foot of prepared bed, is distributed with a watering-
pot as fast as the soil will take it up. The beds are then covered
with bags or tarpaulins for a day or so to keep in the fumes, and
are aired for a week before sowing.
FUNGICIDES AND THEIR APPLICATION loi
Formalin has latelj^ been found to have some appHcation as a
spray. E. Levin reports highly successful control of bacterial
soft rot of lettuce obtained by spraying diseased plants with
0.4 formalin (i oz. to I2| pints), while the addition of formalin
to Bordeaux mixture was at one time claimed to have given
promising results in the control of citrus canker in the Philippines.
SiilpJmric Acid.
Treatment of seeds with concentrated sulphuric acid is a
means of disinfection particularly valuable in some instances, as
with cotton seed, and with most kinds, so far from being injurious,
hastens and increases germination if carried out for the right
length of time. The seed to be treated is well stirred in a glass
or stone jar with a quantity of undiluted acid sufficient to cover
it and to give freedom of movement. The commercial grade of
acid is suitable. After 15 to 45 minutes' soaking (according to the
resistance of the seed to injury) the contents of the jar are poured
through a strainer of wire gauze and the seed then washed in a
liberal amount, preferably a stream, of water. It may be sown at
once or dried. This method is recommended to be used for cotton
seed whenever possible, as it clears away all the lint and fuzz.
Copper Sulphate.
One or two per cent, solutions of copper sulphate (1-2 lb. to
5 gallons) are used for the disinfection of seeds, especially of
grain for smuts. Preferably the grain is immersed in a barrel
of the solution, the floating matter skimmed off, and then the
seed quickly dried and sown. Another method is to sprinkle
a heap of seed with the solution, turning and mixing until all is
wetted, then drying. If the seed is treated with weak milk of
lime after the copper solution there is less resulting injury.
Bleaching Powder.
The so-called chloride of lime or bleaching powder has recently
been recommended as very effective for disinfecting some kinds of
seeds. The method of preparation advised by J. K. Wilson is as
follows : — " 10 grams of commercial chloride of lime (titrating
28 per cent, chlorine) is mixed with 140 cc. of water, (5 oz. to
3I pints). The mixture is then allowed to settle for five or ten
minutes and the supernatant liquid decanted off or filtered.
The solution or filtrate, which contains about 2 per cent, chlorine,
is used as the disinfectant. The volume of the solution employed
should be about five times or more the volume of the seed." The
permissible period of exposure must be learnt by experiment
for each kind of seed. Washing is not necessary.
Bleaching powder has recently been mentioned as a disin-
fectant for soil.
Eaii de Javelle.
Recent experiments by Duggar and Davis indicate that com-
mercial Javelle water (chlorinated potash), diluted i in 10 and
used for soaking seeds for 3 hours or upwards, may prove an
exceedingly effective means of seed sterilisation.
CHAPTER XI
WOOD-ROTS : THE TREATMENT OF TREES
Structure.
For the purpose of this section a tree or shrub (excepting
coconut and other palms) may be regarded as a more or less inert
framework of wood, composing the root, stem, and branch systems,
over which is stretched in a thin layer the active living substance
of the tree. The woody tissue, once it is matured, contains
relatively few living elements, and is capable of no further
growth. The wood functions as mechanical support and for
the passage of the soil solution from the roots which absorb
it to the leaves which discharge its surplus water. In some
species only the outer layers of the stem (sapwood) are concerned
in conduction, the remainder, the heartwood, being quite lifeless,
in other species all the wood continues to function to some extent.
Outside the wood lies what is popularly known as the bark,
which consists of (i) the inner living layers, the bast, and (2) the
outer dead layers of covk, the bark proper. An important
function of the bast is the transference of the food materials
elaborated in the leaves, including the supply of the necessary
nourishment to the roots. The bark is protective in its functions,
as will be noted later. Between the wood and the bast is an
extremely thin layer of delicate actively dividing tissue : the
cambium layer. From this the growth in thickness of all the
permanent parts of the tree proceeds. From its inner face
continuous new layers of wood are deposited on the old, and from
its outer face new layers are added to the bast.
Increase in length of any woody organ takes place only by
additions to its outer end, and these are made at intervals by the
outgrowth of buds into new leafy shoots.
Wood-rots.
It will be readily understood, from what has been previously
said of resistance, that the wood, with its relatively few living
elements in the sapwood and none in the heartwood, will be sus-
ceptible to the attacks of fungi. There is in fact a very large
number of fungi which specialise in wood destruction, many of
them saprophytes, infesting dead wood with effects universally
familiar, others with various grades of ability to attack the wood
of living trees.
When a tree is intact the wood is completely enclosed, first
by the outer bark, which is impervious to water, and even to air
except through the special breathing organs (lenticels), and is
WOOD-ROTS: THE TREATMENT OF TREES 103
difficult tor fungus hyphffi to penetrate ; secondly by the bast,
largely made up of active living tissue, and thus resistant to any
but specially adapted parasites. In very many cases, therefore,
the fungi attacking the wood of Uving trees are perforce wound
Dravm by W. Buthn
Fig. 30 Healing of Pruning
Wounds (Mandarin Orange)
\} ' » ':
Drawn by W. Buthn
Fig. 31 Effects of Bad
Pruning (Cacao)
parasites, able to make an entry only when the outer envelopes
are broken.
The Healing of Wounds.
When the outer wood is exposed, by the cutting or breaking
of a branch or by the removal of a patch of bark, it has in itself
104 DISEASES OF CROP-PLANTS
no power to regenerate its covering, only the cambium can
produce new material to cover it. The process can be followed
on trees from which branches have been cleanly removed. From
beneath the cut edges of the bark around the margin of the
wound a lateral outgrowth of callus from the cambium creeps
in to cover gradually the exposed wood. If the surface is
smooth enough and the wood lasts long enough without rotting,
this ingrowing callus meets and unites in a more or less central
point or line, and restores the continuity of the cambium, so that
new wood and new bark are formed over the wound. If on the
other hand the exposed wood rots away, leaving a cavity, the
foundation for this process is lacking, the callus curls round on
itself at the margin of the wound, and an open passage is left to
the interior of the tree.
When a branch is cut through or broken some distance from
its base, there is a tendency, especially if the branch is situated
low on the stem where it is in the shade of the leaves, for the
stub so left to be thrown out of the circulation, so to speak, and
die back to its base on the parent branch or stem. So long as
the dead stub remains, the callus which forms around its base
cannot cover its site. If it is left to rot away it usually leaves
a cavity which is also difficult to heal.
If a vertical stem is cut through, the upward growth is usually
taken on by a shoot or shoots arising below the cut, and a process
of healing similar to that just described for a lateral wound
begins.
If the new shoots arise close to the cut, so that no dead
stump blocks the progress of the callus, the wound will cover,
subject to the same condition as before, that the wood resists
decay for a sufficient time. For thick stems the required time
may be many years. Frequently in such a case the covering
remains incomplete, the wood rots and the new stem or stems
sooner or later break down owing to the incompleteness of their
structure on the inner side.
The Treatment of Wounds.
On the facts thus outlined are based the principles of tree
surgery, which are simple enough, but are commonly neglected.
They may be stated as foUows : —
1. When a branch is removed it should be cut close to the
base and the resulting wound shaped into as close conformity
with the form of the parent branch or stem as is possible without
making the wound unduly large. The edges of a saw cut should
be rounded off, especially the projecting lip usually left on the
lower side.
2. The live bark should extend, cleanly cut and not bruised
or torn, close up to the edge of the wound. If there is dead bark,
it should be cut back until eliminated, and the wood shaped
accordingly.
WOOD-ROTS: THE TREATMENT OF TREES 105
3. If experience of the kind of tree, the vigour of its growth,
and the size of the wound indicate the necessity, the wood should
be treated with an antiseptic or a protective covering to preserve
it until the wound has time to heal over.
4. If a cavity has formed from an old wound the rotten
wood should be dug out, an antiseptic applied, and the cavity
filled with cement to the level at which the callus is expected to
extend over it.
It will be found that some species of trees heal wounds very
much more readily than others, that young trees heal more
quickly than old ones of the same species, and of course that
some woods are very much more durable than others when
exposed.
Wounds near the soil level are the worst to deal with, as the
soil moisture greatly increases liability to infestation and rotting
of the wood. Treatment of wounds about the collar and the
crown roots should be accompanied by removal of the soil about
them, and also of sheltering weeds or low branches. In humid
localities where moss accumulates the bark should be scoured by
spraying or swabbing with lime-sulphur or some other detergent.
The treatment of canker, gummosis, and bark rots generally,
consists of cutting back to sound bark, the scraping of the wood,
and the application of a preservative to the latter. The practice
of gouging out the underlying discoloured wood has to the
writer's mind little to commend it. Though fungus hyphse may
be present, the drying out of the wood, its impregnation with an
antiseptic or the exclusion of air by a protecting coating, which-
ever is most appropriate to the particular case, is likely to be
quite as effective.
Antiseptics.
The antiseptic most in favour at present is o.i per cent,
corrosive sublimate solution (see page 99). Its effects are
evanescent and are liable to be interfered with by substances
present about the wound. Applied to the surface of wounds
just previous to the application of a protective paint or filling it
may be expected to have a certain amount of usefulness in
destroying spores or mycelium already present. There have
been grades of carbolineum specially prepared so as not to be
injurious to bark, and if a reliable brand could be obtained this
material should in many cases be the most satisfactory to use.
Bordeaux and Burgundy pastes are to some extent in use as
temporary antiseptics.
The covering of a wound with a good waterproof coating is in
itself sufficient to alter the conditions to the disadvantage of
some kinds of wood-infesting fungi.
Protective paints.
Numerous substances are or have been in use for covering
io6 DISEASES OF CROP-PLANTS
exposed wood until the healing process has time to complete
itself. What is desired is a durable coating which does not
readily crack, which has no ill effects on bark or callus, and
preferably is to some extent antiseptic. Coal tar sometimes
proves definitely objectionable on the second count ; the differ-
ences of experience in this matter probably arise from differ-
ences in composition of the material used. Resin-petroleum,
prepared by dissolving 3 parts of resin in one part of warm crude
petroleum, has been recommended. The simplest treatment,
which in many cases has proved quite satisfactory, is the applica-
tion at intervals of coats of white lead paint.
The writer has experimented with a paint which gives better
results where protection is decidedly needed, as on soft and
spongy woods, than any other which has been tried. It is
prepared by heating over a slow fire until smooth 20 oz. of boiled
linseed oil and 12 oz. of Barbados manjak, a hard bright asphalt.
Probably a preparation of manjak and crude petroleum would
also be satisfactory.
On young and vigorous trees with durable wood, provided
wounds are shaped to cover easily, and provided the conditions
of shade and moisture are not extreme, there is commonly no
need of any artificial protection, though in some localities con-
sideration has further to be given to the habit of some species
of ants in occupying exposed wood, especially Cremastogaster
sp., the acrobat ant, which seems to attack the cambium and
seriously delays the restoration of the bark.
CHAPTER XII
THE CONTROL OF DAMPING-OFF
Damping-off is the term applied to the failure of seedlings due
to their infection while in a tender state by certain soil-inhabiting
fungi. The reason for the name is the association of the trouble
with conditions of more or less excessive moisture, which favours
the development of the parasite at the same time that it in-
creases or prolongs the tenderness of the plants. Though not
necessarily confined to seedlings crowded in boxes or seed beds,
it is amongst such that the affection usually appears and, by
progressive infection, is able to cause extensive losses.
The longer the soil of the seed-bed has been in use, and the
more decaying animal or vegetable material it contains, the more
likely it is to harbour fungi capable of causing damping-off.
Heavy water-retaining soils are more favourable to the affection
than those which are light and porous, and provision for rapid
drainage is one of the most important precautions against it.
Shade and shelter, by maintaining humidity, increase the ten-
dency to it ; in these respects, as in the supplying of water, the
conditions which may favour the seedlings favour the disease,
and a mean has to be struck between slow growth and loss.
The trouble occasioned by damping-off in these islands is not
so great as might be expected by an agriculturist accustomed to
temperate countries, where warm, humid weather, such as is
associated with ideas of the tropics, is greatly feared in this
connection.
In the first place nearly all the staple food plants — sugar-
cane, bananas, tannias, dasheens, sweet potatoes, yams, cassava
—are raised from cuttings of one sort or another, while cotton,
corn and pulses, which are raised from seed, are planted, a few
seeds together, in tneir permanent positions in the open ground.
Of agricultural as distinct from garden crops, tobacco, onions,
and limes, which are raised in seed-beds, have been the plants to
suffer most in the West Indies from the affection under notice.
Merely reducing the density with which the seed is sown is
often sufficient to avoid or reduce the damage by permitting
increased ventilation, preventing the drawing up of the seedlings,
and making the spread of infection more difficult. Further
measures to this end are the use of sandy soil, of subsoil, or of a
surface dressing of sand or fine gravel, and the choice of an open
rather exposed situation for the seed-beds. The use of wood-
ashes or lime is not to be recommended.
The one means so far discovered which can be relied upon to
give satisfactory results under any reasonable conditions is the
lo;^
io8 DISEASES OF CROP-PLANTS
disinfection of the soil. For this there are two kinds of treat-
ment available : by heat and by chemicals. Primitive methods
sometimes used are the roasting of the surface soil by covering
the bed with trash and brushwood, which is fired ; or, alternatively,
the heating of the soil in some kind of pan. The more modern
way is to use live steam for the purpose. A large shallow box
of sheet iron (about lo inches deep) with one or more perforated
pipes in its base, is inverted over the bed and the edges pressed
into the soil. The pipes connect with the boiler of a portable
engine, and steam at 80-100 lb. pressure is forced into the
soil. About 30 minutes is the average exposure given.
Alternatively a stationary boiler may be used and the soil
brought to it in a covered box which has perforated pipes ar-
ranged for the passage of the steam.
The formalin drench method is carried out with a one in 50
solution of commercial formalin, applied to the prepared bed
slowly enough to sink in, and at the rate of two quarts to the
square foot. The soil should be covered to retain the fumes
for a day or two, and then left open several days before planting.
The steaming method is the better of the two, but both are
too expensive to use with any but a valuable and delicate crop.
A cheaper method, using sulphuric acid, is described in papers
by C. Hartley and R. G. Pierce. The experiments of these
authors relate to the seedlings of coniferous trees, and its suit-
ability for any other type of plant should be determined by
preliminary trials. For the requirements of nurserymen the
treatment consists in the application, in standard soils, of three-
sixteenths of a fluid ounce of commercial sulphuric acid to each
square foot of seed-bed, applied in solution in water immediately
after the seed is sown and covered. This has proved more
reliable than the more expensive methods mentioned above.
The amount of water used to carry the disinfectant does not
appear to be a matter of importance, provided that the necessary
amount of the acid is applied to each unit of area. The quantity
used by the authors varies from i pint per square foot when the
soil is wet to 2 pints when the soil is dr3\ There is a possibility,
especially in light soils, of a concentration of the acid by evapora-
tion to a strength injurious to the root tips, which in practice has
been found to be completely avoidable by watering the beds
frequently during the period of germination. When the root-
tips have penetrated to a depth of half an inch this is no longer
necessary.
There are differences in the amount of acid required for
successful results in different soils. In sandy soil which was
probably somewhat alkaline, a heavier application, one-fourth
to three-eighths of an ounce, was indicated. In a fine sandy soil
which was probably already acid, chemical injury to seedlings was
more difficult to avoid and reduction of the acid to one-eighth of
an ounce was advisable. On heavier soils the use of five-eighths
THE CONTROL OF DAMPING-OFF 109
of an ounce produced no injury and reduced losses by damping-
off to less than i per cent.
On a soil with a high carbonate content, evidenced by a
vigorous effervescence when the acid was applied, the method
was found to be ineffective. On this soil the use of copper
sulphate, one-fourth of an ounce per square foot, gave good re-
sults. This substance was applied in the same way as the acid,
and the same precaution to avoid chemical injury was found
necessarj'.
An interesting indication was given by the experiments of
the effectiveness of cane sugar, 2| oz. to the square foot, in the
control of damping-off. The authors point out that, if some
unrefined sugar-bearing substance were available, it is possible
that for certain soils the application of sugar would become an
economically satisfactory treatment. Experiments on this
subject might well be carried out in the West Indies.
There are secondary advantages to be expected from sul-
phuric acid disinfection which may be of considerable impor-
tance in some cases. Under appropriate circumstances a larger
germination percentage is secured, the number of parasites in the
soil is reduced and the well-known effect of disinfection on fer-
tility results in increased growth. Another valuable effect has
proved to be the reduction of weeds owing to the greater sus-
ceptibility of their seeds. Similar advantages are secured from
steam treatment.
Some hints are given as to the method of handling the acid.
It should always be dissolved by pouring it into the water,
reversing the process may cause a serious accident. The solu-
tion should be made up in wooden or earthen containers and
applied with watering cans which have been coated inside with
paraffin wax. Boots may be protected by being heavily greased.
Wooden containers should be washed out, immediately after
use, with water containing washing soda.
In respect of sterilisation methods generally note should be
taken of the evidence which has recently come in from several
directions that the condition of sterilised soils favours their more
rapid and complete infestation with the parasites concerned when
reinfection takes place.
With herbaceous seedlings it is usually quite vain to apply
fungicides to the affected plants, but in the case of lime seedlings,
which are very subject, among other seedling troubles, to damp-
ing-off in the seed drills during wet and cloudy weather, it has
commonly been found practicable to save a large proportion of
the plants by a dusting process. A mixture of equal parts of
finely powdered sulphur and lime is used for this purpose, and
success depends on the promptness of its application when the
first signs of trouble appear.
CHAPTER XIII
PREVENTION OF FRUIT ROTS
The rots affecting fruit stored or in transit are mostly caused
by saprophytic or weakly parasitic fungi. As fruit approaches
ripeness the natural resistance associated with tissue in a state
of activity is reduced, and the mechanical protection of rind
or cuticle has to be mainly depended upon to prevent the invasion
of the tissues.
The fungi most concerned in decay of fruits ordinarily shipped
in the West Indies are the Penicilliums (blue moulds) Diplodias,
and, on pineapples, Thielaviopsis. It has been repeatedly
demonstrated that the occurrence of blue moulds on citrus
fruits — the best known type of fruit rot — depends entirely on
the presence of injuries, in which either the surface of the rind
is broken by scratches, cracks or punctures, or the cells of the
rind are burst by injuries of the nature of bruises, which may
not be visible even on careful inspection. If neither form of
injury is present the fungus is unable to make an entry. Certain
types of injury are not preventable, insect punctures and thorn
pricks for example, but experience in countries where this matter
has been closely followed up in the interests of important indus-
tries has shown that a very large part of the damage occurs in
the handling, and hence elaborate care is taken at every step of
the process which transfers the fruit from the tree to the distant
consumer. Such precautions as the wearing of gloves by pickers,
the use of padded collecting boxes, and the avoidance of falls
of even a few inches are at the opposite pole from local methods,
but they are adopted for strictly business reasons, and if ever a
successful fruit industry is to be established in these islands it
must first be realized that greater care is needed in the handling
of fruit for shipment than is necessary for handling eggs.
In many fruits weak spots of another type are provided by
the cut end of the stalk, as in pineapples, or the socket from which
the stalk is pulled, while, if the end of the stalk is expanded by a
persistent calyx, to leave it attached provides cover under
which moisture collects and spores may germinate. In the first
type there may be advantage in leaving the stalk long or sealing
the freshly cut end, in the second dryness is the main protection,
and in some cases a touch of shellac varnish has been advocated.
The best general protection is dryness, begun by packing
surface-dry fruit and continued by thorough ventilation. Even
injured fruit may carry if it is kept free from humidity. Packages
IIO
PREVENTION OF FRUIT ROTS iii
and storage should be such as to allow the moisture given off to
get away quickly, and in this respect the warehouse should be as
carefully considered as the ship. Conditions on the latter have
often been blamed for mischief which has got well started ashore.
The practice of curing or quailing, where comparative tests
have been made, has been usually found to be the reverse of
beneficial. Washing citrus fruit has been shown greatly to
increase the percentage of loss. With some types of infection, the
exclusion by inspection of fruit showing injury or incipient decay
is of very great importance. Delicate fruit should not be packed
more than one layer deep. All packing should be tight, but
squeezing must be avoided.
In particular cases fruit may be fumigated with formaldehyde
(see page loo) or disinfected with copper sulphate (one in 5,000 to
one in 50,000), formalin (one in 10,000), or potassium perman-
ganate (one in 10,000).
Packing sheds should be light and well ventilated. If they
can be kept clean it does not appear that periodic disinfection or
fumigation is necessary, but rejected fruit should never be
allowed to remain long in the building or its vicinity.
CHAPTER XIV
PLANT DISEASE LEGISLATION
The subject of plant disease legislation, in the present connection,
has reference to the enaction and administration of laws for the
protection of plant industries against diseases and pests. It
divides naturally into consideration of protection against external
sources of infection and of the limitation of affections already
more or less established.
Plant Importation.
In island communities such as these of the West Indies, the
question of protection against outside infections resolves itseli
into that of the admission and inspection of plant material at the
one or more ports of entry. Such material may be seeds or
cuttings for planting, rooted plants with or without soil, fruit,
forage, grain or seed for feeding or manufacturing purposes,
cordwood, litter accompanying animals, and packing or wrapping
of various kinds. When these things, in their nature and origin,
constitute a possible source of infection, it may with confidence
be stated that the only method approaching safety is that of
total exclusion, and even this is qualified by the impossibility
in practice of strictly enforcing it. Any system of inspection,
however rigorous, is very fallible. Many examples might be
quoted in illustration of this statement. The mosaic disease of
sugar-cane is highly infectious and is carried by cuttings, yet
after careful and long-continued search no visible organism has
been found to be associated with it. It can be introduced in
material which might be examined with the utmost care and
passed as healthy. Citrus canker so closely resembles the com-
paratively inoffensive and widespread scab disease that it was
passed over as such until its characters became known as a result
of a disastrous outbreak following its introduction into Florida.
The dangers of admitting unknown and therefore unrecognised
diseases, of the introduction of infection on resistant plants
which do not show the presence of disease, and the impossibility
of determining beforehand what effects a disease or pest may
have under new conditions, should by this time, in view of the
many instances which have become notorious, be well enougii
understood. The local besetting sin in this matter is the tendency
to regard the making of an ordinance as the end rather than the
beginning of action.
112
PLANT DISEASE LEGISLATION 113
There are few communities which would consent, or which
could afford to consent, to exclusion in any wide sense, but, as
regards any plant industry of considerable importance, the only
safe way is to reduce importations which have any visible con-
nection with the diseases of the plant concerned to the narrowest
possible limits. As regards the exclusion of known diseases, and
for general purposes, the effectiveness of regulation depends
on (i) the thoroughness with which the pests and diseases of the
country of origin are kept surveyed, (2) the promptness with
which information is communicated to those responsible, and
(3) the efficiency of the inspection cariied out. These are matters
which in turn depend on the organisation of agricultural depart-
ments and of co-operation between them, and the extent to which
they receive attention should govern the freedom of exchange
permitted.
The regulations relating to the exclusion of plant diseases in
force at the present time in the islands of the Windward and
Leeward Groups comprise : —
(i) Submission of all material imported for planting to
inspection by the local agricultural department, with provision
for (a) its disinfection, if thought necessary ; (6) its growth " in
quarantine," i.e., in a specified place subject to inspection ; (c)
its rejection.
(2) Exclusion of material related to the existence of specified
diseases in certain countries. At the present time these include,
in the islands where the industries affected exist, coffee rust,
citrus canker, lime anthracnose, cacao witch-broom, Panama
disease of bananas, bud-rot and red ring disease of coconuts, and
mosaic disease of sugar-cane.
Internal Regulation.
The ordinances concerning the control of plant diseases
already present apply mainly in the direction of compelling noti-
fication and the adoption of measures for prevention or treatment.
Their object is the protection of growers in general against injury
which may be done to their crops through the existence of
infected plants upon which a parasitic disease is uncontrolled.
Obviously compulsion can only be justified if the danger is
substantial and if some reasonably practicable means of control
can be prescribed.
It may be taken as a general principle that such orders
can only be successfully enforced so far as they have the support
of public opinion. Where general apprehension exists as to the
effects of some particular disease the difficulty is comparatively
small, the bud-rot disease of coconuts being a case in point,
but where this kind of action is taken without the education
of the planter as to its necessity it is certain to be looked upon
by many as vexatious interference, and to cause an undesirable
reaction against the body responsible for its application. For
I
114 DISEASES OF CROP-PLANTS
this reason the power of compulsion should be exercised with
great restraint, should be based on thorough investigation of
the subject, and be accompanied by a campaign of education.
On the other hand there are certain spectacular troubles, such as
black blight and love-vine, with regard to which the demand for
governmental action may be stronger than the evidence of damage
necessary for its justilication.
If the more menacing diseases are made notifiable and classed
as such, there remain the non-notifiable infectious diseases,
which, if they exist on one man's property unchecked, may cause
injury to his neighbour. Probably the best suggestion that has
been made for dealing with cases of this kind is that power
shoutd be given, when a direct request has proved of no avail,
to apply to a magistrate fc-r an order directing the person re-
sponsible for the property on which the disease exists to take
steps to remedy the matter. Before granting the order the
magistrate would have the opportunity of learning from evidence
given before him the nature of the grievance and the measures
reasonably applicable for its removal.
The legislation of the kind here discussed now existing in
the Lesser Antilles comprises (i) provision in the cotton-growing
islands for a close season for that crop, and (except in Barbados)
for the eradication of trees which support cotton stainers, the
carriers of internal boll disease ; (2) provision in St. Lucia,
Grenada, and Trinidad for the proclamation of specified diseases,
with powers of inspection, for the declaration of quarantine, and
for compulsory treatment or eradication. In Trinidad the
proclaimed diseases are bud-rot, little-leaf, and red ring disease
of coconuts, mosaic disease of sugar-cane, anthracnose of limes,
love-vine and bird vine. In St. Lucia they comprise bud-rot,
love-vine, and bird vine.
PART II
INTRODUCTION
AGRICULTURAL CONDITIONS IN THE LESSER
ANTILLES
The chain of islands which with Httle irregularity of distribution
stretches from the Virgin Islands to Trinidad (Lat. i8° to io° N.)
affords much greater variation in the conditions governing
agriculture than is generally realized. It is true that the
difference in temperature, which is perhaps the first consideration
to occur to the mind, is not sufficient to have an appreciable
influence on the choice of crops, but temperature, in its higher
ranges, is among the least effective of environmental factors.
First in this respect comes the amount and seasonal distribution
of the rainfall, and closely related to this in its influence on
crops is the physical nature — lightness and permeability, or
heaviness and retentiveness — of the soil. In both these factors
there is a very wide range from island to island, and in regard
to the rainfall the difference from district to district even in the
same island may be extreme, and commonly is distinctly marked
in its influence — the degrees of contrast depending on the height
and distribution of the hills, and on situation in relation to
the north-east trade wind.
The diversity of the conditions produced by the various
combinations of soil and climate in the West Indies has come
much more into evidence since economic changes rendered it
widely necessary to substitute other cultivations for that of the
once universal sugar-cane. That plant, with its produce de-
pendent only on vegetative growth, is adaptable to a wide range
of conditions which its successors, cacao, limes and cotton, with
their crops dependent on flowers and fruit, are quite unable to
face.
The more intimate that one's knowledge of local conditions
becomes, the more one is impressed with the extent to which an
agricultural " natural selection," often at cross purposes with
the planter and his advisers, has determined and is proceeding
to determine what crops shall be grown in each restricted locality.
The principal part of the art of agriculture consists in artificially
modifying the environment so as to induce plants to grow in
situations and to an extent foreign to them in a state of nature,
115
ii6 DISEASES OF CROP-PLANTS
but the limits within which this may be done and especially those
within which it may be done profitably are, in respect of many
tropical crops, rather strictly drawn. This is the more pro-
nounced because neither management nor labour are as yet
capable of going beyond somewhat crude agricultural practices.
The relevance of these reflections to the consideration of certain
types of plant diseases will appear in the following pages.
To make clearer to outside readers the conditions and crops
of the various islands the following summary is given. The
islands are arranged in geographical order from north to south.
St. Kitts-Nevis.
The cultivated land in St. Kitts covers a broad belt of usually
gentle slopes stretching from the sides of the central range
(highest elevation 3,700 feet) to the coast, and is mostly occupied
by estates of the old sugar-plantation size. Sugar-cane is
universally grown, with cotton as a catch crop. The steeper
slopes for some distance above this belt are occupied in places
by gardens in which the usual West Indian provisions — sweet
potatoes, yams, bananas, corn, cassava, groundnuts, etc. — are
raised. For the greater part of the necessary supplies of this
nature St. Kitts depends on the adjacent island of Nevis. The
upper slopes of the mountains are covered with rain-forest or
tree-fern.
The soil is light and deep, and particularly easy to work.
The average rainfall on the cultivated areas is some 40-50 inches,
according to position.
The only serious affection of sugar-cane is root disease, which
occasionally causes local reductions of crop. The light soil is
quickly affected by drought, and lends itself to depletion of
fertility when not systematically manured. The prevalence
of either condition allows the development of root disease.
The same quality of the soil particularly favours cotton-
growing by greatly reducing liability to shedding, and the
average yield is high. The plants grow to a large size and the
consequent humidity in wet years induces some losses from boll
diseases.
The cultivated land in Nevis is arranged around its central
mountain (3,500 feet) in much the same way as that of St. Kitts
around its central range. Many of the old sugar estates have
been divided into peasant holdings. Provisions are raised in
quantity for local consumption and export to St. Kitts. The
staple crops are sugar-cane and cotton. There is a rather
large coconut estate near Charlestown, and vanilla is grown
in some quantity near the edge of the upland forest at Maddens.
The average rainfall at Charlestown is about 50 inches ; its
seasonal distribution is erratic.
The soil is rather heavy, and cotton is consequently more
subject to losses from shedding than in St. Kitts. Sugar-cane
INTRODUCTION 117
does well, but in the smaller holdings receives, as is usual in
peasant agriculture, irregular attention and manuring. Root
disease is in consequence rather prevalent,
Antigua
Antigua is an island of the open type, lacking forest, with a
low or moderate rainfall averaging 45 inches on returns from the
whole island, and subject to occasional severe droughts. Agri-
culturally the island divides into three districts, the north-
eastern limestone area, with a rather light and stony soil and the
lowest rainfall, the southern section of low volcanic hills, with
more rain, and the central plain, the soil of which is typically a
heavy clay, containing salt in places, generally low-lying, from its
nature and situation difficult to drain, and having a tendency to
lose tilth quickly in ratooned fields. The lowness of the rainfall
is partly offset by the retentiveness of the soil, and there are many
hollows in which water lodges for the greater part of the year.
The principal crop is sugar-cane, the cultivation of which was
enabled to persist through the years of depression by the early
adoption of the central factory system. Scarcely any rotation is
practised, the usual course being to grow plant canes and one,
two, and in some cases three crops of ratoons, followed by a
catch crop of potatoes or corn and by replanting the same year.
Crops of corn or of onions are grown with the young canes.
The yield of an average estate is about 20 tons of plant cane to
the acre and 9 or 10 of ratoons, but local yields up to 40 tons of
plant cane can occur under very favourable circumstances.
These results are only obtainable by the selection of the
better parts of the land, and it is characteristic of Antigua that
large stretches of scrub and pasture exist which have been more
or less permanently thrown out of cultivation. Could the more
intensive methods characteristic of Barbados agriculture be
applied, it is probable that a much closer approach could be made
to the universal state of cultivation of that island.
Most estates are short of manure, so that not all plant canes
receive a share. Chemical manures are used on the ratoon
crops. It is the practice to range the trash on alternate banks
and cultivate those which intervene. The trash from fields due
for replanting is available for the pens. Labour is fairly plentiful,
but its efficiency is low owing to prevalent ill-health. Planting
is frequently badly delayed by the inability of the factories to
deal with the crop in time for the most favourable season to be
utilized.
Under the general circumstances described there is no reason
for surprise at the tendency for development of root disease, the
effects of which are more pronounced in Antigua than in any
other island, though the cause goes to some extent disguised as
soil grub infestation.
Cotton is usually a separate cultivation, located in the more
ii8 DISEASES OF CROP-PLANTS
suitable districts, and partly in the hands of peasants. The
production has never reached large proportions. There is one
working lime estate, situated on the eastern slopes near English
Harbour. Coconut palms occur in small patches round the
coast. Pineapples are produced with some success on the slopes
of ridges in the volcanic area, and where there is good natural
shelter the humidity is sufficient for orange trees to exist and for
scale insects to be fairly well controlled by the common species
of entomogenous fungi.
MONTSERRAT
Montserrat is one of the smaller islands, about ranking with
Nevis. It has a central district of rather bold mountains (highest
elevation 3,000 feet), steep and rocky near their summits, falling
off in cultivable and often gentle slopes to coastal flats or to the
sea. Its streams are few and inconsiderable in size but have
cut considerable ravines (guts) in places.
The higher slopes are in forest, secondary bush, or vegetable
gardens where these are feasible. Three small cane estates
survive, making muscovado sugar, of which a small quantity is
exported. Montserrat is the island that has benefitted most of
all from the introduction of Sea Island cotton, the growing of
which is now the staple industry, engaging both the estates and
numerous small holders. Near Plymouth there is about 1,000
acres of land in scattered lime cultivation, under climatic con-
ditions marginal for this crop, which shorten the duration of
bearing fields and make their re-establishment difficult. There
is another 50 acres of limes at Roaches, in a gut from the sea to
the hills. About 50-60 acres of hill land is planted with bay
trees, and the area is extending. A peculiarity of Montserrat
is its peasant industry in the gathering of papain. There is no
papaw cultivation, but in certain districts the plants spring up
abundantly when the bush is cut. The milk is collected daily
from fruiting trees by women and children, and the coagulum
dried in ovens. The annual value of this material is about
£2,000, but the industry has been receiving less attention with
cotton at its recent high prices. With this stimulus the latter crop
is planted even on rocky slopes where little cultivation beyond
the cutting of the bush is possible, and does very well under these
conditions.
Not much of the cotton land is as yet manured, even on
estates. Occasionally a field is put in a green manure crop,
grazed, or given pen manure. The seed is exported to Barbados,
and excepting the produce of one group of estates only a small
proportion of the meal, for use as stock food, is returned to the
island.
Windbreaks, temporary or permanent, are desirable for many
cotton fields, but as a rule their provision is neglected.
The provision crop consists mainly of sweet potatoes. Yams,
INTRODUCTION 119
dasheens, tannias and cassava are but little grown. There are
plantains in the hills, reported to be diminishing ; probably, as
in Dominica, owing to weevil infestation. Bananas are
fairly plentiful in the hill districts, and a coarse species u?ed
green is a common article of food. Mangoes, avocados and bread-
fruit, in their seasons, are staple foods ot the peasantry. The
principal leguminous food-crop is the pigeon pea, and the pro-
duction of black-eye peas is increasing.
The average rainfall is mostly from 40 to 60 inches, according
to locality. One estate gets up to 90 inches. In or near the hills
the conditions for vegetative growth are very distinctly better,
apart from differences of rainfall, owing to increased humidity.
This effect is marked enough to exclude cotton from certain
districts.
The soils of Montserrat are exceedingly various, ranging from
tough hard clays, almost unworkable, through numerous varieties
of heavy and light loams to light fine volcanic soils resembling
those of St. Kitts and St. Vincent. Trass soils also occur.
There is, however, a predominance on the wider slopes and levels
of soils hght enough to be well suited to cotton in a season of
normal rainfall.
Dominica
Dominica is an exceedingly mountainous island (highest
elevation 4,750 feet), much broken by ravines and steep-sided
valleys. The older cultivated lands are close to the coast or m
the uiouths of the valleys. The rest of the country is in forest
save for an area of scrub known as the Grand Savannah, and a
series of clearings situated along the Imperial Road which
represent an attempt made some 20 years ago to open up the
interior for cultivation. The rainfall ranges from an annual
fall of 200-300 inches in the wetter parts ot the interior to 80
and 120 inches on the Leeward and Windward coasts respec-
tively. The son is irregular, rich m places but often distinctly
poor over considerable areas. Most of the estates are com-
plete.'y occupied in lime cultivation. There are fairly extensive
areas of cacao in some districts, from which the yield is rather
small. An attempt is bemg made to increase the production
of coconuts.
The established lime cultivations do well when cared for and
have been remarkably free from any active disease or disability.
Semi-parasitic fungi hasten the dying-back of trees failing through
age and disrepair or from poor nutrition. Mistletoe becomes
troublesome when neglected. The newer forest clearings are
subiect, as is usual in such conditions everywhere in the tropics,
to root diseases derived from decaying stumps and logs. Similar
trouble exists in the cacao cultivations in connection with damaged
or failing shade trees.
During 1922 it was foimd that the anthracnose of limes had
120 DISEASES OF CROP-PLANTS
become established near the Windward coast, since which
discovery the disease has already extended widely and constitutes
a grave danger to the future of the principal industry of the
island.
Many districts are too wet for the successful growing of
cacao; the crop proved a complete failure in the interior with
this apparently as the underlying reason. The principal pro-
vision crops are dasheens, yams, plantains and bananas, bread-
fruit and various species of peas and beans. They are produced
almost entirely by small holders. Temperate vegetables do
well at elevations of 1,000-2,000 feet. Coffee was largely
grown up to about 1850. The reasons for the failure of the
industry are discussed in the introduction to the chapter on
the diseases of that plant.
St. Lucia.
St. Lucia is a decidedly mountainous island, approaching and
in some districts equalling Dominica in this respect, but with its
contours more systematically arranged. There is a central
range some 1,500 feet in height, with peaks near its southern end
rising above 3,000 feet. From this range ridges run out to the
coast in all directions, with numerous narrow, well-sheltered
valleys between. Practically the whole island is of igneous
material. The average rainfall at Castries is 90 inches, with a
range of about 60 to 120 in other districts where records are taken.
The area of land suitable for arable tillage is for the most
part confined to the lower slopes near the coast and to the lower
reaches of the valleys. There are several considerable discon-
tinuous areas of this kind under sugar-cane, covering about
3,000 acres, with four factories of the modern type in operation,
and sugar is the highest of the colony's exports in value. The
diseases of sugar-cane have had little attention.
Some 6,000 acres, well scattered through the island, are
under cacao, including numerous estates and a large number
of small holdings. Pod-rot and canker, due to Phytophthora,
are fairly common, but reach to serious proportions only under
conditions of exceptional humidity. Dieback is verj^ prevalent
among the more neglected of the peasants' plots. Rosellinia
root disease is continually troublesome in some situations,
and unless control measures are carefully applied slowly bu
surely kills out a group of trees around each centre of infection.
During the last decade the lime industry has attracted
considerable attention. An area of some 3,000 acres is planted
in the crop, and is still expanding under conchtions which appear
to be very suitable. So far the trees have suffered little from
specific disease. Some foot-rot has been reported on ill-drained
land. Scale insects are well controlled on established trees by
entomogenous fungi.
In the dryer south-western district some trials of cotton
INTRODUCTION 121
have been made, but shortage of labour and the incidence of
disease discouraged further development.
St. Vincent.
St. Vincent, like Dominica, is mountainous and broken, but
its physical features are on a somewhat smaller scale (highest
elevation 4,000 feet) and communication is easier near the coast.
A great part of the interior is occupied by forest. The rainfall
is heavy, averaging about 100 inches at Kingstown, and more in
some districts, but this is largely offset by the lightness of the
soil, a combination which produces conditions of a rather hybrid
character, so that the island cannot be classed with those in
which the conditions are specifically favourable to limes and cacao,
or placed without reserve in those which are distinctly confined
to an open arable type of agriculture. Such conditions are well
suited to crops of a vegetative nature and to short-term flowering
crops which can endure or evade damage from rain. In the
former class arrowroot yields the principal product of the island,
with cassava as another starch plant supplementing it to some
extent. The cultivation of sugar-cane is local in character,
and difficulties of transport seem to preclude the centralisation
necessary for the appUcation of modern methods of manufacture.
Of the crops dependent on flowers cotton is grown to a very
large extent. The yields have been usually small, partly as a
result of disease induced by the heavy rainfall and partly as a
result of insect-borne diseases, but the high value of the product
and the returns of occasional fortunate years have enabled
the cultivation to continue, and in recent years the control of the
cotton stainer has considerably lengthened the crop period,
and increased the yield. Black-eye peas and corn as catch
crops, and pigeon peas and groundnuts as rotation crops are of
considerable importance. There are a few small areas of cacao,
but no citrus cultivations. There are small but well-established
coconut groves on alluvial lands, and some new plantations
are being tried elsewhere ; in one case on some 1,000 acres of the
old sugar land of the Carib country, devastated in 1902 by the
volcanic eruption.
Baebados.
Barbados lies outside the chain of islands of volcanic origin ;
its surface rock is coral limestone except in the north-east corner,
where the underlying estuarine and oceanic beds of sandstone,
clay, and chalk are exposed. Practically the whole island is
cultivated, and the fields are mostly level or on gentle slopes,
the land rising by abrupt steps in successive long and rather
wide terraces up to the central maximum of 1,100 feet. There are
no streams on the limestone ; the fields are drained centripetally
into sump wells situated in the hollows. The soil is heavy and
retentive, and requires frequent thorough cultivation to maintain
a good tnth. The average annual rainfall is about 60 inches.
122 DISEASES OF CROP-PLANTS
Considerable fluctuations occur from year to year, and a distinct
difference is maintained between the higher and lower levels.
Sugar-cane has remained the universal crop in default of any
other, and the industry was able to survive the period of depres-
sion largely by virtue of the North American market for syrup.
Ratooning for one or two years is the rule in the uplands ;
on the drier coastal districts only plant canes are grown. Heavy
manuring is everywhere practised, including the use of large
amounts of imported sheep-manure and chemical fertilisers.
The common rotation crop is sweet potatoes. In the drier districts
cotton had for a few years and may have again an important
place in this connection. Yams, tannias and eddoes, and cassava
are also grown. Corn is grown as a catch crop ; sorghums are
raised for fodder and to some extent for grain. Bananas, peas
and beans, breadfruit and other local foodstuffs are produced
in a scattered way.
As elsewhere in the islands the only serious affection of
sugar-cane is root disease, induced in this case by low rainfall
and extreme exposure.* The system of agriculture made necessary
by these conditions, as also the epidemic of disease which in the
'nineties drove the Bourbon cane out of cultivation, are discussed
in the chapter on sugar-cane diseases.
Cotton is even more than usual an uncertain crop in Bar-
bados, owing primarily to the heaviness of the soil. During
the flowering and boiling periods a short spell of wet weather
is sufficient to cause heavy shedding, and it is common to see
fields in which good bearing is confined to borders or patches
which happen to be better drained or lighter than the rest.
Under these conditions the land needs much more careful pre-
paration for this crop than it usually gets or than is necessary
in lighter soil. Success over a term of 5'ears has only been
obtained in the driest districts and this is qualified by short
crops both in wet years and in periods of continued drought.
It has been the custom to depend for an important part of the
yield on the secondary growth brought out by showers between
December and July. The bolls so produced are usually small,
but there is much less liability to shedding. The advent of
leaf-blister mite has to a considerable extent qualified the success
of this system, though the recent adoption of a close season
may go some way towards restoring it.
The usual parasitic diseases of cotton occur, but are not com-
monly the source of any very considerable losses. The absence
of the cotton stainer causes the internal boll disease to be rare.
Owing to the dry climate and the open wind-swept nature
of the country fungus and bacterial diseases of plants in general
are not numerous or usually important. The exceptions are
mainly affections of the type of root disease of sugar-cane which
depend on the weakening of resistance in their host rather than
on humidity.
* Mosaic disease has now become generally distributed.
INTRODUCTION 123
Grenada.
Among the hilly, typically wooded islands Grenada represents
a decline in ruggedness of contour as compared with St. Vincent,
as the latter does in comparison with Dominica.
Grenada has a central area of forest-clad hills, of which the
highest is 2,750 feet, but a great part of the island combines
easy slopes for cultivation with good natural shelter from low
ridges and has good or fair means of communication. Much
of the soil is very suitable to orchard crops, being of good depth
and heavy enough to be retentive. The average rainfall at
St. George's is about 75 inches, but totals of 100-150 inches are
not uncommon in some situations.
Grenada M^as fortunate in being early to find in cacao a satis-
factory substitute for sugar-cane, and has since remained almost
completely a country of that one product. To the extent of the
limited demand, the growing of nutmegs provides an allied
industry, and in recent years the planting of coconuts on open
lands has been considerably stimulated.
One or two sugar estates remain, and fair quantities of com
and the usual food-crops are raised by small holders.
As a general rule the conditions are very suitable for the
growing of cacao without much loss from pod-rots or root
diseases induced by excessive humidity, or, on the other hand,
from dieback induced by poor conditions or drought.
This balance of conditions is of course liable to be disturbed
by periods of more than usual wetness or dryness which lead
to the temporary increase of affections of the one type or the
other.
Estates situated on the wetter hill slopes of the interior are
liable to have trouble, which in some cases is serious, with root
disease caused by Rosellinia Pepo. This species is rare near
the coast, but another member of the genus occurs which while
slower and less definite in its action appears to be capable of
causing root disease under drier conditions.
The cacao estates in Grenada are relatively small and a more
intensive cultivation is practised than is the rule elsewhere.
Considerable use is made of pen manure, and it is the custom to
bury dead leaves, weeds, etc., about the roots of the trees. The
yield per acre is distinctly greater than in Trinidad. Very many
fields are grown without shade.
Trinidad and Tobago.
The island of Trinidad, which has an area of 1,862 square
miles, lies about 16 miles to the eastward of Venezuela, between
10° 3' and 10° 50' N. latitude. The main body of the island has
a form approaching to a rectangle about 55 miles long and 35
broad, with large projections from the N.W. and S.W. angles
towards the South American coast. Enclosed by these is the
shitUow Gulf of Paria, the entrance to which from the south has
124 DISEASES OF CROP-PLANTS
a width of about ii miles and the largest channel from the north
a width of only 8 miles.
The island is traversed by three ranges of hills running
approximately east and west. Only the elevations of the
Northern Range, which lies immediately behind the north coast,
have the character of mountains and rise to a maximum height
of 3,000 feet. This range is an obvious continuation of a similar
range in Venezuela, and is connected with it by the promontories
and rocky islands which form the northern boundary of the Gulf
of Paria. The Southern Range of low and broadly scattered
hills occupies a somewhat similar position relative to the south
coast, while the Central Range, made up of hills mostly well below
1,000 feet, runs about midway across the island. Between the
ranges lie the Northern Plain, an unbroken alluvial flat drained
by the River Caroni, and the Southern Plain, which is dissected
into shallow troughs and ridges.
From the physical and biological points of view Trinidad
must be regarded rather as a detached part of South America
than as a member of the Antillean Chain, and its structure clearly
indicates a history in which it has formed part of the continent
and has been traversed by an outlet of what is now the Orinoco
River.
The average annual rainfall is about 64 inches. The dry
season extends from January to the middle of May with an aver-
age precipitation of about 3 inches per month. From June to
November there is an average fall of about 8 inches per month.
As regards distribution within the island there is a distinct
reduction from east to west ; the line separating the area with
50-75 inches from that with 75-100 inches almost bisects the
main body of the island from north to south.
There are a few small areas in the north-east with recorded
averages over 100 inches, while the north-western peninsula and
its islands fall below 50 inches.
A great part of the island being covered with forest or with
almost equivalent cacao cultivations, the climate is distinctly
humid and very favourable to the development of a rich vegeta-
tion, much more so than the recorded rainfall would suggest.
The most important agricultural industry of the island is the
growing of cacao, of which the production amounts at present
in an average year to about 27,000 tons. The crop next in im-
portance is sugar-cane, from which an annual output of roughly
50,000 tons of sugar is obtained. The area under coconuts has
been rapidly extended in recent years, and the present production
is in the neighbourhood of 25 million nuts per annum. In the
minor industries rice is grown for local consumption to the extent
of about 2,500 tons and there is a considerable production for
local use of Indian corn, peas and beans, cassava, yams, sweet
potatoes, dasheens, tannias, eddoes, bananas, mangoes and
oranges. The supply of these foods is more than adequate in
INTRODUCTION 125
their seasons, but is rendered irregular by the difficulties of
storage. Efforts to establish the rubber industry in the island
were prejudiced at the critical period by the selection of Castilloa
as the tree to be grown, and there is only one considerable plan-
tation of Hevea. Several plantations of limes were made a
few years ago, but although the trees grow well the production of
fruit is seriousl}^ reduced by the prevalence of anthracnose.
The most favourable cacao areas are among the hills of the
Central and Southern ranges and the valleys and lower slopes of
the Northern Range. Immortelle shade is every\\'here used, the
species known as " anauco " mainly in the hills and the " bocare "
on the plains. There is very little approach to intensive culti-
vation and the production per acre is consequently low. The
reasons assigned for this condition are the inadequacy of the
labour supply and lack of capital. The size of holdings ranges
all the way from two or three acres to several hundred. Most
estates have been built up by the combination of small holdings
or by extension on the " contract " system, in which the con-
tractor takes over a new area of small extent, clears and plants
it with cacao and temporary crops and hands it over as the
trees come into bearing.
The areas occupied by sugar-cane lie immediately behind the
western coast : the northern district situated in the basin of the
Caroni River, the central about Couva, and the southern in the
undulating country south of San Fernando. About half the cane
is grown on estates grouped about local factories, of which there
are about half a dozen of notable size, and the other half by a
class of small cultivators known as cane tarmers who own or
rent land and sell their produce by weight to the factories.
The cane is mainly planted in the second half of the wet
season (August to November) and the reaping season is from
early February to June. Ratooning is a general practice, but, as
elsewhere, the number of ratoon crops is undergoing continuous
reduction. It rarely now goes beyond three and is in many
cases reduced to two and sometimes one.
Coconuts have long been grown on the lands bordering the
sea wherever the physical conditions permitted, but considerable
areas have recently been established inland, often on old cane
lands and to some extent on the site of forest. On the depleted
soils of the former type growth is somewhat slow, but fertility
may be expected to increase as shade and cultivation take effect.
The island of Tobago, which has an area of 114 square miles,
lies 26 miles N.E. of Trinidad. It has a dorsal ridge 18 miles in
length and rising to 1,800 feet, and, except for a district in the
south-east, is much broken by hills and ridges. The agriculture of
the island closely resembles that of Trinidad, except that the
sugar industr}^ on which it at one time depended, has practically
died out. The estates are occupied by cacao and coconuts,
and there is a large body of peasant proprietors who grow food
and raise stock for their own support and for export to Trinidad.
Section I -General Diseases
CHAPTER XV
ROOT DISEASES
RosELLiNiA Diseases
In the West Indies as in other parts of the tropics where forest
land has been opened up to agriculture, destructive diseases
make their appearance as the result of infections arising from
native fungi which occur on buried roots, on logs, and on other
debris remaining from the forest. A number of different fungi are
responsible in various parts of the world for diseases arising in
this way but the characters of the affections produced are essen-
tially the same.
In the West Indies the fungi concerned are mainly species of
Rosellinia. Other forms occur but are comparatively rare. In
cultivations of an open nature where sun and wind have access to
the soil, and especially in those of arable crops, there is a rapid
dissipation of the decaying material necessary for the develop-
ment of the fungus and the diseases as a rule soon disappear.
They can persist, however, in cultivations such as that of cacao
in which the conditions, especially where shade trees are abundant,
approach to those of the forest in respect of shade and humidity.
They also occur in windbreaks and hedges of certain susceptible
trees and shrubs in wet or sheltered districts. The causative
fungi infect shaded soil rich in decaying vegetable matter and
spread slowly through it, destroying every plant with which
they come in contact.
The existence of such diseases is traditional in the West
Indies, and those under discussion have in all probability been
met with from the time of the first settlement. A description of
disease of this type occurring on coffee in the French Antilles was
published in Paris as early as 1842. The first account seen
having reference to the British West Indies is that by C. A.
Barber, in 1893, of root disease of cacao in Dominica, which
contains a list of twelve hosts observed. The disease has been
from time to time described by agricultural officers since the
formation of the Imperial Department of Agriculture. F. W.
South (1909-13) was the first to recognise the genus of the causa-
tive fungi. Five species have been met with by the present
writer, of which three have been found associated with root
126
RosELLiNiA Pepo :
Fig. 32
CoNiDiAL Fructifications
X 4^
^^
-l^§
^^^^
^
^^^
^^^^^^
^fesc*"^i
^B^^i^l^MB
^^^^^^si
-■^'^iSV"
^Wiii
^«#i^'* £"
w^ ^^^m
^^»e
^llw^\ . ^-
RosELLixiA Pepo
33
Peritheci
ROOT DISEASES 127
diseases. They will probably prove to be general in their occur-
rence in the islands with forest areas.
A. Rosellinia Pepo. Pat.
This species was originally described by Patouillard in 1908
from material on the bark of Hymena^a Courbaril collected by
Duss in Guadeloupe. In the British Antilles the perithecia have
been collected in Dominica, St. Lucia, Grenada, and Trinidad.
From the published accounts of root diseases in Jamaica, Porto
Rico, and Martinique, it seems probable that the species also
occurs in those islands.
Its most general importance is due to its attacks on cacao,
but it is capable ol producing destructive effects on any of the
ordinary crop plants, herbaceous or woody, which are planted on
land recently cleared from forest. Under such circumstances,
limes in Dominica have suffered severely from this species as well
as from R. bunodes. Where cacao trees have been killed by the
fungus, and other plants have been put in for temporary shade,
dasheen, banana, pigeon pea, horse bean (Canavalia) and cassava
have all been seen attacked. Sugar-cane seems to possess con-
siderable resistance, and coconut seems almost or entirely immune.
The conidial fructifications (the Graphium stage) are developed
in great abundance on all kinds of dead vegetable material lying
about on the infested spot, and occur from an early stage on the
exposed roots and the base of the stem.
They appear some time before the development of perithecia
and are borne on the black surface mycelium which develops
wherever the fungus reaches the open under damp conditions.
Each has the form of a black bristle-like stalk 2 to 3 mm. long,
built up of perpendicular hyphae which branch out freely at the
top into a tuft, which is white or whitish to the naked eye from
the conidia which cover it. The conidia are borne laterally
towards the terminations of these branches ; the cells which bear
them have a somewhat zig zag or corkscrew appearance from their
tendency to bend away from the point of attachment of a coni-
dium. The conidia are rounded or oval, one-celled, about 5
microns in length.
The perithecia are borne, usually at the base of the stem,
amongst and in succession to the conidial fructifications, on the
somewhat carbonaceous layer which is formed on and in bark
which has become thoroughly infested.
The perithecia are formed much less freely than in the case
of R. bunodes, and, in spite of long-continued search, material
containing ripe asci has only once been obtained in the British
islands ; this was found by the writer on a dead lime tree in
Dominica, in a situation with an annual rainfall of some 250
inches. Examination of this material at Kew resulted in the
identification of the fungus as Patouillard's species.
The perithecia are normally slightly verrucose but are some-
128 DISEASES OF CROP-PLANTS
times found smooth. Apparently this is due to weathering,
though possibly (cf. R. hunodes) there is some variation in the
amount of roughness developed.
The most striking characteristic of this species is the produc-
tion of fans or stars of white mycelium in the region of the cam-
bium (between bark and wood) of the roots. The presence of
these distinguishes it at once from R. bunodes. A somewhat
similar appearance may be produced by R. paraguayensis,
but, in the examples of the latter fungus seen, the growth has
been very much less vigorous.
On the roots the mycelium forms an irregular coating over the
surface. In its early stages it is smoky grey in colour, but soon
becomes black. It is gathered at first into rather loose branching
strands with spreading hyphae between them. Later the whole
is combined into a mass which is more or less woolly on the sur-
face, and tending to be carbonaceous below. It forms dense
layers and pockets in the outer bark, and is connected with
whitish strands more or less vertical to the surface which every-
where penetrate the cortex. On reaching the surface of the wood
these repeatedly branch and spread in all directions over it,
so that, when " bark " and wood are separated, a conspicuous
white pattern of branching lines, stars and fans is seen on the
" bark." From this layer strands penetrate the wood in radial
lines along the medullary rays, and send out hyphae which invade
and fill the large vascular elements. In this species the mycelium
in the wood, owing to its lack of colour, is not apparent to the
naked eye unless a cut surface is exposed for a day or two, when
the hyphae grow out and turn black. In long-infested wood,
thin plates, seen as black lines in a section, mark off certain
areas, and are believed to belong to this fungus. Such plates,
however, are very common in dead wood, and so have little
diagnostic value.
The external mycelium is most fully developed and conspic-
uous when the fungus reaches the base of the stem and appears
above the surface of the soil. It is then seen in broad spreading
fans, or an advancing sheet, which from the beginning or at
an early stage encircles the stem (Fig. 34). The margin while
the fungus is actively advancing is light grey in colour for a width
of about half an inch, behind which the colour shades off to brown
or black with a greenish tint. On smooth bark the sheet is even
and glossy, rather like wet fur.
The height to which the fungus reaches is determined by the
moisture conditions. When the stem is well exposed, it reaches
no further than the few inches for which the moisture of the
soil can affect it ; if the stem is enclosed by weeds or low branches,
or sheltered by a log, it commonly goes up for a foot or more.
The limit of the external mycelium is also the limit of the ultimate
infestation of bark and wood. If a diseased tree is uprooted,
or is cut below the limit of infestation, and thrown aside where
Fig. 34
RosELLiNiA Pepo : Smoky Mycelium on Surface of Bark xl^
^W-^
m^
Fig. 35 RosELLiNiA Pepo : Mycelium under Bark. Nat. Size
Fi.g. 36 Rosellinia BuNODEs : Conidial Fructifications X 4J
ROOT DISEASES 129
weeds grow up and shelter it, the fungus extends to the whole
of the stem and branches thus kept moist.
In the rootstocks of herbaceous plants, as in thick, soft bark,
the firm round strands of mycelium, buff-coloured without,
white within, penetrate the parenchyma in all directions.
B. Rosellinia bunodes, B. et Br.
This fungus is recorded as the cause of root disease of coffee »
pepper, and associated plants in Southern India, of hibiscus in
Ceylon, of coffee and associated plants in Porto Rico. In the
Lesser Antilles, South found it on CastiUoa and camphor from
Grenada, and on limes in Dominica. The present writer has seen
it on hibiscus in Grenada, on limes, hibiscus, acalypha and
several unidentified native shrubs in Dominica. In St. Vincent
a disease of arrowroot is caused by a Rosellinia with the general
characteristics of this species, and very probably identical with it,
in connection with which examples have been seen of attack on
banana, plantain, cassava, tannia, yam, corn, pigeon pea. Sea
Island cotton, coffee, and avocado. Sugar-cane and Guinea grass
are among the few economic plants which appear to show marked
resistance.
There is no doubt that this list could be extended indefinitely.
The writer has not met with R. bunodes on cacao, but South
records an instance of its occurrence on young cacao plants
growing near diseased arrowroot.
R. bunodes was originally described from Ceylon by Berkeley
and Broome. It is at the present time responsible, where proper
precautions have not been taken, for steady and serious losses
of lime trees on recently cleared estates in Dominica. Since
the species previously discussed is about equally abundant
there on the same host, the records made previous to 1915
regarding black root disease of limes cannot usually be referred
to one species as distinct from the other.
Under the conditions prevalent in the uplands of Dominica
and Grenada the perithecia are formed very readily and in great
abundance on the surface of exposed roots, around the base of
the stem of infested trees, and on infested woody material lying
about in damp situations.
They are easily distinguished by the coarse, more or less
pyramidal warts with which they are densely covered. In one
instance only a nearly smooth variety has been seen. The spores are
exuded from the perithecium in a narrow thread without much
cohesion, and lie in a clump on its summit. The writer has no
evidence to offer as to the method or possibilities of their dis-
persal, but they would be easily carried away by heavy rain.
The conidial fructifications are of the same type as those of
* According to an opinion recently given by Fetch on material from Porto Rico the W.Indian
species may be found to agree more closely with A', goliatk Speg. described from Brazil. The two
species are in any case very close and not certainly distinct.
K
I30 DISEASES OF CROP-PLANTS
R. Pepo described above, and no character has been detected
by which they may be distinguished in the two species.
In respect of the general course of its development and in its
main characters R. bunodes closely resembles R. Pepo. There
are, however, certain definite specific differences in the appearance
of the mycelium which enable the two to be readily separated
in the absence of perithecia.
At the point of infection on a root a dead patch extending to
the cambium is formed in the cortex. From this, the fungus
spreads over the bark and through the cortex, the infestation of
the wood following later. The mycelium on the surface consists
mostly of closely appHed, firm-textured, branching black strands
which thicken into irregular knots along their course. On
examples of Castilloa roots examined such knots (the sclerotia
of some writers ?) had formed over and infiltrated the
iarge transverse lenticels, and on hibiscus similar bodies in the
form of dome-shaped papillae have been seen.
At a later stage the space between the strands may fill in so
that a dense layer is formed, which on its lower surface grows
between and encloses the outer layers of the bark, and on its
upper surface bears in relief a closely ramified system of rhizo-
morphic strands. From the lower surface abundant cylindrical
strands, with a black periphery and a white core, penetrate the
cortex in a direction more or less vertical to the plane of the
cambium. On reaching the wood they run for a short distance
over its surface, and distribute themselves into it by vertical
branches which penetrate by way of the medullary rays. From
these the adjacent large tracheids are invaded and filled with
bundles of parallel hyphae, hyaline at first, black later.
The result of this distribution is that the strands are seen
in the wood of a transversely cut root as black dots and radial
lines ; in a median longitudinal section as vertical and horizontal
lines ; in the bark as black threads of irregular length and course.
As in the case of R. Pepo, the surface mycelium ascends the
stem for some distance above ground as an encircling sheet.
In this species the whole sheet when quite fresh may be creamy-
white in colour, but eventually turns black. The " bark "
beneath commonly remains healthy for some time after it has
become covered, but it is eventually penetrated and killed.
By the further development of the mycelium a sort of crust is
formed on which the conidial fructifications and perithecia
successively arise.
The characters by means of which R. bunodes may be readily
separated from R. Pepo at any stage may be gathered from a
comparison of Figs. 35 and 38, and by noting that in contrast with
the condition shown in Figs. 39 and 40, the mycelium of R. Pepo
does not show in the wood until very late, and then only in long
zigzag lines formed by cutting through a continuous black
film or plate. There is only one caution to be observed : where
ROOT DISEASES 131
R. Pepo has formed a crust on the bark and is fruiting, some
blackening may extend to the mycelium under the bark at
that point. The examination of a root will remove any un-
certainty. In herbaceous plants the distinction between the
yellowish white strands of Pepo and the black ones of bimode'; is
usually quite clear.
The disease on arrowroot is a special case, brought into promin-
ence by the estate cultivation of that plant in St. Vincent, of the
general effects of Rosellinia spp. on herbaceous plants with
succulent rootstocks. It is further described under the diseases
of that crop.
Rosellinia {Paraguay ensis, Starb. ?)
A species which agrees fairly well with the description of
R. paragiiayensis occurs on cacao trees in Grenada and St.
Vincent. The perithecia have also been found by the writer
on the wood of an Erythrina on a cacao estate near Soufriere, St.
Lucia.
During a visit to Grenada in 1915 several groups of trees were
met with which were dying out, although growing in appar-
ently good soil and in favourable situations. The appearances
were such as to suggest root disease as the cause. The district
in each case was in the drier lowlands where the disease due to
Rosellinia Pepo seldom if ever occurs, and some of the character-
istic features of that disease were absent. The trees were slowly
dying back from the top, and putting out new suckers from
below, which in turn failed until the tree was completely dead.
In various stages of this process examined, the collar and upper
roots bore no sign of disease, but on the lower roots a mycelium
was found which formed a white radiating pattern between bark
and wood closely resembling that of Rosellinia Pepo but much
more scantily developed. These differences might have been
put down to the effects of drier conditions had not the finding
on dead cacao trees in two of these localities of the perithecia of a
distinct species of Rosellinia, associated with a similar scanty
white mycelium, brought the existence of a separate, though
closely allied, disease into question.
In one instance of a diseased group, the only one on an other-
wise healthy estate, the trouble seemed to have had its origin
some sLx years previously in the felling of two examples of the
tree known in Grenada as tendre acailloux {Pithecolobium Berteria-
num, Benth.), in the others no such source was apparent. In
another instance bananas planted as cover where trees had
died in this way never reached the bearing stage, and examples
were seen of banana plants in various stages of a root disease
which may have been due to Rosellinia, but which could not
definitely be recognised as such.
Some confirmation of the view that this species of Rosellinia
attacks cacao was obtained on an estate near Georgetown, St.
132 DISEASES OF CROP-PLANTS
Vincent, where it was found fruiting abundantly on dead cacao
trees occurring in patches suggestive of the effects of Rosellinia
disease. The degree of parasitism of which this species is capable
requires to be established by further investigation, but on the
Grenada evidence the conclusion that under certain circum-
stances it can produce a root disease of cacao seems justified.
As seen so far, the disease is much slower in its action than
that due to R. Pepo, and the failure of the trees much more
lingering. It is capable, however, of existing under conditions
of relative dryness such as appear to exclude the more familiar
form.
The fruiting habits of the fungus are the same as those of the
species already described, and the conidial fructifications have
the same form. The perithecia form a dense layer on the sur-
face of the wood, looking like fine shot dusted thickly upon it.
Occurrence of Rosellinia Diseases in New Clearings.
Serious outbreaks of the diseases due to R. Pepo and R.
bunodes are liable to occur in clearings recently made from the
forest. Room for these exists at the present time, in the smaller
islands, only in mountain districts. The high rainfall and reduced
sunshine of such situations produce conditions particularly
favourable to the fungus, but experience in cacao plantations
indicates that similar troubles would be met with in new clearings
made at lower levels.
The fungi concerned have not been found in a purely natural
habitat, i.e., in undisturbed forest, although sought for to some
extent. From the manner of their appearance in recent clearings
there can, however, be little doubt of their existence there as a
part of the natural flora.
It is the usual custom in these islands, when clearings are made,
to burn as much as possible of the smaller material, but to leave
the logs on the ground to rot, and to make no attempt to remove
the stumps. Many of the latter send up suckers which have to be
cut away from time to time. Under these circumstances it
takes very many years before the logs and stumps finally dis-
appear, and during the course of their decay they make the soil
around them dangerously rich in crumbling woody matter and
humus.
On land so prepared, as soon as the felling and burning are
finished, nursery plants are set out in the spaces between the
stumps and logs. To get anything like regularity of stand many
have to be placed close up to these obstacles. The writer has
not seen new plantations of cacao, but has had ample opportunity
in Dominica for examining all stages of lime cultivations thus
begun.
Fungi are, of course, very abundant on the dead logs and
branches, but the Rosellininas are far from conunon on these or,
in general, on the decaying stumps. But each newly infested
Fig :)7 RosELLiNiA BuNODES : Perithecia : X i
Fig. 38 RosELLiNiA BuNODES : Mycelial Strands in Lime Bark: x 1^
ROOT DISEASES 133
clearing affords unmistakable examples of the association
between the diseased trees and forest stumps and sometimes
these are very striking, as when the infection of a group of five
or six trees can be traced to the spreading roots of a single large
stump. In many cases the connection has been traced in detail
and newly diseased roots found in contact with infested roots of
the forest trees.
Relatively few trees are lost in the first three or four years
after planting. This is in part due to the time taken (a) by the
fungus to develop on and about the stumps ; {b) by the roots of
the planted trees to grow out into the infested area, and (c) by
the fungus to kill a tree after infection. But observation shows
that when the period of heavy losses ensues, most of the trees
are infected from one another, so that the characteristic distribu-
tion of the disease is in scattered patches, the number of which is
very small compared with that of the stumps and logs in the
clearing.
There is every reason to believe, and experience in other
countries agrees, that the number of species of forest trees of
which the stumps are readily infested by the fungus is quite
limited, and that the presence of the stumps of these especially
susceptible trees in clearings has a good deal to do with the ap-
pearance of the disease. It is a well-known fact that there is
such a special susceptibility in the case of certain cultivated
and semi-cultivated trees. Accurate information as to the
identity of the forest trees most concerned is difficult to
obtain. In Dominica the largest number of cases occur in
connection with Chataigniers {Sloanea spp.), while Mahoe cochon
{Sterculia caribaa), Mahoe piment {Daphnopsis tinifolia) and
bois cabrit or goatwood {AlgipJtila martinicensis) are also recorded
as susceptible.
The disease does not develop in clearings made on poor and
shallow soils, or on land which remains in a swampy unaerated
condition.
Occurrence in Established Plantations.
In lime plantations no examples have been met with of the
persistence or recurrence of Rosellinia disease after the disappear-
ance of the remains of the forest growth from the soil. In cacao
plantations, and probably the same applies to those of coffee,
the disease is liable to occur in fields of any age. The liability
is greater in degree according to the rainfall and humidity of the
situation and is also affected by the kind and condition of the
accessory trees in the cultivation.
There is usually no evidence of the disease having persisted
from the time of the original clearing, which in many cacao
estates is a remote one. Its occurrence seems spontaneous and
is mostly unexpected. In a few cases it has seemed to start on
the cacao trees themselves, but much more usually it has its
134 DISEASES OF CROP-PLANTS
origin in trees grown for shade or as windbreaks, or shrubs grown
as marginal hedges. The most susceptible of the trees commonly
grown in or about cacao are avocado, breadfruit, and pois-doux
{Inga spp.) Mango and the immortelles are sometimes but less
frequently concerned. A year or two after these trees have been
cut down, after a large root has been cut in digging a drain,
or when they have died from natural causes, the fungus is
commonly found to be established on their roots in the
same way that it occurs on the stumps in new clearings, and
the adjacent cacao trees begin to pick it up. In valley
cultivations the disease most frequently appears in situations along
the lower courses of ravines or on flats where flood water over-
flows, a distribution which suggests that the infective material
is often water-carried, probably from the upland forest. On
hill estates the distribution is usually more irregular.
The cases sometimes met wiili of cacao trees becoming
diseased in the absence of stumps can be accounted for by
infection of the surface type. The instances noted have been in
the wettest districts, where production of organic matter is at a
maximum, and where cloudy days, the depth of shade produced
by luxuriant growth, and the frequent saturation of the soil
with water, all reduce the rate of its destruction. Where rainfall
and shade are not excessive the addition of organic matter to the
soil does not keep pace with its decay, and the soil met with on
the ordinary cacao estate is usually not particularly rich in humus,
nor is there much in the way of leaf-mould on its surface.
The spread of the disease along closely planted hedges and
windbreaks, of which some striking instances have occiurred in
St. Lucia, is characterized and probably mainly effected by
infestation of the surface soil. The leaves and twigs which
accumulate along the base of the windbreak, and decaying,
enrich the soil with humus, and the shelter aftorded by the trees
and by the vegetation which grows up under their protection,
provide conditions especially suitable for the fungus. It creeps
along the line like a smouldering fire, killing off the trees and
their seedlings, and most of the shrubby and herbaceous vegeta-
tion, as it comes to them. Windbreaks of pois-doux and galba
{Calophyllum Calaba) and hedges of Hibiscus and Aralia are
very susceptible to the disease.
The Course of the Disease.
The course of the disease is much the same in old or new
plantations. Typically it starts from one or from several scattered
centres and extends slowly but persistently in a widening circle.
Its present or past existence in a field may always be suspected
where irregular groups of trees are missing or are notably of
lesser age.
The fungus spreads at two levels. It works its way along the
roots and passes from one root to another in the network that
ROOT DISEASES 135
extends below the surface of the soil, and can do this at any
depth provided there is aeration. It can also infest and extend
through the surface soil, independent of roots, where this is rich
in decaying matter, and through accumulations of leaves and
other decaying material on the surface, provided that in either
case the situation is sufficiently shaded and watered to remain
continuously moist.
It does not appear that direct spore infections take place.
In all cases studied the mycelium has first been developed on
dead material before attacking living organs.
Infection by contact or from the soil takes place on the
surface of the bark without the necessity for a previous injury.
When it has obtained a foothold on a root the fungus has three
lines of advance : (a) over the surface of the bark, (b) in the bark
and on the surface of the wood, and (c) in the wood itself. These
appear usually to follow each other in the order named. If,
as is commonly the case, the infection starts on an outlying root
the fungus spreads along it infecting other roots attached to it
or in contact with it until the crown is reached. Here the
mycelium can cross over on to adjacent main roots, which are
infested in turn. In nearly all cases there is sufficient moisture
and cover about the base of the stem to enable the mycelium to
attack the bark of the collar, and commonly to extend for some
distance up the stem. It is at this stage that the fungus becomes
visible on undisturbed trees. A smooth coating of mycelium,
which in the case of R. Pepo is black with a light grey margin
about half an inch wide, and in R. bunodes is at first white, then
black, is seen extending up the bark above soil level.
In those trees of which a considerable proportion of the root
system has become affected before this stage is reached, a gradual
yellowing and thinning of the foliage will have been visible.
Where on the other hand the fungus has reached the collar
without much destruction of roots, as in the case of infection of
short surface roots or possibly the collar itself from the soil,
the envelopment of the stem may be seen taking place while the
tree is apparently in full health and vigour. The death of the
tree soon follows upon the complete infestation of the collar,
and in the latter type it takes place with startling suddenness.
In a few hours an apparently vigorous tree will stand naked save
for the fruit in various stages of development, and a few young
leaves on the tips of the shoots. Some weak shoots may after-
wards be put out, but they come to nothing. The only previous
indication of disease which is commonly present in such cases
is the production of an unusually large crop of flowers and fruit.
The dependence of the fungus on continuous moisture usually
limits its development to the roots and to the base of the stem,
but living branches and the undersides of recumbent stems are
attacked if they come in contact with infested soil, while if dead
trees become thickly enclosed with weeds or are felled and left
to lie they become infested as far as they are kept moist.
136 DISEASES OF CROP-PLANTS
"Wherever a tree has died, unless strict measures have been
taken to control the fungus, the adjacent trees almost inevitably
contract the disease sooner or later. Such cases may develop
years after the origmal tree has been removed and the cause of
Its death forgotten, so that they have the appearance of being
sporadic. Examination of the position and age of supplies, and
the keeping of records of surveyed areas enable them to be
linked up with considerable certainty to previous losses. How
far such belated cases are evidence of the slow progress of the
fungus along the roots, or how far they are due to delayed in-
fection has not been ascertained, but the general evidence points
to the conclusion that the process of investment of mature trees
is a distinctly slow one. In a case definitely recorded a fully
infested dead lime tree was removed in October, 1914, and the
fungus {R. bimodes) was just coming up round the collar of the
next tree in the row, a very large and vigorous specimen, in
August, 1916. The variety of circumstances must produce wide
difterences, but it is judged that the two years taken in this case
is not an unusual period, and is in some cases considerably
exceeded.
Typically an infested clearing in which the disease is of
several years' standing shows a few large open patches, each
representing perhaps a score of trees, with usually two or three
around its margin dead or dying, and several more on which the
fungus may be found. Sometimes two or three such patches
have coalesced. Scattered about are fresh centres in various
stages : a single tree, a gap of two or three trees in a row, with
another going, or a group of two or three trees in different stages
about a large stump. Of the supplies put in, some are several
years old and still thriving, others are dead within a few months
of being planted. On one cacao estate in a wet mountain district
the loss of about 150 trees appeared to be clearly traceable
to two original centres of infection.
Counter Measures : I. Prevention
(a). In New Clearings.
It would be a counsel of perfection to recommend the removal
of stumps or even of logs from new clearings in their earliest
stages. In most West Indian plantations such a policy is not
economically possible. But, in arranging and planting new
clearings for orchard crops, the probability that root disease
will occur should be kept in mind, and certain precautions can
be taken which will considerably reduce the trouble to be faced
when disease appears.
First among these is provision for the construction, immediate
or when occasion and funds permit, of a complete and close
system of trench drains. To this end the arrangement of the
trees should be planned so that each block shall be isolated from
RosELLiNiA (Paraguayen'sis Starb ?) Perithfxia X 3
ROOT DISEASES 137
the rest by main drains, and, where possible, each row, or two
rows if the planting is close, separated from the next by a trench.
There will be many patches encumbered with logs and stumps
over which for some years the system will extend only in the
plan, but it is necessary to take long view s, and too many cases
have been seen where, when the need for a trench has urgently
arisen, the line has been brought up against trees irregularly
planted and now too valuable to be destroyed. Drains are
being considered here, of course, not in view of their primary
function of removing water, but as isolation trenches preventing
the spread of root disease from tree to tree. At the same time,
thorough drainage may be considered to have direct value from
the point of view of Rosellinia disease, for although, as has been
pointed out, the fungus does not thrive in water-logged soil,
neither do the trees, and, when that condition has been passed,
the drier the soil can become the quicker is the decay of organic
matter and the less favourable the conditions afforded to the
fungus.
It has been not uncommon in both lime and cacao fields,
when a large stump has appeared to be a centre of disease, for a
trench to be dug around it, cutting off the widely extending
roots. The best plan would probably be to dig a trench some
two feet wide well away from the stump, remove all sections of
roots, and fill it in again.
In planting the trees, positions in close proximity to stumps
should as far as possible be avoided, and a good deal may be
done in the way of rearranging the smaller logs so that they do
not form too close a shelter about the stem of the prospective
tree. Beyond these measures little, for the time being, can be
done.
When the first few years have passed and the logs are more
or less rotted, it has been shown to be quite feasible to hasten
very materially the time of their disappearance by cutting or
breaking them up so that they can be easily handled. Merely
to dispose of them better by dragging them away from the
stems of the trees is a considerable gain, and in some cases it
has been found possible to stack much of the material, and even
to make good use of it for fuel. The great convenience, quite
apart from root disease, of having the ground clear should be
an additional inducement to the planter to make every possible
effort to this end.
A point to be always kept in mind is that wood which can dry
out from time to time, e.g., logs, stumps, or parts of them which
are raised above the soil and not shut in by weeds or overhanging
branches, is in little danger of harbouring the fungus.
The same consideration applies to the soil. In damp, shel-
tered clearings, such as are favoured by the fungus, all that
is possible should be done to encourage the free circulation
of air beneath the trees. Low hanging branches which maintain
138 DISEASES OF CROP-PLANTS
a closely sheltered circle around the base of the tree induce
conditions which invite surface infection, and should most
certainly be cut away. Grass and weeds should be kept short
all the time, not merely in the crop season.
It is highly desirable in infested clearings, and especially
in the neighbourhood of infested spots, to go further and clear
away the soil and weeds about the base of the stems, completely
baring the collars, and liberally exposing the main roots so far
as this may be done without forming a saucer in which water
will stand. The treatment of the collar and the recesses between
the roots with lime-sulphur solution to keep the bark free from
moss gives an added protection of considerable value.
All these measures are summarized in the word ventilation,
and cannot be too strongly emphasized. The fungus cannot
tolerate dry conditions and it is for the planter to take every
advantage he can of this weakness.
(b) In Cacao Plantations.
There are probably few managers of cacao plantations who
would nowadays plant breadfruit or avocado trees through
their fields. Where these trees are already established, the
planter who has learnt respect for root disease has two courses
open to him. The one is to take the utmost care of such trees,
to prune out dead branches, to avoid injuries to the roots, and
generally to try to preserve them in health as long as possible ;
the other is to take them out as occasion offers, removing the
stumps, and following out and removing every root that can be
got at. Striking cases have been seen where the cutting down
of avocado, breadfruit, or pois-doux has led to the appearance of
the disease in places from which it was thought to be far removed.
These trees are the chief victims, but there is none that can be
considered to be really safe. It should be the settled policy
of cacao planters in wet districts to leave no woody material
to rot in the soil if it can possibly be avoided. Where there is
deep shade and high humidity, any vegetable matter used as
mulch may be a source of danger, though one that may have to
be risked in view of manurial requirements. The more careful
and prompt the attention that is given to any appearance of the
disease on the plantation the less will the risks of this nature be.
With regard to windbreaks : where experience has shown
that there is danger of disease arising, it is advisable to keep
them as free as possible from undergrowth, and to scrape away
any accumulations of leaves or other material from hollows or
other sheltered places. This can be burned or used in the pens.
II. — Treatment.
The prospect of saving trees with deep-seated or well-estab-
lished infections is too remote to pay for the labour and skill
required. But experience has shown that in fields in which the
ROOT DISEASES
139
Fig. 42 RosELLiNiA Spp.
(A) ASCUS OF R. BUNODES. (B) ASCUS OF R. SUBICULATA.
(C) AsCUS OF R. PARAGUAYENSIS (?) (E) AsCUS OF R. PePO.
(F) Terminal Hyphae from Conidial Fructification of Pepo.
(G) External Hypha characteristic of Rosellinia Spp.
140 DISEASES OF CROP-PLANTS
fungus is established in the soil and surface infection is taking
place, an examination of the trees, involving no more than an
initial clearing and periodic inspection of the crown and collar,
will directly save many lightly infected trees, and, indirectly,
by preventing the formation of new centres of distribution, save
very many more. All that is needed is to have the lower branches
pruned sufficiently for access to the trunk, a couple of labourers
to go on ahead and clear away weeds and earth for a foot or so
around the junction of roots and stem, preferably using their
hands or a blunt instrument to avoid wounding the bark, and
then to make a few minutes* investigation of each tree so pre-
pared. In addition to saving trees, such a system enables the
hopeless cases to be detected long before they otherwise would
be, and makes it possible to deal promptly with them. It is
justifiable to insist that in every clearing in which the disease
has become established such an inspection ought to be made at
least twice a year.
The principles of the actual treatment are two in number
and very simple : {a) complete excision of diseased tissues ;
(b) exposure as complete as possible of the affected part and
its surroundings to freely circulating air, with the object of making
the conditions too drj^ for the fungus to exist.
The cases which repay treatment are those arising from
surface infection, in which on inspection the local nature of the
infestation appears to be clearly defined. Severed roots should
as far as possible be removed ; cut surfaces should be cleanly
trimmed and treated with wound dressing ; excavations made
to get at the seat of injury should be left open. It should be
hardly necessary to say that the material removed must be
carefully disposed of. In many cases a few cuts with a sharp
knife will save a tree from an infection which would have meant
its certain death.
Where the disease has been detected advancing along one or
more large roots but has not reached the collar, and it is desired
to give the tree a chance of recovery, after the diseased roots
have been cut away as much soil as possible should be dug out
about the junction of the main roots and as far back as they can
be conveniently followed, leaving the tree more or less on stilts.
This prevents the crossing over of the fungus at or near the collar,
and may very considerably delay the loss of the tree ; but if the
fungus, as is probable, exists on the outer roots, it will in time
get round in that region and come up each of the main roots
in turn as far as the point to which it has been bared. It is not
a measure to be recommended. It is better to have the tree out
and be rid of the fungus, but the temptation to get an additional
crop or two from some specially fine tree is sometimes hardly
to be resisted.
The directions here given as to treatment apply equally to
limes and cacao, but as has been previously indicated, with the
ROOT DISEASES 141
latter crop, so far as the writer's experience goes, the cases are
more usually of the deep-seated kind, which does not permit of
successful treatment.
III. — Control.
Removal of Diseased Trees.
When an infested tree is found it is well that its destruction
should be prompt, but it is an advantage to choose a dry day
for the operation. If the disease is so far advanced that the
fungus is producing conidia, either on the stem or on dead leaves
and twigs lying about, then before it is disturbed the spore-
bearing surfaces should be flamed with a torch or by burning
trash of some description around them. This disposes for the
time being of the means of aerial dispersal of the fungus, and
lessens what is otherwise a real danger of the carriage of infection
on the persons and implements of the labourers.
The diseased tree should be cut down, the stump dug out, and
the roots followed up and removed as completely as possible.
One planter gets out the fragments with a builder's sieve, and
has had unusual success with his supply plants. It is convenient
for the labourers to have baskets handy into which the small
roots can be thrown as they are dug out, otherwise they are in
danger of being scattered.
The diseased material should be destroyed by fire, preferably
on the spot. It has been formerly stated that the whole tree
should be cut up and burned, but experience has shown that the
necessity for this, which has often been a real difficulty to planters
trying to follow instructions, may be avoided. It is easy to
note how far the fungus has reached, and if the stem is cut above
this point, and the top is disposed of in such a way that it can
dry out, it will not develop Rosellinia. This has been verified
on trees thrown into the edge of the forest, provided they did
not rest upon the ground, and on others perched on large stumps
and on rocks. It is better to dispose of even the stump in some
such way than simply to leave it lying about as is too frequently
done, but all material actually infested certainly ought to be
burned. No material, root or stem, infested or uninfested, should
be left lying on the ground. In one instance the stem of a cacao
tree, some 5 feet long, which had been charred and thrown aside
in the grass, was found sheeted from end to end with conidial
fructifications, and stems and branches of dead lime trees have
been frequently seen, where they have rested for some distance
on or close to the ground, in a similar condition. Had they been
wedged among the branches of a tree they would have been
quite harmless.
Under some conditions it may be necessary to choose a
convenient situation and carry to it material to be burned, but
such transportation has obvious dangers in the way of scattering
bits of diseased material. It should certainly be preceded by the
142 DISEASES OF CROP-PLANTS
scorching of any spore-bearing surfaces. A kerosene blow-lamp
is a useful appliance in connection with this scorching, and in
starting fires.
Treatment of the Soil.
Attempts have naturally been made to arrest the progress of
the disease in the soil, or to sterilize situations from which dis-
eased trees have been removed, by the application of disin-
fectants.
There is not much scope for the use of disinfectants in the
control of this disease. It is safe to say that at the present time
there is no substance available which is cheap enough to be used
in sufficient quantity to sterilize any considerable area of ground.
When dealing with a fungus which penetrates to every part of
even the thickest roots, it cannot be expected that chemical
agents will kill it out, or, being necessarily of a transient nature,
stop its progress, unless all woody material is removed. When
this has been done, exposure of the soil to sun and air is probably
just as effective as any disinfectant could be. Where it is re-
quired to deal with infested soil or accumulations of vegetable
matter under shade, as, for example, in the case of the trees with
surface infections specified above, it is better to scrape away
the surface soil with a hoe than merely to extinguish the fungus
with a disinfectant, leaving material liable to reinfection. Such
a scraping of the soil, combined with removal of low branches
and of any other hindrances to the tree circulation of air, has in
the cases under the writer's observation been sufficient. An
application of lime or sulphur would give an additional margin
of safety.
The addition of lime to the soil when clearing up a spot from
which infested trees have been removed is commonly practised,
and is believed to contribute to success in establishing supplies.
It seems probable that the results obtained are not so mucn due
to the sterilizing effect of the lime as to its action in hastening the
disappearance of organic matter. There is a further probability
that by neutralizing soil acidity lime produces conditions less
suitable to the survival of the fungus.
If the view here taken of the function of lime is the correct
one, the main effect may be obtained by the use of slaked lime,
which is the only form that can be conveniently obtained in some
islands. Quicklime, because of its additional sterilizing effect,
is to be preferred when equally available.
Where sulphur has to be imported, its price will not enable
it to compete with lime, but there would appear to be possibilities
in some places of obtaining it cheaply from local sources. Sulphur
has a more definite fungicidal action than lime, and one which
may be expected to develop slowly and have considerable per-
sistence.
The effect of sulphur on the soil reaction is to increase acidity,
ROOT DISEASES
143
and from this point of view it is desirable to follow its use with an
application of lime.
Isolation of Contacts.
The diseased tree having been removed, it is necessary to
take steps to prevent the spread of the disease among the trees
around. In a regular field, with ordinary distances of planting,
the roots of the trees form a continuous system of contacts,
interrupted only by the deep drains. By the time a tree is so
far diseased as to be noticeable, the probabilities are great that
the roots of one or more of the neighbouring trees have also
become infected. It is required to cut the connection between the
infested roots and those of the surrounding healthy trees, and to
break the continuity of the possibly infested surface soil. To be
reasonably sure of doing this, it is necessary to carry an isolation
trench outside the trees in contact with the one diseased. Thus,
in an undrained field, if D in the first diagram (I.) represents a
diseased tree, C the possible contacts, and H the healthy trees,
the isolation trench should take the course indicated by the
unbroken line, and will include nine trees.
H H H H H
H
C
C
C
H
H
C
D
C
H
H
C
C
C
H
H
H
H
H
H
H
H
H
H
C
H
H
D '
H
H
"'c*
H
H
H
H
I.
II.
Neither the roots of the trees nor the course of the disease
follow regular lines, and it is quite improbable that all the eight
contacts are infected, but there is no practical means of knowing
which remain free. In order to save those which are still un-
infected, it is advisable to cut up the enclosed area chess-board
fashion as indicated by the dotted lines. If these secondary
trenches are omitted it is likely that the contacts will all go off
sooner or later in the manner already described.
In situations where there is a risk of soil infections the trees
still fiurther out (H H in the diagrams), should have some
special attention, especially if D is an advanced case when dis-
covered.
In situations liable to root disease there is much to be said
for the general adoption of the system represented in the second
diagram (II.), where a permanent drain (denoted by a double
line) exists between every row and the next. All that is required
for isolation is the cutting of cross trenches between C and H, to
144 DISEASES OF CROP-PLANTS
separate contacts from healthy trees, and secondary cross trenches
between D and C, to save the contacts if they do not prove to
have become infected. The length of isolation trench required
is reduced to one-sixth. Where close planting is adopted or the
whale-back beds which this system produces are objected to, the
trenches may be made to include two rows instead of one, the
protection obtained being then less complete. Like the preven-
tive measures already indicated, this system should be applied
first of all in the situations where the losses are greatest, and
extended as opportunity permits.
It is not necessary, from the point of view of root disease,
that the trenches should conduct water : indeed, where loss of
surface soil by wash is feared more than the alternate danger
of water-logging, it may be advisable deliberately to check
their function in this respect. They may then be periodically
cleaned out, and the deposit returned to the soil. It gives a
clearer point of view if the system is regarded as one of per-
manent isolation trenches with a secondary function as drains,
rather than as one of drains in the first instance.
Returning to consideration of the treatment of infested
spots, there are two commonly existing situations which need
to be dealt with. These are (i) the case where a large patch of
trees has already been eaten out of a field, and the disease is
spreading outward around its circumference, and (2) the case
where one or more trees have been attacked around a large forest
stump, infested or likely to become so, which together with its
heavy roots prevents the cutting of trenches over an area
which may include quite a large number of trees. In both cases
the procedure has to be modified to suit each individual set
of circumstances but still follows the simple principles set oat
above. Where roughly parallel main drains or water courses
exist on each side of the area, they should be joined across above
and below to establish an outward limit, even though it may be
a wide one, to the spread of the infection. Then working inward
from this, successive trees or rows of trees which appear healthy
may be separated off wherever it is possible to dig a trench, and
the disease thus confined to the narrowest limits.
In the situations sometimes met with, especially in cacao
plantations, where the trees are growing amongst a confusion
of fallen rocks, the possible measures are limited to early removal
of so much of an infested tree as can be got at, and the liming
of the soil about adjacent trees, with a view to preventing spore
infections of the material which accumulates in the enclosed
pockets of soil.
It has been commonly recommended that a trench be carried
in a circle around diseased trees and their contacts. This
method has practical disadvantages. The extent of the existing
infection can never be ascertained by inspection, and a wide
circle, while enclosing many healthy trees, may prove too narrow
ROOT DISEASES 145
to include some line of infection that has run off in advance of
the general spread. A circle leads nowhere, whereas a system
of squares may be added to at any point and be carried in any
direction, and is capable of any subdivision. It has moreover
the great advantage of linking up with an existing or prospective
drainage system.
As regards the form of the trench itself, there is but one
essential so far as root disease is concerned : that it should be
deep enough to cut through all the roots passing across its
situation. When digging near an infested tree, the earth re-
moved should be thrown inward as a precaution against the
scattering of possibly diseased material among the healthy trees.
This refers more particularly to secondary trenches ; the outer
trench should be put far enough away to avoid, in general,
the chance of finding such material. The earth should be dis-
tributed, not banked at the edge of the trench, unless put there
in exceptional circumstances with the express purpose of avoiding
wash. Under no circumstances, however, should the collar of
the tree be earthed up. Roots passing into the healthy area
should be followed up and removed, so far as this can be done
without much injury to other roots. It is better for cut ends
of roots to be protected with a coat of tar.
Survival of Supply Plants.
On the thoroughness with which the clearing up is done de-
pends the chance of survival of an early supply plant. It has
been found in St. Lucia, where the results of several years of
experience of the treatment of this disease on cacao are now
available, that, where the work has been carried out under the
personal supervision of the planter, supplies put in a month
afterwards have remained healthy. The longer the delay the
greater the chance of survival. It must be remembered that a
supply may do well for a few months while its roots occupy the
site from which the stump was removed, and then become
infected from some outlying fragment of the old tree as its
roots spread wider. Such cases have been definitely traced.
The Possibilities of Control.
Rosellinia disease in orchard cultivations is by its nature
capable of the most serious consequences. It is cumulative in
its effects : each tree that contracts it infects as a rule not merely
one but several more, and each tree killed takes at least five or
six years to replace. In new clearings, encumbered with stumps
and logs, and in cacao fields with large shade trees, the disease is
difficult to deal with.
It may be confidently claimed, however, that the application
of the principles of prevention and control set out in the preceding
pages will reduce the annual losses to a minimum that is not
likely to be serious, and one may further expect a gradual
diminution year by year of even the sporadic cases.
L
146 DISEASES OF CROP-PLANTS
Control does not depend on any special apparatus or material,
but on operations of a familiar type, demanding little special
skill. Everything depends on the vigilance and thoroughness
with which they are carried out. Careless or half-hearted work
is of little use against this disease, which is apt to pursue its
course with a disconcerting appearance of deliberation and
inevitability.
ScLEROTiuM Rots.
The genus Sclerotium is an artificial group in which are
placed fungi with no observed process of reproduction by spores,
but in which the mycelium forms solid resistant bodies, often
resembling seeds in appearance, made up of closely compacted
hyphae.
Sclerotium Rolfsii Sacc. is a widely distributed soil fungus,
which under conditions of rather high temperature and marked
humidity vigorously attacks the softer tissues of plant parts
which come in contact with the ground.
It most frequently comes to notice as the cause of crown-rot
and consequent wilting in herbaceous plants. As regards these
it appears to come near to being omnivorous, but in these islands
is perhaps commonest on leguminous herbs. The suscepti-
bility of different hosts is largely determined by the extent to
which they shelter the soil and thus increase local humidity,
and probably also by their relative succulence.
The mycelium usually first attacks the stem near soil level,
invades and rapidly kills the cortex, and then spreads over the
roots. The first sign of its presence noticed is usually the wilting
of the leaves which ensues upon the ringing of the stem. Branches,
leaves and fruits which touch the soil are subject to independent
infestation and rotting. A very copious white mycelium, with
a distinctly radial habit of growth, extends upon the infected
parts and is often freely developed in the immediately surrounding
soil. It shows a distinct tendency to form strands, and when
old develops a buff or brownish tint.
In a short time the exposed mycelium becomes dotted with
tiny white tufts, which develop into smooth spherical sclerotia
white at first, then buff-coloured or brown. Their size when
mature is somewhat variable, but usually comes within a diameter
range of 1-2 mm.
It is assumed with great probability that the sclerotia can
hold over the fungus for a long time, producing a new growth of
mj^celium when the conditions are favourable. The readiness
with which the disease appears in wet weather even in districts
with a severe dry season shows that the fungus is furnished
with effective means of perennation.
No very widespread damage has been reported from the
disease, but temporarily and locally it can be very destructive.
The root disease of ground-nut in these islands described by
Root Disease of Cacao (R. Pepo)
[F. IV. Urich
ROOT DISEASES 147
F. W. South was evidently due, in part at least, to this fungus.
In addition to its wide range on herbaceous hosts the fungus has
occurred in the West Indies on seedling citrus plants, both in the
seed-bed and the nursery. In the United States it has been re-
corded from sweet potato. On sugar-cane it gives rise to a disease
of a special type, red-rot of the leaf-sheath, described with the
diseases of that crop.
Control.
The only apparent hope of dealing with the disease is through
measures designed to prevent its appearance by reducing humidity,
which include close surface draining, wide spacing, and the
provision of support for climbing plants. Access of sun and air
are very effective checks to the fungus. Affected plants should
be promptly pulled up, and in this case should certainly be
burnt. The application of lime to the soil has been recommended.
FoMES Root Rot.
The large bracket fungus Fomes (Polyporus) hicidus (Leys)
Fr. has come to notice from time to time as a suspected parasite
working on the roots and collar of various trees.
F. W. South regarded it as taking part in the destruction of
lime trees in Montserrat and Antigua, but it is impossible to say
what is its real position amongst the other factors contributing to
the death of the trees. (See Diplodia Dieback.)
A case in which, owing to the absence of compHcations, the
connection of this fungus with root disease was more clearly
indicated received some attention from the writer in St. Kitts in
1914. At the Basseterre Botanic Gardens a row of large
Saman trees {Pithecolobium Saman) was thinned out by cutting
down alternate trees. About a year afterwards three of the
remaining trees died with a fungus rot of the roots, and an
adjacent hedge of bread-and-cheese (P. imguis-cati) was similarly
killed. The Saman trees have continued to die one by one.
White mycelium is plentifully developed, in some places in thick
masses, under the bark of the collar and roots, and large sporo-
phores of Fomes hicidus regularly grow out from this when
the tree is dying or dead. The bracket-like fructifications attain
in well-grown examples to a diameter of 30 centimetres. They
are usually supported on short thick reddish-black polished
stalks, single or fused ; the upper surface is irregularly wrinkled and
has the appearance of being coated with brownish-red lacquer ;
while the underside when fresh is whitish or buff in colour.
The fungus has a similar status in other countries. T. Fetch
has found it in association with root diseases of coconut, mango,
and flamboyante in Ceylon ; P. J. van Breemen suspects it to
be the cause of a white rot of Ccesalpinia coriaria and Acacia
tortuosa in Curagoa, and P. A. van der Bijl attributes to it the
death of Acacia spp. in South Africa. It will be seen that there
is some cumulative evidence of its parasitism on leguminous trees.
CHAPTER XVI
STEM AND LEAF DISEASES
Pink Disease.
This is a disease in which twigs and small branches appear as if
they had been dusted over with pink powder, and sustain damage
which commonly leads to their defoliation and death. In the
later stages the pink colour fades to grey, and the thin coating
cracks irregularly. In some cases the fungus assumes the form of
white or pink pustules arranged more or less in lines parallel with
the branches.
West Indian records show that pink disease occurs in Porto
Rico, Dominica, St. Lucia and Trinidad on cacao, and has been
also seen on lime, grapefruit, pigeon pea, oleander and Amherstia.
The fungus to which it is due has been found to be identical
with that of the pink disease of the Eastern Tropics, which has a
very wide range of hosts and gives serious trouble in rubber
plantations.
That the disease has not assumed anything like the same
importance in the West Indies would appear to be due to difference
in climate. The fungus is dependent on a very considerable
amount of humidity for vigorous growth. In even the wettest of
these islands there are few places opened up to cultivation which
are not penetrated more or less by wind and sun all the year
round ; it is notable that the disease on the lime in Dominica
is still known to exist only in one specially wet and sheltered
district, although it was recorded from the same place nearly
twenty years ago. On cacao in Dominica and St. Lucia the
disease has been known the same length of time and remains a
rarity. In Trinidad, according to J. B. Rorer, it has been found
on cacao only at rare intervals in the northern and eastern
districts and has not proved a serious trouble.
Causation.
The causative fungus Corticium salmonicolor B. et Br. (C.
javanicum Zimm.) was referred to for several years in West
Indian publications as C lilacino-fuscum B. et C., owing to a
mistaken identification by Massee of material from Dominica.
The infection probably comes in the first place from some forest
plant. The spores germinate on healthy bark and develop
a superficial mycelium of cobweb-like texture. The fungus later
148
Fhoto by]
Fig. 44
Pink Disease of Cacao
STEM AND LEAF DISEASES 149
penetrates and kills the bark and frequently enters the wood,
in which case a sudden wilting of the leaves may be observed.
In the case of lime trees the affected branches are visible from an
elevation scattered amongst the healthy foliage. The pink layer
is the fruiting part of the fungus, and produces the spores upon its
surface. One or more pink fungi which are harmless occur on the
bark of trees, and are liable to be mistaken for this species.
A second stage of the fungus, originally described as Necator
decretus, Massee, occurs together with or in succession to the
Corticium stage, often on wood killed by the fungus. This
consists of orange-red pustules about 3 mm. in diameter, made
up of irregularly shaped thin -walled spores. This stage has not
so far been recognised in West Indian examples.
Treatment.
The simplest and most effective way to treat the disease is to
cut off the branch on which it occurs at its junction with the
next older branch or with the stem, taking care to cut well
below the affected part. The branch removed should be burned
or buried, with as little handling or transportation as possible,
and the bark for some distance below its point of origin disinfected.
General spraying with Bordeaux mixture or lime-sulphur mixture
has given satisfactory results on citrus trees in the Philippines,
the latter being preferable owing to its lesser effect in protecting
scale insects. No such measure has yet been needed in West
Indian cultivations.
Thread Blight.
Thread BUght is the name commonly given to affections of
shrubs and trees in which white to dark brown compact strands of
fungus mycelium run along the twigs, and send out branches to the
leaves, on the blades of which they ramify, or spread out into a
thin web, generally white in colour.
Distribution.
Diseases of this nature occur widely over the world, especially
in humid situations in the tropics. In the Lesser Antilles
thread blights have been found on various forest trees, and on
cultivated plants have been most noticed on cacao, coffee and
nutmeg. There are in various parts of the tropics several
Basidiomycetes which have scandent whitish mycelium of the
general type under discussion.
Corticium Stevensii, Burt {Hypochnus ochroleucus, Noack).
The common thread blight which occurs in humid situations
in these islands on cacao, nutmeg, and numerous other plants has
been identified by E. A. Burt as due to the fungus above named,
which was first described from Brazil, and later as the cause of a
150 DISEASES OF CROP-PLANTS
thread blight of apple, pear and quince in the southern United
States. The fructification, which is rarely found, consists of a
reticulum or felty membrane, coloured dirty pinkish buff, cover-
ing the whole underside of a leaf and easily separable from it.
The basidia are scattered along the hyphse on short lateral
branches ; they are simple, measuring ii X 7-8 microns, with
four short sterigmata ; the spores are hyaline, flattened or slightly
concave on one side, measuring 8-1 1 X 3-4 microns.
Nature of the Attack.
The strands of the fungus pursue a wavy but generally
longitudinal course on the thicker twigs. They may reach a width
of about a millimetre. The upper surface is somewhat romided,
while the lower is closely adpressed to the bark, to which it
adheres rather firmly ; the colour is white or cream in young,
deep brown in mature examples. The strands are made up of
apparently solid hyphse, which are closely adherent in the brown
integument, and in the body of the strand are loose, hyaline,
remotely branched, straight, of regular thickness, and mostly
following a parallel corurse. Adhesion to the bark seems to be
secured by short rhizoid-like hj^hae. There appears to be no
actual penetration of any twig upon which a cork layer has formed.
A branch is given off from the strand at the base of each
leaf-stalk reached, and runs up the latter to the blade, where
it breaks up into successively finer branches with irregular
flat expansions and finally frays out into its ultimate threads.
The tissue of the leaf is penetrated by connected mycelium
of a different type, thin-walled and with numerous clamp con-
nections. The leaves become spotted where penetration occurs,
and finally dry up and remain hanging by the fungus strands.
The smaller twigs also die. Where two leaves touch the fungus
crosses from one to the other, often making a little knot or tuft
of mycelium which firmly joins the two.
At intervals in the course of the strands along the twigs, and
sometimes on the leaf-stalk, prominent hemispherical sclerotia
are formed. They are creamy white in colour, rather smooth,
and reach 3-4 mm. diameter. They are made up of hyphse
similar to those composing the strands.
Control.
The dependence of the fungus, for its development to an
extent sufficient to be seriously harmful, on what may be regarded
as excessive humidity, suggests pruning and the reduction of
shade as the basis of control. The gathering of infected leaves
and removal of infected twigs may be expected to have some
influence in checking the disease and preventing its spread, but
in the case of trees upon which the infestation is at all general
it is ineffectual for its eradication. Spraying with Bordeaux
STEM AND LEAF^, DISEASES 151
mixture affords the best means of control, but the expense of
periodic application has to be faced as a permanent charge unless
the humidity of the situation can be reduced.
Corticium (Pellicularia) koleroga (Cke.) v. Hohn.
This species occurs on coffee in Porto Rico, Jamaica, Trinidad,
Surinam, Venezuela and Colombia, and has been identified with
a fungus originally described from Mysore, India. As described
by E. A. Burt, the vegetative mycelium forms long, slender
mycehal strands of rather uniform diameter, whitish or pallid
at first, finally fuscous, running along the branches, and the midrib
and veins of the leaves, infesting the leaves and ramifying
between the cells of the leaf parenchyma, finally emerging at
many points on the underside of the leaf to form minute fructi-
fications which give a mottled appearance to the leaf ; fructi-
fications soon laterally confluent into a thin arachnoid, perforate
membrane covering the under surface of the leaf between midrib
and principal veins, drying pale smoke-grey. Spores hyaline, even,
flattened or slightly concave on one side, 10-13 X3I-5 microns.
This species is distinguishable from the preceding by the
absence of sclerotia and the thinner, lighter coloured, more
firmly attached fructifications, which would seem, moreover,
to be much more freely produced.
On coffee in Porto Rico, according to G. L. Fawcett, this
thread blight is common on all plantations except those at
higher altitudes, and is conspicuous from the appearance of the
blackened leaves held suspended by its threads. It reduces
the foliage and often causes the death of twigs. The berries
are also attacked, but the loss from this cause is not important.
The fungus has also been observed in Porto Rico to occur
under favourable circumstances on sour orange. Hibiscus, Croton,
and two wild vines of the genera Luffa and Cucumis. It has
recently been recognised to be rather common in Trinidad on
coffee and " croton " (Codiaeum).
Control.
The information from Porto Rico is that gathering and burning
the diseased leaves gives poor results. Lime-sulphur sprays
have proved ineffective. Bordeaux mixture prepared with an
excess of lime is effective if followed by inspection and where
necessary re-sprapng.
Horsehair Blight.
This is the name given to fungus strands, closely resembling
black horsehair, forming a loose tangle, with occasional attach-
ments, among the leaves and twigs of cacao, nutmeg, and other
trees grown under humid conditions.
It appears from recent investigations by T. Petch that the
152 DISEASES OF CROP-PLANTS
word blight is inappropriate, as the fungus does not enter living
tissue, but derives its nutriment from the materials on which it
occurs only after they are dead.
Horsehair blight occurs on cacao in the West Indies in
Dominica, St. Lucia, Grenada and Trinidad. Similar forms
occur in India and Ceylon on various jungle plants and on tea and
nutmeg, and there are also records from Java, Australia, Africa,
and South America.
Fructifications are occasionally produced from the threads
while they are still on the tree, but more commonly from the leaves
to which they are attached after these have fallen to the ground.
They reveal that the horsehair-like material is a special type of
mycelium pertaining to a group of species of the genus Marasmius.
The West Indian specimens at Kew, according to Petch,
represent the species M. sarmeniosus, Berk. The ideas as to
its parasitism have apparently arisen from confusion with the
effects of thread blight.
Algal Disease, Red Rust.
An algal organism, best known as the cause of the troublesome
affection of tea twigs in India and Java named Red Rust, is
widely distributed and common in the tropics, including appar-
ently all the West Indian islands, occurring especially on the
leaves of numerous trees. The twigs of cacao are occasionally
attacked, and dieback due to this cause is not uncommon in
Trinidad, is recorded from Jamaica and St. Lucia, and probably
occurs to some extent in the other cacao-growing islands. This
form of the disease occurs also on mango both in India and the
West Indies.
Causation.
Cephaleuros mycoidea, Karst. (C. virescens, Kunze, Mycoidea
parasitica, Cunn), the causative organism, is one of a group of
mostly epiphytic algse, and affords an instance of partial para-
sitism, which outside this genus is exceedingly rare in the algse.
The organism is very common on leaves, especially those with
a smooth surface and somewhat leathery texture.
In this type of situation it forms, generally on the upper
surface, orange or rusty red, roundish or less often irregular,
slightly raised patches up to a centimetre in diameter. As a
rule the presence of the alga on leaves causes little or no damage.
In the case of tea, cacao, and mango alike it is in its occurrence
on the twigs that destructive powers may be developed.
The first outward sign of the disease on cacao shoots is the
occurrence during the dry season, on the twigs of the previous
season's growth, of dark purplish or black spots ranging up to
1-2 centimetres in diameter. With the coming of the rains
these take on a rusty-red colour, due to the development of a
Photo by]
Fig. 4()
Algal Disease of Cacao ; Active Stage
STEM AND LEAF DISEASES 153
dense pile of fine hairs bearing tiny globular heads, the sporangia
of the alga. The latter when ripe discharge upon being wetted
numerous biciliate zoospores, which after a short active period
come to rest and are capable of germination. The ripe sporangia
are capable of being broken off and transported by wind before
this discharge occurs.
A second type of sporangium, essentially similar, is formed
in the body of the thallus as it occurs on the surface of leaves.
The alga is itself liable to be invaded by fungus hyphae, the
combination producing for a time a type of structure resembling
that of a lichen.
Nature of the Attack.
On various leaves the alga exists in all conditions between
the common case of mere attachment to the cuticle and complete
penetration leading to the development of thalli (cellular ex-
pansions) on both sides of the leaf. The epidermal cells beneath
an algal patch are usually but not invariably discoloured and
dead. The only importance attributed to the occurrence of the
organism on leaves lies in the provision of material which can in-
fect the stems.
Infection of the twigs appears to take place from germination
of the zoospores in the surface cracks which develop in connection
with the formation of the first layers of bark. The alga pushes
in among the cortical tissues, successive layers of which are
sloughed away. The irritation caused by the presence of the
parasite sometimes results in hypertrophy (abnormal swelling)
of the twigs. In severe cases the separate patches first occurring
on the twig may coalesce and completely envelop it for some
distance.
If growth is not sufficiently vigorous to throw off the infection
by new and deeper bark formation the result is the failure of the
leaves and the death of the twig, and even if this does not occur
the lesions in the bark are liable on weakly trees to afford access
to the dieback fungus, Diplodia.
Influence of External Conditions.
The accounts of the diseases caused by this organism agree
as to the dependence of serious injury on lack of vigour in the
host. On tea the disease occurs suddenly and constantly when-
ever bushes become unhealthy from any cause whatever. On
cacao most harm has been observed where the trees are in poor
condition as a result of exposure or insufficient drainage, or in
young plantations where favourable conditions have not yet
become established.
The reasons for this are believed to lie in the inability of
weakly twigs, as indicated above, to get rid of the parasite, and
in the greater susceptibility to infection of twigs in which bark
154 DISEASES OF CROP-PLANTS
formation is weak and slow. It has been also observed that,
at the other extreme, very quick-growing shoots produced on
tea after severe pruning develop cracks in the bark which are
liable to infection.
Control.
Spraying with Bordeaux mixture carried out early in the dry
season has proved more or less effective against the disease, and
is the most satisfactory method of control yet found. The
means of permanent remedy, adopted with great success in the
case of tea, must be sought in close attention to drainage, culti-
vation, and manuring.
Fig. 47
Algal Disease of Cacao : Later Stage
Section II— Diseases of Permanent Crops
CHAPTER XVII
DISEASES OF CACAO
According to Humboldt the home of the cacao tree is the forest
of the basins of the Orinoco and the Amazon. The evergreen
habit, the large thin leaves, the ability to endure shade and the
intolerance shown towards wind are in accord with a moist forest
habitat though it would be quite unsafe to conclude on this
evidence that such a situation is necessary or even most advan-
tageous for the commercial production of cacao from the tree.
When the factors which most directly affect cacao under
cultivation are distinguished it is seen that no single combina-
tion of them can be regarded as necessary for its well-being, for
within limits a deficiency in one may be compensated for by the
surplus of another.
There can be no doubt that a rainfall regularly distributed
throughout the year best suits the vegetative habit of the tree
and leads to full and rapid development. But where frequent
rain is associated with cloudy weather without a dry and sunny
season the crop will usually be far less than in situations less
favourable to vegetative growth. On the other hand a cloudy
sky may be a great advantage when the rainfall is scanty. Any
prolonged dry period, unless the soil is particularly deep and
retentive, or very well protected, is highly unfavourable.
Cacao is naturally a deep-rooted plant, and, in situations where
soil and subsoil permit this habit to be developed to the full,
it enables the tree to pass with little harm through a dry season
which would defoliate it in less favourable conditions. Deep
draining is thus clearly indicated, in order that full penetration
may not be hindered by subsoil waterlogged in the wet season.
Equally clear is the disadvantage of planting over impenetrable
subsoil where marked periods of dry weather are liable to occur.
In the absence of such periods cacao is grown in Surinam, accord-
ing to C. J. van Hall, in soil i| to 2 feet deep. In the coast
lands of Venezuela the dry season is tided over by irrigation.
While the main roots will penetrate great depths to get in
touch with subsoil water, the feeding roots are largely developed
in the soil proper. Under some circumstances a dense layer of
155
156 DISEASES OF CROP-PLANTS
fibrous roots, known in Grenada as bichet, is formed close to the
surface. Tlie use of vegetable mulches is objected to by some
planters on the ground that it encourages this formation of
feeding roots where they are liable to be killed in dry weather.
The effect of such a loss is probably not so serious as it is thought
to be : some plants, notably the vine, habitually produce feeding
roots of short duration.
These effects of rainfall, of depth of soil, and of dry seasons
all depend on the conservation of moisture, and to some extent
this is capable of being artificially regulated by the traditional
method of growing trees for shade. The question of shade or
no shade is often debated as though it were capable of a single
and universal answer, whereas it depends entirely on local, and
sometimes very narrowly local, conditions.
The control of the various pathological conditions arising
from lack of vegetative vigour on the one hand and from infec-
tions favoured by too great humidity on the other, is largely
dependent upon the adjustment of shade conditions, so that it is
relevant to consider the subject somewhat closely.
A correct estimation of the factors involved and their effects
requires systematic physiological studies which so far as the
writer knows have never been made. In default of such studies
the indications obtained from empirical observation and from
analogy must be accepted.
That shade in some circumstances exercises a most beneficial
and even essential influence is certain. The most essential fact,
which is quite clear, is that the value of shade depends on its
effect in remedying otherwise defective conditions and thus
extending the range of situations within which cacao can be
successfully grown. Where the natural conditions are in all
respects good the introduction of shade is a definite disadvantage ;
it is generally admitted that where shade can be dispensed with
the trees bear more heavily. With soils of equal quality the
estates which have least need of shade are those situated in
sheltered humid situations, with no strongly marked dry season
— conditions found in the neighbourhood of hills and forest —
while the need becomes more and more imperative with the
increase of exposure and of inequality in the distribution of
rainfall. Contributory differences in the same direction are seen
in the transition from deep to shallow soils and from strong
retentive soils to those which are light and very permeable.
The most convincing case of dependence on shade known to the
writer is that of plantations on soils of the last-named character
in St. Vincent. In spite of deep roots and of a high rainfall, the
trees are poorly nourished, and the response they make to pen
manure suggests that lack of organic matter, due to the non-
retentive nature of the soil, is largely the cause of this. The
provision of effective shade very distinctly increases the vigour
of the trees.
DISEASES OF CACAO 157
While the writer believes that the retention of humidity is
an important contributory effect, he is disposed to agree with
van Hall in the view that it is the protection of the soil, and not of
the tree, that is the most important function of shade trees.
The most pressing anxiety of the cacao planter— be his fields
with or without shade — is to fill up with quick-growing leafy
plants any gaps which may occur in the canopy. The rapid
loss of humus which under tropical conditions takes place in
exposed soil is well known. This is avoided over a large part of
the year by the provision of shade, and a longer period of activity
is permitted to the biological processes in the soil which lead to
the nutrition of the plant. It is noteworthy that where no shade
trees are grown, as on many estates in Grenada, their absence is
compensated for by close planting and the use of large quantities
of pen manure.
There is another well-recognised requirement of cacao, that
of shelter from wind. Its effects are related to those of shade,
and shade trees to some extent perform its function. It cannot
be dispensed with, and if it is not provided by natural conditions
must be artificially supplied. Its influence, without much
question, comes from the preservation of a humid atmosphere
about the trees.
DiEBACK.
The term dieback in connection with cacao has been generally
used in connection with the effects ascribed to the fungus Diplodia
theohromce, which there is little room for doubt often kills back
the twigs and not infrequently large branches of trees when these
are for some reason not in a vigorous condition. Since in practice
it is impossible to draw a line between the cases of failure directly
brought about by adverse conditions of growth, in which this
fungus almost invariably appears sooner or later, and those cases
in which the fungus plays a more active part, the term may be
usefully extended to cover the general type of failure it implies.
The cacao tree is by its nature suited to humid conditions,
to a considerable depth of soil, and a fairly abundant supply of
humus. To secure these conditions a cacao field is usually so
managed as to approximate to forest conditions in respect of
atmospheric humidity, shading of the ground, and supply of
organic material. This is usually accomplished by interplanting
with shade trees or, where trees are dispensed with, by taking
advantage of natural shelter, by close planting, and by the
liberal use of mulch and pen manure. A fairly high and regular
rainfall is required to maintain the necessary humidity, and where
the rainfall is on the margin of adequacy the means of conserva-
tion referred to require to be the more perfect. On light, well-
drained soil, again, more protection is required than on soils which
are heavy and retentive.
158 DISEASES OF CROP-PLANTS
When the conditions fall short of any of the requirements
outlined the effect is to induce in the tree a more or less severe
manifestation of the condition referred to above as dieback.
The lightest form is shown only in diminished size and harden-
ing of the leaves. Next comes the condition in which the out-
standing twigs lose their leaves and dry off, new foliage being
produced later from short shoots which develop further back.
Trees in this state are familiar objects and present a more or less
dense head of foliage with numerous twigs projecting above it.
In yet more severe cases the dying back extends to the larger
branches and may gradually involve them all. At every stage
in this retrogression any temporary improvement in the con-
ditions stops the process and new shoots are sent out behind the
margin of injury, but with the recurrence of unfavourable con
ditions the process may continue until the tree is wholly dead.
Some cases of root disease have occurred, in trees un-
favourably circumstanced, in which Diplodia has been the only
fungus detected.
Such failing trees, and trees in which the condition is as yet
only incipient, are particularly liable to severe attacks of the
cacao thrips, which contribute materially to their decline. There
are good grounds for believing that it is only on trees lacking
in vigour that infestations of thrips persist and do more than
passing harm.
The part played by the fungus is impossible to define as it
varies with every circumstance. It is safe to say that dieback
would take place under appropriate conditions if the fungus
were entirely absent, for this only means that the range of con-
ditions to which the tree can successfully adapt itself is limited ;
but with the fungus universally present as it is there is no doubt
that it often takes a definite part in the process. This is especially
seen when its mycelium coming down a smaller branch which is
dead or dying enters and infests the wood of the larger branch
to which it joins, cutting off the water supply and causing the
death of that part of the branch above the junction. It may be
said in general that the effect of the fungus when it has obtained
a footing on the dead twigs is to extend the range of the injury,
and give it a progressive nature in weakly resisting twigs or
branches which might otherwise have remained for the time
being alive. Where the conditions which induce the attack
remain unameliorated the result in the end is much the same,
but where they are temporary in their nature or exceptional the
action of the fungus may greatly increase the resulting loss of
branches.
Associated Fungus.
Diplodia Theobromce (Pat.) : Fructifications consisting of black,
carbonaceous, more or less rounded pycnidia immersed in the cortex ;
single or combined in groups or sometimes in a stroma ; pilose when grown
Fig. 4S
DiPLODiA Rot ok Cacao Pod
DISEASES OF CACAO 159
in moist air; 0.2-04. mm. in diameter. Spores on short simple sporo-
phores accompanied by paraphyses ; ovate or ellipsoid ; when young,
one-celled, hyaline, with granular contents, and often extruded in this
condition ; when ripe, uniseptate. slaty- or brownish-black, often longi-
tudinally striate, usually 25-30 x 12-15 microns.
Mycelium septate, abundantly developed in the wood, hyaline when
young, becoming successively grey, slaty- and brownish-black. Commonly
gives to infested wood a slaty-grey tint.
The spores germinate in water whether in the one-celled or two-celled
condition.
Synonyms: Botryodiplodia Theobromce Pat. 1892.
Macrophoma vesHta Prill, et Del. 1894.
Diplodia cacaoicola P. Henn. 1895.
Lasiodtplodia nigra App. et L.iub. 1906.
Botryodiplodia elasticoe Petch 1906.
Chaelodiplodia grisea Fetch 1906.
Lasiodiplodia Theobromct Griff, et Maub. 1909.
Diplodia rapax Massee 1910.
The fungus is very widely distributed and common on a
wide range of vegetable material. It is regarded as the cause of
one type of cacao pod-rot and occurs as a semi-parasite in ripe
sugar-cane. The same or closely similar species cause rots of
citrus fruits and cotton bolls in these islands. A species not cer-
tainly distinct is associated with the dieback of lime trees.
The cacao species affects various other trees in a manner
similar to that described above in reference to cacao, and in
this way has come into prominence on Hevea in Ceylon and
Malaya. It appears to be generally distributed throughout the
tropical world, having been recorded from North, South, and
Central America, the West Indies, Tropical Africa, Ceylon,
Malaya, and the East Indies. It has been described under very
many names, and it is still in some degree a matter of choice as
to which shall be accepted. The genus Diplodia covers a large
and ill-differentiated group of fungi which form sunken pycnidia
containing two-celled dark spores, and has been subdivided by
splitting off new genera under the names Lasiodiplodia, Botryo-
diplodia, etc., based on the grouping of the pycnidia and the
nature of their surface. The species under notice varies in these
characters and has been placed in one genus or another according
to the state of the actual specimens examined. The first descrip-
tion assignable to it seems to be that of Patouillard, who called the
fungus Botryodiplodia Theobromce. Later P. Hennings described
it again under the names of Diplodia cacaoicola, by which in
consequence of the writings of Howard it became generally
known in the West Indies. Bancroft in 1911 claimed to have
found the perithecial fructification and described it under the
name Thyridaria tarda, which according to rule should rep ace
any name given to the pycnidial form. The connection, which was
not fully demonstrated, lacks confirmation and the name has
not been generally adopted. The fungus is perhaps most :om-
monly referred to at the present time as Lasiodiplodia Theobromce ,
but it is evident that this genus is not based on constant characters
i6o DISEASES OF CROP-PLANTS
and the combination Diplodia Theobromce would seem to be the
soundest to use until a better subdivision of the genus is worked
out.
Control.
The symptoms of dieback often appear in a group of trees or
along the borders of a field, and occasionally in individual trees.
It will be obvious from the foregoing account that for purposes of
control it is first of all necessary to form some idea as to which
of the many causes capable of bringing about this condition is
responsible in the particular case. Where a group of trees is
affected, especially if the trees around its margin are progressiv^ely
involved, search should be made for the usually well-marked
evidence of Rosellinia root disease. If this is not found the
adequacy of the drainage and aeration on heavy soils should be
next questioned. On light soils the conditions as to exposure
both lateral and vertical should be considered in relation with the
amount of rainfall received and the retention of sufficient moisture
in the soil during periods of dry weather. If deficiencies in these
latter respects are suggested by the circumstances they may be
met by closing in the edges of the cultivation with quick-growing
screens, for which purpose Aralia Guilfoylei has proved very
suitable, by providing overhead shade, which may be most
rapidly effected by putting in large cuttings of Gliricidia maculata
(madura) and by the addition of pen manure or heavy vegetable
mulch to the soil. A great deal of dieback, especially in peasant
holdings, is directly due to neglect of reasonable measures of
cultivation and manuring. The same is true of some neglected
estates, while another cause is to be found in injudicious removal
of shade. Cacao trees are sometimes planted where the soil is
too shallow to maintain them, and the subsoil too dense for root
penetration, in which case the trees reach in a few years the limits
of their development and begin to fail.
In an affection such as this, which may be due to any of the
numerous causes which react unfavourably on a tree which is
tolerant only of a distinctly limited range of conditions, it is
sometimes very difficult to find the cause of an unthrifty con-
dition. The measures suggested above are all along the lines
of good agricultural practice, and will not be wasted if they are
applied in turn until the directly appropriate measure is dis-
covered.
Diplodia Pod-rot.
The rot of pods brought about by the fungus Diplodia
TheobromcB has commonly been known in the literature referring
to the West Indies as the brown pod-rot, an unfortunate choice
of name, since the Phytophthora pod-rot in its early stage
colours most pods attacked by it a clear brown, and the antbrac-
Photo by]
\J. B. Rorer
Photo by]
[J. B. Rorer
Fig. 49
Cacao Canker
Produced bv Inoculation
Fig. 50
Cacao Pod-Rot : Tip Infection
DISEASES OF CACAO i6i
nose fungus produces a similar effect. There can be little doubt
from the present aspect of affairs that a great deal of damage
which has been attributed to Diplodia was really due to Phytoph-
thora, which is not always conspicuous on the pods it destroys
and is quickly succeeded by saprophytes.
On the other hand Howard and succeeding investigators
have found Diplodia capable of destroying pods to which it has
gained access by way of injuries and it very commonly attacks
pods which are over-ripe or have been left some time after
being picked. It is usually abundant on heaps of cacao shells.
This role agrees with that which it takes in connection with
sugar-cane and citrus fruits.
History and Distribution.
A. Howard, writing in 1901-2, regarded the pod-rot met with
in Grenada, Dominica, and St. Lucia as due to the fungus under
notice, and described it accordingly. He failed to recognise
the Phytophthora rot, believing it to be more or less confined
to Trinidad. He secured artificial infections which quickly
rotted the pods, but only through wounds. Van Hall and Drost,
who conducted infection experiments in Surinam, found the
fungus incapable of infecting sound, healthy pods, but able
to cause the decay of picked pods or pods which by some means
or other have been wounded or have already been attacked by
other fungi. With this statement Petch entirely agrees as
regards Ceylon experience, and in Trinidad Rorer has appeared
to attach little importance to the disease. It is recorded each
year from Grenada, St. Lucia and Dominica, but in view of the
difficulties of diagnosis it is uncertain how much reliance can
be placed on the differentiation between this and other pod-rots.
Cailsative Fungus.
This is described in connection with cacao dieback.
Symptoms.
The absence of the fructifications of the fungi described
under Phytophthora pod-rot and anthracnose, and the presence
from an early stage of the very different ones of Diplodia afford
the best evidence of this affection. Small pustules, densely
distributed, appear below the brown epidermis of the pod and
raise small projections which cause the surface to be very dis-
tinctly rough. From each pustule as it ripens is emitted a string
of spores which are white, grey or black according to the stage
of development they have reached. The white unripened spores
are one-celled, the black ripe spores are divided by a single
septum. Both forms are capable of immediate germination.
In damp air they hold together when emitted and form a tendril,
when dry they form little powdery heaps. The whole pod is
i62 DISEASES OF CROP-PLANTS
penetrated by an abundant dark slaty- or brownish-grey mycel-
ium, which envelops and quickly destroys the beans.
Control.
The main points in the prevention of loss from this affection
are indicated by the conditions of attack, i.e., avoidance of
wounds (most of which are made by the pickers) and prompt
collection of ripe or injured pods. The fungus is so abundant
and the materials for its development so varied that attempts
to avoid infection, once a favourable opening exists, seem to a
mvcologist rather hopeless. Against this has to be set, however,
the fact that experienced agricultural officers in close touch
with local conditions attribute visibly good results to sanitary
disposal of husks and deplore any looseness in this matter. A
further means of reducing the amount of infective material
lies in the regular removal of dead and sickly branches from the
trees.
Phytophthora Pod-rot and Canker.
It is regarded as established that the common types of
pod-rot and stem canker are due to the attacks of the same
fungus, and that there is in many cases a connection between
them; they may therefore be most conveniently discussed
together.
Distribution.
Apparently these diseases occur in all cacao-growing coun-
tries, though there are very considerable differences in their
incidence. In the Western Tropics and in Ceylon both canker
and pod-rot are prevalent, but pod-rot is the more prominent
trouble, while in Java canker is a very serious disease and pod-rot
relatively uncommon. These differences have not been ex-
plained, but may possibly be accounted for by the varying
resistance of different types of tree and by the close dependence
of the fungus on humidity. The loss of ripe pods during the
rainy season in Trinidad is estimated by J. B. Rorer at 30 to 60
per cent. There are also heavy losses of young pods, as is shown
by the increase obtained from spraying, and, further, the injuries
to the trees themselves are a matter of serious consideration.
History.
The existence in Ceylon of a disease of the nature of canker
was recorded in 1833. At a much earlier date, in 1727, the
plantations in Trinidad were crippled by an epidemic of blight
on the pods, which more likely than not was due to the affection
under notice. The industry did not recover until the Criollo
then grown was replaced by the more hardy Forastero 30 years
later. In recent times J. B. Harrison called attention to pod-rot
Fig. 51
Cacao Pod-rot produced by Inoculating Stem
DISEASES OF GACAO
Drawn by]
Fig. 52
Phytophthora Faberi
i64 DISEASES OF CROP-PLANTS
in Grenada in 1895, and in 1897 there was a serious outbreak
of canker on Criollo in Ceylon. The examination by G. Massee of
material sent to Kew led, as in the case of the West Indian
sugar-cane epidemic, to a wrong diagnosis, and the researches of
J. B. Carruthers, who was engaged through the action of the
Ceylon Planters' Association, were inconclusive as to the causative
organism. In 1899 the fungus associated with pod-rot in
Trinidad was identified by Massee as Phytophihora omnivora
De B., and shown by J. H. Hart to be capable of producing the
affection. The cause of canker remained much longer in doubt.
It was attributed by a succession of mycologists to various
fungi, mostly to the Nectrias, or their conidial stages, which
quickly appear on diseased material. In 1910 J. B. Rorer in
Trinidad proved by inoculation experiments that the Phytoph-
thora from the pods could give rise to typical canker, a discovery
which was confirmed, and extended to the similar disease on
Hevea, by T. Petch in Ceylon. Rorer also called attention
to the susceptibility of the soft tissues of the stem shoots to
attack by the same fungus, giving rise to an affection named by
him chupon-wilt. The identity of the parasite was later decided
as Phytophihora Faheri Maubl.
Symptoms, (i) Phytophthora pod-rot.
The fungus is responsible for the loss of pods at all ayes,
without the necessity for any preliminary injury. The cacao
tree normally sets a good many more fruits than it proceeds
to develop, and in this way the loss of young pods by disease is
usually masked. On large pods the course of the disease is
plain to see. Usually starting at either the tip or the stem
end as a discoloured spot, a browning of the surface (or
blackening in the case of dark red pods), with a clear line of
demarcation, spreads along the pod until it is all covered. The
rot involves the whole of the fleshy tissues of the husk and
extends to the pulp and the contained seeds. Only in pods
approaching ripeness in which the seeds are no longer in close
contact with the shell can they remain for a time unattacked. •
The fungus appears on the surface of the pod as a whitish
down which later may develop into a white mould-like covering
on which masses of the sporangia accumulate. The pod quickly
shrivels and blackens, and at this stage is usually conspicuously
infested with secondary fungi.
(2) Canker.
As canker arises from the infestation of a patch of the living
tissues of the stem, it is difficult to detect in its earlier stages
owing to its effects being hidden by the outer bark. The occur-
rence of a succession of diseased pods may give rise to the first
suspicion of its presence. Sometimes it can be detected by the
presence of a dark patch on the bark, and at a later stage there
DISEASES OF CACAO 165
is commonly a discharge, from cracks or borer holes in the
diseased bark, of a reddish fluid which dries to a rusty deposit.
When the outer bark is shaved off the diseased tissue can be
detected by the difference of its colour from that of the healthy
tissue surrounding it, which further is commonly marked off by a
dark brown line. The colour of the affected tissue may be
brownish or claret-coloured. With the latter indication, which is
the one commonly referred to, the deep pink colour normally
present in some trees is liable to be confused.
The adjacent outer layers of the wood are usually more or less
discoloured, and narrow dark lines may extend for a considerable
distance from the point of origin.
The injury to the tree is consequent upon the killing of the
patches of bark, and its severity is determined by the size and
position of these. The main stem or a branch may be girdled and
killed outright above the situation of the lesion ; the interference
with the functions of the bark may cause either local defoliation
and death of twigs or a general sickly appearance ; or the effects
may remain localized in the bark and natural recovery occur.
(3) Chupon Wilt.
The affection on the suckers is thus described by Rorer :
" the chupon is generally first attacked in the soft tissue near the
tip. A small water-soaked area can be seen on the stem which
gradually becomes sunken and darker in colour and spreads up
and down the stem, frequently girdling the shoot and causing
the upper part to wilt. The same disease has been observed on
young shoots on the upper branches of the tree. The point
of attack is generally in the axil of a leaf, though the leaf blade
or petiole may be the first part affected, the disease afterwards
running down into the stem."
The Fungus.
The causative fungus is Phytophthora Faheri, Maublanc (1909) .
According to the original description the conidiophores are 150-200
microns in length, non-septate, hyaline and terminated by an apical
conidium, more rarely branching and bearing two conidia ; conidia
(sporangia) of various forms, generally lemon-shaped, with a thin smooth
membrane, a little thickened at the apex, 30-80 x 25-42 microns ; oospores
rounded, smooth, with a thick membrane, 45 microns in diameter. Rorer
gives the most usual dimensions of the conidia as 30-50 X 25-27 microns,
the number of zoospores as 15—30, the diameter of chlamydospores as
30—50 microns, oospores 33-40 microns. There is no swelling of the
sporophores below the conidia.
J. Rosenbaum concluded after study of the bodies classed as oospores
(from which antheridia are always absent) that they are properly to be
regarded as multinucleate vegetative bodies with the function of chlamy-
dospores. He found their greatest frequency of diameter to be 40 microns.
The fructifications of the fungus as seen on the pods consist
of a whitish down appearing behind the margin of the brown
i66 DISEASES OF CROP-PLANTS
discoloration of the surface, made up of slender stalks bearing
the minute oval conidia. These when produced in mass form a
white deposit. Fresh conidia continue to form on the stalks
if the air is humid, and a felted white covering may be formed
over the whole or part of the pod. The thick-walled resistant
spores are formed in this layer and in the underlying tissues.
Infection.
The most abundant, but least resistant, means of dispersal
is afforded by the conidia, which may be washed by water or
blown by wind on to other pods. If they come to lie in a film or
drop of water they function as sporangia, producing lo to 30
zoospores, which swim about by means of the two cilia with which
they are provided. The zoospore settles down and produces a
germ tube which penetrates the epidermis either directly or by
way of a stoma. The whole process occupies only an hour or two.
If the surface is merely damp with no free water present the
conidium produces one or more germ-tubes without an inter-
mediate zoospore stage.
The thick-waUed spores are set free by the decay of the pod
and are capable of remaining dormant in the soil or elsewhere
until conditions are favourable for their germination. This takes
place by the production of a short germ-tube bearing a conidium
at its top from which infection may ensue as described above.
By these means pods on a healthy tree may become infected.
How far or under what circumstances direct infections of the
bark, producing canker, may occur does not seem to be known.
It is, however, well-established that diseased pods commonly
infect the stem on which they are borne, giving rise to canker,
and, vice versa, that pods borne on cankered trees may become
infected from the stem.
According to Rorer's experiments, " when the tissue of the
pod at the stem end becomes well invaded by the fungus, the
mycelium runs back into the cushion either through the bark
or the central woody cylinder of the stem of the pod, or through
both. If it has gained entrance to the tree through the thin
bark layer of the pod stem it generally spreads out in all direc-
tions so that the whole cushion and surrounding area of bark
becomes cankered ; but, on the other hand, if the mycelium
grows into the tree through the woody part of the pod stem the
cushion may not become cankered. The mycelium will extend
in narrow lines for some distance up or down the stem in the
cambium layer but sooner or later wiU grow outward into the
bark and then spread out rapidly, forming larger cankered areas."
The canker spots are quickly invaded by secondary fungi
which occupy the tissues and complete their decay.
Influence of External Conditions
The parasite under discussion belongs to a group of fungi
DISEASES OF CACAO 167
particularly dependent on humid conditions, consequently the
diseases due to it are more prevalent in the rainy season and
on trees with a heavy canopy, whether of their own foliage or
that of shade trees. The conditions favouring the fungus are,
up to a certain point, the conditions which favour vigorous
growth of the trees, and a balance has to be struck between the
losses and gains which may ensue from pruning and the reduction
of shade and shelter. To one operation the antithesis does not
apply : the provision of abundant deep drainage is beneficial
in both directions.
Inherent Resistance,
The Criollo types are very susceptible to canker, and it is
due to this fact and to their generally less hardy nature that they
have mostly been replaced by the much more resistant Forastero
and Calabacillo. Alligator cacao {Theobroma pentagona) grown
in the Dominica Botanic Gardens proved to be so highly sus-
ceptible to canker as to discourage any attempt to use it com-
mercially.
The suggestion that the susceptibility of Criollo is accounted
for by the influence of the deeper shade under which it is grown
would appear to be disposed of by the fact that in the manurial
plots in Dominica Forastero and Calabacillo thrive, whereas
under the same conditions the Criollo first planted was attacked
by canker when 8-12 years old, and has been almost entirely
killed out. The same conclusion may be drawn from the observa-
tion that in grafted plants canker on a susceptible scion is sharply
arrested when it reaches a resistant stock.
Control.
Beyond the general effects of the measures already indicated
the control of pod-rot and of canker infections following pod-rot
depends upon systematic spraying with Bordeaux mixture.
Rorer has conducted large-scale experiments with this treatment
for several years and has shown that in Trinidad the protection
given to the young pods by spraying soon after a crop has set
greatly increases the number of pods which come to maturity,
while this and a subsequent application 4-6 weeks later during
the development of the pods greatly reduces the amount of
" black cacao " from diseased pods present in the crop. The
results have been distinctly profitable from a commercial point
of view, without taking into account the protection of the trees
from canker. No hard and fast rules can be laid down as to
times of spraying, they depend on the times of heavy setting and,
as to the commercial return, on the extent to which losses occur.
By combining nicotine sulphate with the Bordeaux mixture the
spraying is made effective against thrips. The amount of
4-4-50 Bordeaux to be used per tree is estimated at i gallon
and the number of trees sprayed per man per day at 100. The
i68 DISEASES OF CROP-PLANTS
cost per 100 trees, with materials at pre-war prices, was about
$10 per application. An addition of nearly 7,000 pods per
1,000 trees, or 635 lb. dry cacao, has been obtained from sprayed
as compared with unsprayed trees.
Van Hall has issued striking figures as to the success attained
in the direct treatment of canker on a large and badly infested
plantation of Criollo in Java. The practice of deep excision and
treatment with carbolineum and tar having failed to arrest
the losses, a change was made to a policy comprising (a) reduction
of the heavy canopy by thorough pruning ; {b) treatment of the
cankers by merely shaving off the outer bark to allow the infested
tissues to dry out, the use of tar being avoided as preventing
this consummation ; and (c) the more careful control of boring
beetles, the tunnels of which afford openings for infection.
By these means the number of trees attacked was reduced from
4,400 in the first to 318 in the third year, and the annual loss
of trees from 5.4 to 0.7 per cent.
Rorer has pointed out that the collection and destruction
of diseased pods, on which a good deal of stress has been laid as
a sanitary measure, cannot be held to have much effect on the
prevalence of this disease, and has been given up as unremunera-
tive by planters who have tried it. There is an early and enor-
mous production of spores from the affected pods while they are
on the tree which is quite sufficient to maintain an epidemic,
and the only feasible means of prevention during its continuance
is the protection of the healthy pods by spraying. On fallen pods
and husks the parasite seems unable to maintain itself against
the competition of numerous fungi more efficient as saprophytes.
In this respect it differs from Diplodia. It should be noted
however in this connection that marked infestations often exist
on the trees in the neighbourhood of the sites to which the pods
are brought to be broken.
The Surinam Witch-Broom Disease.
The principal feature of this disease is the hypertrophy of
young shoots, which assume a monstrous form and shortly die.
To this feature are also added (i) the production of hardened
(indurated) and malformed pods, which either fail to come to
maturity or have a part of their contents spoiled, and (2) star-
blooms, which are crowded blossoms developed on hypertrophied
cushions and usually resulting only in a few misshapen pods.
History and Distribution.
The disease was first reported in the Saramacca district of
Surinam, about 1895. It spread by degrees through the colony
and caused an enormous amount of damage. The average
annual export of cacao for the 5 years 1893-7 was 3^ million
kilos, that for the five years igo8-i2 was reduced to i^ million.
Fig. 53
Surinam Witch Broom Disease
[J.C. Rore
DISEASES OF CACAO
169
The disease now occurs throughout Surinam, and has ex-
tended to the adjacent parts of British Guiana. It has not been
recorded outside of the South American continent. A wild
After van Hall and Drosi
Fig. 64
A Typical Witch Broom
Theobroma {T. speciosum) which occurs in the forests of southern
Surinam is very Hable to the disease and is thought to have
communicated it to the cacao.
Ritzema Bos (1898, 1899), Massee (1901) and Howard (1901)
examined preserved material. The first-named found traces
170 DISEASES OF CROP-PLANTS
of what he regarded as a fungus of the type associated with
some forms of witches-brooms in Europe and described it as
Exoascus TheohromcB. Massee did not find this fungus present,
and Howard observed only Fusarium spores. Went was the
first mycologist to see fresh material. He found a mycelium
in the tissues which did not fruit in cultures or reproduce the
disease from inoculations. Van Hall and Drost (1907) made an
elaborate study of the disease and concluded that it was due
to CoUetotrichum luxificum, which appears abundantly on the
affected shoots and indurated pods ; inoculations failed.
Rorer (1913) pointed out that C. luxificum is common in
Trinidad although the disease is absent, and, further, that the
mycelium in the affected shoots has clamp-connections which are
characteristic of the Basidiomycetes. Inoculations with the
mycelium were unsuccessful.
G. Stahel (1915) confirmed Rorer's findings, and examining
the old witch-brooms for fructifications, found those of a Maras-
mius commonly present. The spores from this fungus allowed
to fall on buds enclosed in test-tubes gave rise to the disease in
{a) 14 out of 46, {h) 7 out of 9, (c) 9 out of 10 cases, and infections
were produced on a plantation previously free from the disease.
Mycelium in pure cultures from the Marasmius spores agrees
with that found in the shoots. The indurated pods contain
an exactly similar mycelium and also bear the Marasmius fructi-
fications, and both these and the diseased cushions have been
readily produced by infection with Marasmius spores.
Nature of the Disease.
Van Hall and Drost's description of the appearance of a
hypertrophied twig (witch-broom or krulloot) is as follows
(Fredholm's translation) : " It is generally two to six times as
thick as a healthy twig, its surface is rough and somewhat
furrowed, its base turgid, at times exhibiting longitudinal cor-
rugations. The leaves do not become fully developed, but
remain soft and flimsy like recently unfolded leaves on sound
twigs ; often they are of a darker colour. Among other char-
acteristics should be mentioned the bending over of axillary
buds, even before the witches-brooms are fully developed,
the strong tendency to produce side-shoots, the retention of the
supporting leaves, the growth of the shoots in a vertical direction
and their short duration."
The shoot never becomes woody ; it grows quickly, and in
three to six weeks it dries up from the base. Both axillary and
terminal buds and flowering shoots are liable to the affection.
The bud infection does not in all cases involve the growing
point, in which event the top of the shoot may develop normally.
The indurated pods, which always occur in association with
witch-brooms, are of three types : (i) in which the principal
symptom is the hypertrophy of the stalk — these arise from
Fir;. 55
Surinam Witch Broom Disease
J I'.Ro
DISEASES OF CACAO 171
infected cushions ; (2) in which there is hypertrophy of the pod
itself, producing humps or other malformations ; such pods
never ripen, but drop as a rule before they are half-grown ;
these arise from the infection of young pods ; (3) in which black
blotches appear on pods beginning to mature, which have been
previously unaffected ; the damage consequent on these spots
varies according to humidity ; there is a tendency for the affected
beans to degenerate into a mucilaginous mass, in which the
remainder may begin to germinate ; the apparently sound
beans obtained from such blackened pods are light and not
fully ripe.
Effects of the Disease.
It does not appear that the direct effects of the development
of witch-brooms are particularly severe. Each, of course,
involves the loss of a shoot, and in the extreme cases in which
the tree is infested with the growths this results in partial de-
foliation. Wlien cut back, however, the trees exhibit consider-
able vigour in the production of new growth. The dead witch-
brooms are liable to become infested with Diplodia, which then
gains access to the branches. Canker-like diseased areas fre-
quently form where they have been attached. From one cause
or another many trees do succumb, and supplies are generally
quickly attacked and prevented from developing.
The cause of diminished production is mainly to be sought in
the infection of the flower cushions and the fruits.
Causative Fungus.
The fungus has been described by Stahel as a new species
under the name Marasmius perniciosus, as follows : —
Mycelium intercellular on the living plant ; entering the cells and
forming abundant clamp-connections oniy in dead tissue ; when externally
developed turns lemon-yellow in a few days and later may show dark
crimson patches.
Fructifications on their first appearance spherical, somewhat pointed.
Cap successively bell -shaped, fiat parasol-shaped, and frequently, when
old, concave above ; av. diam. 5-15 mm., max. 25 mm., thin, somewhat
fleshy. Primary gills 8-20 in number (avar. 15), corresponding with furrows
on the upper surface, 1-15 mm. wide and about 0.2 mm. thick. Upper
surface of cap tinted crimson, usually only faintly, with a dark-red central
spot surrounded by rays of the same colour.
Stalk white to lemon-yellow according to age, hollow, 5—10 mm. in
length ; base swollen, mealy or scaly, light lemon-yellow changing to dark
brown or brownish red ; arising from a hyphal mass, not from rhizomorphic
strands.
Spores 4-5 X 10-11 mic, pure white in mass ; germinate in rain water
after 30-40 minutes ; killed by one hour's desiccation (if spread out in
a thin layer).
Occurs in damp weather on hypertrophied shoots and indurated pods
of cacao in Surinam.
172 DISEASES OF CROP-PLANTS
The fructifications, small white toadstools closely resembling
those of Marasmius Sacchari, appear in rainy weather on the
blackened witch-brooms while still on the tree and also after
they have fallen, on the indurated pods, especially small ones
which have dried up on the tree, and on infected cushions.
They have only been found on dead twigs of normal form when
these came from the incompletely affected buds before mentioned.
They have not been recorded from any other plant, living or
dead. Dried-up fructifications return to their natural form
when moistened and resume spore-formation.
Mode of Infection.
The witch-brooms are produced from buds infected by the
spores of the fungus, which, being produced in large numbers
from fructifications on the trees, can readily fall or be carried by
air-currents into situations favourable for development. The
pods are infected directly by way of the stomata, or indirectly
from infected cushions.
The spores readily lose their vitality on drying, and it is found
that unshaded cacao is very much less susceptible to the disease.
There is no knowledge of any varietal resistance, and both
Theobroma hicolor and T. speciosum are susceptible to infection.
Control.
The recommendations for control based on the discovery of
Marasmius as the cause of the disease comprise the following . —
(i) Admission of more sun and air by pruning and by reducing
shade trees. (2) Repeated and frequent clearing of the trees
of witch-brooms and other diseased material, including infested
cushions, all of which must be effectively disposed of by burning
or burying so that the fungus cannot fruit. (3) Spraying with
Bordeaux mixture once or twice annually as the season of
growth approaches has been recommended, but is not found to
be necessary for satisfactory control.
The method of cleaning up the trees by drastic cutting back
of the branches before spraying is more quickly effective, but
this advantage does not balance the loss of more than a year's
crop which it involves.
MoNiLiA Pod-Rot.
The Monilia Pod-Rot or Watery Disease has in recent years
become prevalent to a serious extent in some districts of Ecuador.
A general account of it was given by J. B. Rorer in 1918 ; no
technical description has yet appeared. The disease is doubtless
indigenous to Ecuador, where it is found on two wild Theobroma
species, and has not been recognised elsewhere. It is especially
present in localities with a large rainfall and under such con-
ditions may destroy almost the entire crop. In drier districts
much less damage is caused.
DISEASES OF CACAO 173
Symptoms.
The first positive symptoms are manifested on pods about
8-10 cm. in length, and may take the form of a small protuberance
or a certain amount of discoloration. When the pods which
show these external signs are cut open it is observed that dark
brown veins are present in various parts of the interior. In
more developed pods the external signs of the disease may be
slight, there may be one or more dark spots on the surface, or
they may be more or less completely invested with a whitish
growth of the fungus.
In the first case the pods show abnormality in form or appear
somewhat discoloured, presenting areas of a bluish or yellowish
colour as if premature maturity were being attained. In the
second form the diseased pods display on their surface dark grey
spots of different sizes from 0.5 to 3 or 4 cm. in diameter. In the
third form the pods from an early stage are partly or completely
covered with a dense and shaggy growth of the fungus 1-2 mm.
thick, white on the margin and slightly yellow or ashen in the
centre and bearing an abundance of conidia.
When fully diseased pods are cut open it is observed that the
interior of the husk, the central tissues, the pulp, and the seeds
have become enveloped in watery matter and in some cases a
quantitj' of water can be drawn off from the pod. The contents
of the pod are found surrounded by a dark brown coating, which
carpets also the interior partitions of the husk. The husk itself,
when cut, exhibits black or dark brown strands or spots in its
tissues and sometimes small watery cavities or areas surrounded
by a definite dark-brown boundary. In addition there may be
areas which present a striped or mottled appearance.
Observations on diseased pods have indicated that infection
can only occur in their younger stages.
The causative fungus is a species of Monilia probably undes-
cribed. Monilia fructigena is well known as the cause of the
wide-spread brown rot of plums and other rosaceous fruits in tem-
perate countries.
Sph^eonema Black Spot and Bark Rot.
Two affections previously undescribed, caused by a fungus
of the genus Sphaeronema, have been found by J. B. Rorer to be
generally distributed in Ecuador. Like Phytophthora, the fungus
produces a disease of the pods — black spot — and a bark rot which
is so closely associated with wounds from the too ready cutlass
of the labourer that Rorer has given it the name Enfermedad
del Machete.
Black Spot.
This affection first declares itself on pods which are almost
ripe. One or more dark spots appear on the surface, and gradu-
174 DISEASES OF CROP-PLANTS
ally increase in size up to diameters of 2-4 cm. In the centre of
the spot the mycelium occurs in the form of a very white, almost
shining network, and sooner or later there appear a large number
of slender vertical black stalks about a millimetre in height, each
terminating in a minute white pearl-like head of spores.
When a pod so attacked is cut open an appearance of dry-rot
is seen, and such pods are always lighter than normal. Occas
ionally the seeds germinate inside the pod. The appearance of
the spots within the tissues is very dark, and not striped or
marbled as in the case of Monilia attack. The damage is much
less than is caused by the disease mentioned, not all of the seeds
are affected, and owing to the late appearance of the spots it is
possible to save the pods by timely reaping.
Bark Rot.
Examples of the bark infection are stated to exist on almost
aU the cacao trees in Ecuador. The results are much less serious
on the Nacional variety than on the Venezuela, of which thousands
of trees are said to have been destroyed. The fungus is unable to
attack unwounded bark, unless it be that of a tender twig or
shoot, but occurs in practically every cutlass wound made.
The external surface of the bark around the cut becomes
discoloured, and if the weather is damp a dark wine-coloured
liquid may be observed to drip from the wound. Exposure of
the interior tissues shows a yellowish or reddish discoloration
which frequently extends some distance above and below the
wound and occasionally around the branch. Ajdark line separ-
ates the healthy tissues from those attacked. '^-^ ^f.
The Fungus.
The fungus which has been proved by numerous inoculations
from pure cultures to be the cause of these affections is a species
of Sphseronema probably as yet undescribed. The black stalks
mentioned above are the slender elongated necks of buried
pycnidia, producing pycnospores ; the white mycelium on the
spots forms conidia in simple chains, and chlamydospores are
produced on the hyphae.
Anthracnose
At least one definite anthracnose of cacao pods occurs in the
West Indies, existing quite apart from other types of pod dis-
eases. Little importance has been attached to it, and it has not
been made the subject of any detailed investigation.
Symptoms.
With the following description by F. W. South, later observa-
tions, made by the writer in St. Vincent, agree. " Infection
usually shows itself in the form of small spots on the surfaces
Phoio by] \J. B. Rarer
Fig. 51) MoNiLiA Disease of Cacao (Ecuador)
fe
iii^^.,^
Hi
©
w
l>holo hy\ [J. B. Rote
Fig. 57 Sphaeronema Black Spot (Ecuador)
DISEASES OF CACAO 175
of the fruits, which may be of any age. These spots are at first
moderately light-brown in colour, but later become darker and
are sunken below the general surface level of the pod ; they are
usually limited in extent and irregular in outline, while the
tissues in the centre of the spots become hard and dry. The
tissues of the rind below the spots are brown and diseased, and
the causative fungus penetrates as far as the beans. When the
spots are commencing to dry, small yellow pustules, not as much
as I mm. in diameter, break through the epidermis. These are
closely crowded and become pink as they develop, so that the
centres of the spots are covered with an almost continuous
pink coating consisting of millions of small, colourless, unicellular,
hyaline spores belonging to a species of Colletotrichum. In
some cases the spots fuse with one another and produce large
discoloured areas, while in extreme cases the whole pod may
become discoloured and completely destroyed ; this is especially
the case with young pods."
The disease is much less prevalent than Phytophthora rot,
but can occur under conditions of dryness in which the former
disease is absent.
Identity of the Fungus.
A disease which closely resembles the West Indian anthracnose
in its characters has been recorded by Busse from cacao in the
Canieroons, and is attributed to Colletotrichum incarnatum Zimm.
(C. TheohromcB App. et Strunk), originally described from coffee
in Java. The same fungus occurs on cacao pods in Ceylon.
In the West Indies C. Theobromicolum Delacroix, C. kixificum,
Van Hall et Drost, and C. Cradwickii Bancroft have been des-
cribed from cacao pods. Whether these are really separate from
each other and from the Old World species, and how far any or
all of them give rise to disease cannot at present be stated.
" Male " Cacao
In each of the cacao-growing islands it is a familiar experience
that an occasional tree occurs which persistently produces
enormous and quite abnormal numbers of flowers thickly dis-
tributed over its stem and branches, without setting more than
a very occasional and often imperfectly developed pod. Trees
of large size and high vigour have been seen in which this con-
dition was reported to have prevailed throughout their history,
and this persistence appears, so far as the information collected
by the writer goes, to be characteristic. Beyond casual attention
no investigation of the condition seems to have been made, its
rarity having allowed it to be passed over for matters of more
direct importance. Appearances are against the idea of com-
municability and hence of parasitic origin, since surrounding
trees, in cases observed, have shown no sign of alteration after
years of close contact.
176 DISEASES OF CROP-PLANTS
Woody Excrescences
Instances are sometimes observed in which large woody
nodules or rounded excrescences are produced on stems and
branches. In some instances at least, the habit persists through-
out the life of the tree, the trunk and branches of which have in
consequence a very distorted aspect.
The cause of the condition is so far unknown, but as in the
case of " male " trees it has been kept under observation for
years without any sign of communication to adjacent trees having
appeared.
Root Diseases
The principal root diseases of cacao are caused by species of
Rosellinia, and are described under the heading of general
diseases. Occasional instances of root disease are attributed to
Diplodia (see Dieback). At least one other form of root disease
has been seen in Trinidad, but not yet investigated.
Algal Disease
Thread Blight
Horsehair Blight
Pink Disease
{See under General Diseases).
10000(3
^
^
Fig. 58 COLLETOTRICHUM CrADWICKII
Photo by] [J. B. Roret
Fig. 59 Flower Proliferation (.Male Cacao) Trinidad
CHAPTER XVIII
DISEASES OF COCONUT
The Red Ring Disease
The disease for which the above name is now used was first
brought to notice by J. H. Hart, in 1905, as occurring in the
district of Cedros on the west coast of Trinidad, where it had
apparently been in existence for many years. The disease was
investigated in Trinidad by F. A. Stockdale, in 1906, and dis-
tinguished, under the name of root disease, from the specific
bud-rot which it resembles in its later stages. It was attributed
by him to a fungus of the genus Botryodiplodia. J. B. Rorer
about 1910 made an extensive mycological investigation of
material from diseased trees, and reached the conclusion that the
affection was not due to any fungus parasite but that it was a
physiological trouble resulting from unfavourable conditions in
the soil.
The present writer first met with the disease in Grenada in
1918, and finding a nematode to be constantly present in the
affected tissues put forward the theory that it was due to the
parasitism of this worm. This was supported by later observa-
tions in widely scattered localities in Trinidad, Tobago and
Grenada, and has been confirmed by numerous experiments in
which the introduction of small pieces of nematode-infested
tissue among the leaves of healthy trees has invariably led to their
rapid infestation with the worm and the reproduction of the
characteristic symptoms of the disease.
The disease is widely prevalent in Trinidad and Tobago, and
has occurred in the last few years in various districts of Grenada.
Several cases are now known to have been met with in St. Vincent
about the year 1912 and a recent occurrence is reported. In
1920, W. R. Dunlop found the disease to be very prevalent in
the inland plantations of British Honduras, causing losses which
appear to be on as wide a scale as those in Trinidad. F. Stell
has found typical cases in British Guiana, and J. R. Johnston
reports its presence to a serious extent in Panama. There can
be little doubt, from various accounts seen of the losses of young
trees attributed to bud-rot, that the definitive description of
red ring disease will lead to its detection in several other localities
in tropical America. The writer has seen typical specimens
from Panama and Venezuela.
177 N
178 DISEASES OF CROP-PLANTS
The disease has not been found in other palms with the
exception of a case in an unidentified species of Cocos.
External Symptoms.
The failure of the tree is marked by a progressive yellowing
and browning of the leaves in order of age, commencing at the tip
of each leaf reached in the process. Commonly the lowest living
leaves are the jfirst affected, but occasionally one or several
of these may remain green while the discoloration, commencing
above them, successively involves the younger leaves. In the
early stages the process is indistinguishable by external appear-
ances from the natural dying off of the old leaves, especially
where this is accelerated by drought, water-logging, or poor
conditions of growth in general. The difference becomes apparent
however from the steady march of the discoloration, which
soon begins to involve leaves in full vigour, and continues to
those in which the leaflets are not even fully expanded. A tree
at this stage is exceedingly conspicuous from its rich colouring
of brown, orange, and yellow, involving all but the central
tuft of young leaves. This condition may be reached in three
or four weeks from the time of appearance of the first symptoms.
The shedding of green nuts in all stages of immaturity may
take place concurrently with or slightly in advance of the dis-
coloration of the leaves and is thus in some cases the first
external sign of trouble, though it must be remembered that
shedding of small nuts often occurs from healthy trees.
Secondary infestation of the failing leaves with Diplodia sp.
results in the development of patches of brown or black rot,
wet but firm, in situations on the leaf-stalks, especially towards
the base. This appears to be the chief reason for the tendency
of the leaves to break across and hang down. Such infestations
are common in healthy trees on leaves approaching senility,
but have much more scope when the leaves are failing in rapid
succession from disease.
In the final stage of the disease there ensues a bacterial
soft rot of the tender tissues forming the apex of the stem, as
also of the similar tissues at the base of the young inflorescences.
There is no connection between this and infectious bud-rot
disease : death of the coconut palm from widely differing causes
is regularly followed by bacterial putrefaction of the enclosed
soft tissues of the bud.
Internal Symptoms.
The following description applies to trees during the process
of failure of the leaves before the secondary rotting of the bud
begins. A cross section towards the base of the stem shows a
well-marked zone, usually i-i^ inches wide and beginning 1-2
inches from the periphery, in which the ground tissue is dull red
Fig. (51 Red Ring Disease: Fully Infested Tki
DISEASES OF COCONUT 179
or red mingled with mustard yellow. A vertical section shows
the zone completely surrounding the base of the stem, extending
upwards as a solid band of reddened tissue to about half its
height, then breaking up into longitudinal streaks and finally
into scattered dots about i mm. in diameter. In the latter form
it extends to the soft meristem underlying the bud, in which
region the dots appear generally dispersed below the flattened
apex of the stem. In very exceptional cases the discoloration
in the stem as a whole is central.
When the leaves of a tree in this stage are split open in the
median line a similar red-spotted or red and yellow streaked
discoloration is revealed in the leaf-stalk, extending in the fully
developed leaves from near the base to a distance of from 6
inches to 2^ feet outwards. In the older leaves it becomes
masked by the development of brown rot or by the natural
browning of the tissues. In the leaves of the central column
which still remain green, the internal discoloration of the petiole
appears with great intensity, and often extends to the unde-
veloped leaves which they enclose.
In the oldest leaves the infestation extends to the base,
but has no apparent connection with the red zone in the stem.
As the successive leaves are examined in order of age the dis-
coloration recedes upwards, but has been frequently seen to
retain at first a narrow connection with the base on the dorsal
side. In the soft white central leaves the petiole for a distance
of from several inches to a foot from the base is usually normal
in appearance, and then often contains an enclosed central
infestation marked by vivid red and yellow spots and sometimes
extending for two or three feet within the rachis.
In the roots the visible effect of the disease is confined to
the cortex, which consists of radial lamellae loosely packed into
the region between the horny hypoderm and the central woody
strand. This tissue is pure white and soft when healthy, but
when infested it becomes dry and flaky, and is discoloured,
first light yellow or pink, then dark yellow or reddish brown.
The infestation may extend for any distance from an inch or
two to 10 or 12 feet from the point of attachment ; it is con-
tinuous, and decreases in intensity in the outward direction.
Infested trees vary very widely in the extent to which the roots
are affected. A tree which is fully infested in stem and leaves
may show only the very beginnings of infestation in the roots.
Causation.
While fungi and bacteria are rare and casual in their occur-
rence in the infested tissues until an advanced stage is reached
and decay begins, there is a perfectly constant association of
the worm with the lesions existing in the stem, leaves and roots ;
an association which begins with the first trace of discoloration
and persists until the infested tissue is dead and decayed. The
i8o DISEASES OF CROP-PLANTS
scattered red dots in otherwise sound living tissue, which mark
the upper extension of the red zone in the stem and the beginning
of infestation in the leaf-stalk, are initial nests containing usually
a few adult worms together with larva and eggs. The red zone
in the stem is in its every part simply a vast infestation with
myriads of active worms.
In sections of the tender white meristem of the upper ex-
tremities of the stem and of the embryonic leaves the worms may
be seen in large numbers threaded along between the cells like
fungus hyphas or lying coiled up in the larger intercellular spaces.
In this kind of tissue they appear to be able to travel in any
direction and are by no means confined to the discoloured spots
which mark their breeding places.
In the roots the worms are present in numbers which have
been estimated by N. A. Cobb to exceed 20,000 to the linear foot.
These are adult and larval stages, eggs having been rarely found
in the roots. On the other hand abundant reproduction takes
place in the red zone of the stem, being apparently most active
in its upper sections.
The infestation is in all cases confined to the ground tissue,
the vascular bundles remaining to all appearance unaffected
in any way. Neither the species concerned in this disease, nor
any remotely comparable infestation, has been found in careful
examination made of trees failing or dead from other causes.
Infection experiments, carried out by the writer in Grenada
(with the help of J. C. Moore) and later in Trinidad have given
uniformly consistent results, while the equal number of controls
have been unaffected. Introduction of a fragment of infested
tissue (i) into the stem, by way of an auger hole immediately
closed by a tarred plug, (2) into a wound made by cutting out a
small cube from the petiole of an upper leaf, (3) into one or
several leaf axils, without artificial wounds, produces typical
infestation which becomes externally evident usually in about
60 to 70 days. The characteristic red zone in the stem is de-
veloped in infections by each method even in trees 10 or 12
years old, and is dense at the base and lighter above as in natural
cases. In leaf axil infections purposely examined at an early
stage infestation of a small number of leaf bases, without evidence
of extension into the stem, has been found, and very occasionally
such cases have been met with by chance in the field. The next
phase of infestation is the appearance of the red ring at the base
of the stem.
It is rare for trees to show signs of infection at less than four
to five years of age. The writer has met with one plant judged
to be not more than three years old which was changing colour
and was found to be infested in the leaves only. In the great
majority of cases the trees develop the disease during the bearing
of the first few crops of nuts. The most critical period may be
said to extend from the fourth to the seventh year, but cases
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Fis. 64
Red Ring Disease. Longitudinal Section of Segment
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Red Ring Disease: Transverse Section of Segment of Stem
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Fig. 66
Red Ring Disease: Section near Upward Termination of Ring
Fig. 67
Red Ring Disease : Infestation of a Young Leaf
DISEASES OF COCONUT i8i
occurring up to ten years are not particularly rare and odd cases
have been verified in trees over twenty years old.
Examples of the disease have been seen by the writer in all
grades of soil, from beach sand and rich vegetable mould to
exceedingly stiff clay. No influence on liability to the disease
has been detected as proceeding from these variations or from
any unfavourable conditions of growth.
A very large number of cases have occurred in certain planta-
tions on virgin soil cleared from forest, and infestations as severe
are found on land which from a remote period has been cultivated
in sugar-cane. On the other hand, one large estate in Trinidad
converted from sugar-cane to coconuts ten to fifteen years ago has
so far not had a single case.
Mode of Infection.
Information as to the beginning of the disease is as yet scanty
and uncertain. The rapidity of infestation shown in the infection
experiments renders untenable the hypothesis first put forward
that infection takes place at an early age without its effects
becoming outwardly visible until the tree matures. Subj ect to the
results of further investigation, it may be said that infection
probably takes place at the base of the leaves. The dorsal
connection which is very regularly visible between the infestation
in a leaf base and the external natural fold which is formed near
its junction with the stem suggests that the cracks which occur
in this situation may provide an opening for infection.
It is considered probable that the worms ascend from the soil
by way of the surface of the stems and gain entrance at the base
of the leaves. This is known to occur in nematode diseases
of rice and black currant. (See Chapter V.)
The location of the red zone in the stem cannot yet be ex-
plained. Its position is the same in trees artificially infected
by way of the leaves or through holes bored in the stem.
The Parasite.
From material supplied by the writer the worm has been
described by N. A. Cobb as a previously unknown species, and
named Aphelenchus cocophilus. The length of the adults,
male and female, is i mm. (oV inch). The eggs, which are cylin-
drical and slightly curved, are deposited in the living tissue.
Nothing is known as to the existence of the worm outside of the
tree
Control.
Until the process of infection becomes more certainly known,
advice given regarding control measures must be largely con-
jectural. The manner in which the disease spreads in lines and
groups from the scattered cases which first arise is conclusive
i82 DISEASES OF CROP-PLANTS
evidence of infection from tree to tree. When a tree dies the
soil in its neighbourhood must become heavily infested, and it
is reasonable to assume from analogy with the known habits
of similar parasites that it remains so for a considerable period.
It cannot be otherwise than desirable to prevent as far as
possible the worms in infested trees from reaching the soil, and
for this reason trees recognised to be affected should be dealt
with as promptly as possible, before the infestation of the roots,
which cannot well be dealt with, has become general. It is
necessary not merely to cut the tree across at the base, but to
dig round it, cut the roots, and turn it out — which is not a very
difficult operation in most soils. When the leaves have dried an
attempt should be made to roast the stem sufficiently to kill
the worms. Planters may have ideas of their own as to the best
way of doing this. A slow fire would be most likely to penetrate
to the required depth, and it is suggested that the stern be sawn
or chopped into convenient lengths, piled in the excavation
with the chopped up leaves and added fuel, and banked by
scraping up the loose soil and any chips that have been scattered,
so as to give something approaching to the structure of a charcoal
pit. It is recognised that the problem of disposing of such large
masses of material in a satisfactory way is a difficult one, and fire
in some form would seem to be its only solution. The worms
have been found to be alive in large numbers several months
later in a stem which had been merely scorched.
It is conceivable and sufficiently probable to be the basis of
precaution until the facts are known, that the worms maybe
harboured in the crevices of the husk of fallen nuts and be trans-
ferred with these when they are used for planting. It would,
therefore, be safer to plant nuts which come from areas with no
record of infestation, or otherwise to use them only when taken
direct from the trees and not picked up from the ground.
It is not believed that any method of soil disinfection with
chemicals is practicable. Experiments are in progress to test
the protection of trees by banding and by placing a slowly
diffusible repellent substance in the top. The common practice
of putting lime or salt among the leaves may possibly be of
some use in this direction, though nematodes in general are
highly resistant to chemical poisons. It is considered possible,
on the theory that the worms travel on the outside of the stem,
that a band of crude oil may hinder their ascent by producing
a quickly drying surface.
Observations of the spread of the disease suggest that drainage
trenches act as obstacles to its extension and it is advised that
the sites of eariy cases should be dealt with by the method
described in Chapter XV for the isolation of contacts where
Rosellinia disease occurs.
DISEASES OF COCONUT 183
Bud Rot in General
A great deal of confusion exists among agriculturists regarding
coconut bud- rot, and pathologists cannot yet claim to be fully
informed on the subject. The ravages in Cuba and Central
America of the epidemic disease known by this name became
widely known, and a tendency arose and still exists to regard
all cases in which similar symptoms appear as affected by the
same disease. F. A. Stockdale in 1906 pointed out that a pro-
portion of the bud-rot occurring in Trinidad was purely secondary
in its nature, being a consequence of the failure of the palms
brought about by the so-caUed root disease, now known to be
for the most part a specific disease due to infestation with a
nematode worm His contention, though denied at the time,
has been fully confirmed by later investjgatoTS. Nematode
infestation has been recently found to be responsible for a large
amount of so-called bud-rot occurring in British Honduras, and
may be expected with confidence to account for a good deal
more elsewhere. The writer has found that typical bud-rot of
this kind, indistinguishable in appearance from the infectious
forms, follows rapidly on the death of healthy trees from poisoning
or the severance of the stem. The fact is that a characteristic
stinking rot, dominated by bacterial putrefaction, forms the
natural process of decay of the large amount of tender tissues
deeply enclosed by the successive sheathing bases of the leaves.
A similar process can take place within the closely wrapped shoots
of other monocotyledons, as for example in the top-rot of sugar-
cane. Evidence of infectiousness is necessary to establish
the existence in any given situation of a specific bud-rot disease.
It is now becoming apparent that the true infectious bud-rot
existing in the West Indies is of more than one kind. Until
recently the opinion was generally held that the disease occurring
in the Western Tropics is bacterial in its nature, while that of the
East is fungoid. E. J. Butler and his co-workers have shown
that the bud-rot of Palmyra palms in India is caused by Phy-
tophthora palmivora and that this fungus can attack coconut,
and S. F. Ashby in Jamaica has found the same fungus causing
a bud-rot of coconut palms in Jamaica. A. Reinking in the
Philippines has produced a bud-rot which he regards as identical
with the form occurring naturally in those islands by artificial
infection with a Phytophthora obtained from cacao pods and
believed by him to be P. Faberi, which is the common pod-rot
fungus of the West Indies.
After the removal of these fungoid forms it is believed that
there may remain a large residue of bud-rot of bacterial origin,
and it has yet to be shown that the epidemic bud-rot of the
West Indies, in which bacteria alone have been detected, is not
in fact of that nature. A description of this form, as at present
regarded, is therefore retained. The question of causation does
not much affect the description of the course of the disease, and
i84 DISEASES OF CROP-PLANTS
in any case the effects of bacterial invasion are prominent in the
affected tissues from an early stage.
Bacterial Bud-Rot.
The early history of bud-rot disease in the Western Tropics
is obscure owing to the absence of adequate descriptions and the
difficulties of diagnosis. There is a record of epidemic des-
truction of coconut trees on Grand Cayman as early as 1834 >
in Cuba the disease has been making progress for more than
thirty years, and there are large areas once covered with flourish-
ing plantations on which hardly a tree now remains. Heavy
losses occurred in Jamaica from 1891 to 1910, and at about
the same period in Trinidad, and though checked by compulsory
destruction of trees the disease still persists in both islands.
In the Lesser Antilles small groups of trees have died from time
to time with symptoms taken to be evidence of this disease.
Many incomplete attempts at explanation of the disease
were made from time to time. In some cases observers were led
by the presence of insects attracted by the decaying parts to
attribute to one or another of them its causation, but several
of the earlier observers reached the conclusion that bacteria
play the princif)al part. As a sequel to a visit by E. F. Smith
to Cuba in 1904, J. R. Johnston began the study of the disease
in Cuba in 1907, and after visiting Jamaica, Trinidad and British
Guiana, wrote a comprehensive account of his investigations.
J. B. Rorer in Trinidad, and S. F. Ashby in Jamaica, have also
published descriptions of the disease confirming the general
features of Johnston's account.
Nature of the Attack.
The disease does not affect the central bud alone but may
commence anywhere in the crown, either at the base of the
leaves or inflorescences or on the tissues between them. It
never extends far on to the leaf stalk nor does it affect the woody
part of the stem ; only in exceptional cases does it travel more
than a foot or two in the softer central portion of the stem
below the crown.
If the infection starts in or near the central bud it rots the
tissues at the base of the upright column of unexpanded leaves
so that this falls over and comes away, commonly leaving the
expanded leaves still green and healthy and their bunches of
nuts intact. With the only vegetative bud gone there is, of
course, no chance of recovery. The leaves in time dry up and
fall, leaving a bare pole.
When infection begins among the expanded leaves the first
sign is commonly the falling of unripe nuts, due to the infection
of the base of the fruiting branch. The dropping of young nuts
is not in itself evidence of bud-rot. It occurs quite commonly
Fig. (i8
Effect of a Bud-rot Epidemic, Trinidad
[F. W. Uruh
DISEASES OF COCONUT 185
when trees are suffering from drought, water-logging, or any
cause which affects the ability of the tree to nourish the full
number of fruits set. Injury to the base of the spikes such as
may be caused by insects or by careless picking will also cause
young nuts to drop.
Another early sign of the disease is the discoloration of
the flower spikes, which turn chocolate brown and eventually
blacken and wither. Investigation at this time reveals a dark-
coloured wet rot about the base of the affected parts. The
rot works its way through or under the moist strainer, affecting
the leaves or inflorescences as it reaches their points of attach-
ment. Water-soaked areas appear on the leaf stalks, and as
the rot progresses the leaves involved turn jellow and hang
down. On the relatively hard tissues of the leaf base the rot
does not penetrate far below the surface and if the spots become
exposed to the air they dry up. The softer tissues adjacent to
the bud are much more easily penetrated and when the infection
reaches the central column they become entirely in^■olved in a
soft rot which gives off a particularly offensive odour.
Causation.
Apart from the type recently separated in Jamaica, in-
vestigators of bud-rot in the Western Tropics have so far agreed
in reporting the association of bacteria with the disease from its
earliest stages. It has not been possible, however, to correlate
the findings of the different workers in respect to the species of
bacterium regarded as its probable cause. J. R. Johnston
reached the conclusion that a species indistinguishable from
Bacillus coli was causative, but unfortunately his inoculation
experiments were confined to young plants under glass in Wash-
ington, in which, moreover, infectipns were only obtained through
holes made with an auger. Reinking in the Philippines isolated
what appears to be the same organism, and with this and with
B. coli from animals found that infections of seedling palms
could be obtained through wounds made in the growing point,
but only in case of severe injury or excessive dampness. He no
longer regards bacteria as the cause of the epidemic disease in
those islands. Rorer in Trinidad mentions having obtained with
an unidentified but different bacterium typical infections through
wounds and also by pouring material from a culture among the
leaves of an unwounded tree.
Control.
The experience of Jamaica and Trinidad demonstrates that
when supported by powers of legal compulsion and carried out
under eflective supervision, a vigorous policy of cutting down
and burning the tops of all infected trees, with periodical in-
spections for the detection of fresh cases, can quickly reduce the
incidence of the disease from serious to insignificant proportions.
i86 DISEASES OF CROP-PLANTS
In the case of trees which are attacked in the region of the
central bud there is no hope of remedy, but some success has
been attained in the treatment of cases where the infection is
situated in the outer parts of the crown. One kind of treatment
commonly adopted consists in setting fire, if necessary with the
aid of kerosene, to the dry material present in the crown. This
may give the tree a severe check but is sometimes successful in
destroying the infection owing to the shallow penetration of the
rot on the harder parts of the tree.
A method sometimes adopted, which is claimed, with what
justification is not known, to prevent infection and to cure
incipient cases is the tying of a bag containing a pound or two of
coarse salt or of copper sulphate, or both, in such a position that
rain water will gradually dissolve the chemical and carry it down
among the leaf bases.
Spraying with Bordeaux mixtiu^e, to which arsenate of lead
is added to give it insecticidal properties, has been experimen-
tally tried in Trinidad with apparently good results. The bud-
rot of palmyra palms in India has been successfully treated on a
large scale by spraying with resin-soap Bordeaux, applied by
climbers furnished with auto-sprayers, after removing infested
leaf -sheaths.
In the destruction of diseased trees by fire they should be
cut down, the dry material used to start the fire, and the crown
dismembered and thrown on piece by piece, care being taken to
see that everything is at least scorched through. Excepting its
uppermost section there is no need to burn the trunk.
Bud-Rot caused by Phytophthora palmivora.
Incidence.
S. F. Ashby has described a specific bud-rot which occurs in
the eastern and north-eastern coastal lands of Jamaica, where
there are large unbroken areas of coconuts subject to a rainfall
of 90-120 inches. It is to a marked degree most prevalent on
flat lands with fine silt soils difiicult to drain. Periods of heavy
rain are followed one to three months later by a heavy increase
in losses from the disease, which in one instance of a field of
4,000 trees occurred at the rate of 100 a month. Trees from
2-3 up to 20 years old are liable to attack. On one estate the
fungus was found associated with the dropping of full-grown
immature nuts, and at one nursery seedlings were seen with dead
hearts and rotted apices probably due to infection from the husks.
Symptoms.
The existence of the disease is indicated by the discoloration
and withering of the heart leaf or one or more of the youngest
leaves. The heart leaf rots at the base and can usually be pulled
out, which serves as a rough means of distinguishing this affection
Fis. 69
Coconut Bud Rot, Outer Leaves removed to show Collapse
OF Heart Leaves.
Photo by]
[E. Miller
Fig. 70
Bud Rot preceded by Drooping of Leaves (Tobago)
DISEASES OF COCONUT 187
from withering of the leaf-blade caused by Thielaviopsis. The
young leaves next in order are successively attacked and the
central column falls out leaving the ring of outer green leaves.
At the seat of the disease brown sunken spots up to 3 inches in
diameter are found existing on the leaf bases and penetrating
them successively until the apex of the stem is reached. At
the stage of the withering of the heart leaf the heart itself is
usually already soft-rotted.
The Causative Fungus.
The tissues of the leaf-base spots and of the stem apices when
these are not already soft-rotted are penetrated by a stout,
sparingly septate, intercellular mycelium sending fingerlike
haustoria into the cells. Pale brown chlamydospores in the
tissues are not uncommon. The surface of the spots may show
a white mildew amounting sometimes to a dense felt, and a
similar growth is found in connection with spots which sometimes
develop on the unexpanded leaves, and show up later as rows
across the leaflets when these expand.
The surface web or stroma may show only sterile mycelium
or may be abundantly covered with thick-walled brown chlamy-
dospores ; kept in a moist chamber it develops sporangia,
which are oval with broad prominent papillae, and when set
free are usually stalked. They are believed to liberate 30 or
more zoospores. Oospores have not been seen with certainty.
The fungus corresponds with the description of Phytophthora
palmivora [Pythium palmivorum) Butler, the cause of the
destructive bud-rot disease of palmyra and coconut palms in
India, and its identity has been confu-med by E. J. Butler from
living cultures.
Inoculations.
An infection was secured on an unwounded leaf-base, re-
sulting in a spot two inches across and one inch deep, with
typical mycelium at its advanced margin. Developing in bright
weather the infestation did not penetrate to the leaf-base next
underlying in the ten weeks preceding its removal.
The result of pouring suspensions of the fungus from cultures
into the hearts of trees 4-5 years old was the production of rows
of spots on the leaves next to emerge, but no further effect
followed. Complete artificial infestation of the husks of full-
grown green nuts was secured.
Mode of Infection.
Infection begins on young leaf-bases and often the fungus
penetrates several of these before reaching the bud, passing
inwards by a direct course. The rotting of the heart is accom-
panied by secondary bacterial infestation. No definite in-
i88 DISEASES OF CROP-PLANTS
formation has been secured regarding the manner of distribution
of the fungus ; strong winds may detach and transfer fragments
of mycelium, and scavenging insects and birds may carry these
or spores from one tree to another ; it is not thought that transfer
by the pickers is of much account.
Control.
Attempts have been made to prevent infection by appHcations
of mixtures of slaked lime and copper sulphate, Bordeaux paste
and mixture, lime and salt, and rock salt. The results are so
far inconclusive though good effects are believed to have been
obtained in some instances.
The early eradication of diseased trees, when thoroughly
carried out, has been effective with this as with other types of
bud-rot.
Leaf-stalk Rot caused by Phytophthora parasitica.
Symptoms.
S. F. Ashby has described a disease of coconut palms occurring
in Jamaica in which the earliest symptom is usually a yellowing
and withering of the tip of a leaf on the outside of the crown.
The affection spreads to adjoining leaves and finally most of the
outer leaves break down. Progress is slow and typically the
bud is not affected. The bunches of nuts fall owing to the
removal of the support given them by the leaves, but have not
been seen to be infected.
The stalks of the leaves attacked show dark brown somewhat
sunken spots, from one to several inches in diameter, on the
upper and under surfaces ; the lowest spots are usually situated
several inches above the junction with the stem. Long dark
patches are also frequently present on the limbs, and gum may
exude when the affected areas crack. Internally the spots and
patches show either a sodden rot with a narrow red margin or
are light to dark brown, depending on their age ; frequently
they pierce entirely through the limb.
Incidence.
The disease has been so far met with in Jamaica only on
three neighbouring estates in a district of high rainfall. Cases
have occurred in trees from ten to fifteen years of age, but in the
greatest number on trees of five years beginning to bear.
The Causative Fungus.
The tissue of young spots is penetrated by the stout sparingly
septate intercellular mycelium of a Phytophthora, with few
branches entering the cells. In older spots the hypha; show more
frequent septation and grow through the cells. Felts and webs
have not been found on the surface of the spots.
DISEASES OF COCONUT 189
Sporangia develop abundantly on material transferred to a
moist chamber, and chlamydospores and oospores occur in
cultures. The sporangia are decidedly more globose than in
P. palmivora, with a narrower papilla and less tendency to
break away with a stalk.
The fungus agrees with P. parasitica Dastur, described on
castor oil and Vinca in India. It also agrees with P. terrestria
Sherbakoff, on tomato fruits in Florida, which is probably
synonymous.
Control.
Attempts at control by cutting out diseased leaves and
by the use of Bordeaux mixture or Bordeaux paste have not been
successful. More promising results have been obtained by
packing banana or coconut trash round the affected limbs
and firing it.
Stem-Rot.
Stem-rot occurring in local patches on trees of any age is
a matter of some concern in Trinidad and Tobago. A similar
trouble has been noted in Grenada. In the typical form of the
affection the outward sign of its presence is the exudation of a
liquid which runs for a little way down the rind and dries to
a reddish-brown stain. Removal of the rind reveals a patch of
sodden tissue with an early tendency to yellow or reddish dis-
coloration followed by browning as decay progresses.
The starting point of the rot is frequently a cut arising from
the habit of pickers and others of using the stem as a cutlass
rack, or from weeding near the base of the tree. The affection
also arises in many cases in connection with the natural cracks
common in the rind.
In old and hardened trees the rot spreads little and is not
of much account, but in the softer wood of younger trees severe
injury may occur and eventually result in death. This may be a
matter of months or of several years according to the extent and
number of the attacks and the susceptibility of the tissues.
The disease is at present under investigation in Trinidad,
where it is very common in some districts and rare in others.
A small-celled yeast appears to be general in the affected tissues
and F. Stell has recenHy obtained cultures of Thielaviopsis
from the spots.
A small amount of gumming is often associated with the
typical affection as above described, but other cases occur
where a verv profuse exudation of gum from cracks all over the
affected patch is the most prominent symptom. The internal
appearances are similar, but the relationship of the two types
is not known.
T. Fetch has described under the name stem-bleeding disease
igo DISEASES OF CROP-PLANTS
an affection widespread in Ceylon in which the emergence of a
viscid liquid from one or more cracks leads to the formation on
the surface of the stem of black streaks and patches, which may
be small or may extend for several feet, and are found to be
connected with strands or cylinders of rotted interior tissue
which in some cases run nearly the whole length of the trunk.
In old and hardened trees the external patches are small and the
internal decayed part is usually not more than lo centimetres
in diameter. The disease was found to be due to infection of the
cortical tissue through surface cracks with the fungus Thielaviop-
sis paradoxa. The decay is slow and no effect on the health and
bearing of the tree is usually apparent. S^Tnptoms of this type
have not been observed by the present writer.
A condition more nearly resembling the Trmidad affection
described above is also mentioned by Petch and regarded as
distinct, being attributed to the effects of injury by root disease
root asphyxiation, fire, or lightning.
In Jamaica, according to observations made by S. F. Ashby,
natural cracks an inch or more long tend to form on the trunks of
trees when rain sets in following a dry period. Thielaviopsis
has been found, not infrequently, to start a brown rot at these
wounds which may penetrate deeply if neglected.
Extensive yellowish or reddish sodden discoloration of the
outer tissues of the tnink at or near the base with exudation of
rusty fluid and gum also occurs, and may be followed by the
death of the trees. The affected tissues are soaked with gum,
but no definite parasite has been detected unless it be a small
yeast in the intercellular spaces, not always to be found, however.
It will be noted that the description closely resembles that of the
Trinidad affection.
The treatment adopted in Trinidad consists of the paring
away of the discoloured tissues, an operation which, owing to the
confluence of numerous patches, often involves the removal of
large areas of the rind, and the application of a coating of crude
oil or distillate. This, if thoroughly carried out, has the effect
of arresting the progress of the rot, and in most cases the tree
withstands even severe cutting very well. It is obviously
desirable, however, that the treatment should be applied early,
and the site occasionally re-examined.
Leaf-Dwindling or Little-Leaf Disease.
This affection occurs in Grenada and Tobago and is at the
present time quite common in Trinidad, where it is particularly
conspicuous on the wayside trees seen in the neighbourhood of
villages. A similar affection described by S. F. Ashby under the
name Hard or Little Leaf-bitten Disease is reported to be widely
distributed in coconut cultivations in Jamaica.
Photo]
Fig. 71
[Dept. Agri., Jamaica
Phytophthora Bud-rot (Jamaica) : Central Leaves alone
Dead and Broken
Fig.
[Dept. of Agri., Jamaica
Phytophthora Bud-rot, Advanced Stage, Central Column
Fallen Out
DISEASES OF COCONUT igi
Symptoms.
The most obvious character of the disease as seen in these
islands is that after its onset the leaves come out smaller and
smaller in size until in the end, if no recovery takes place, they
are reduced to a ridiculous little crown of aborted stumps not
more than one to two feet long, which may be borne at the end of
a stem perhaps 20 or 30 feet in height.
Examination of the central shoot reveals the young leaves
with brown necrosed spots and patches on the folded leaflets and
on the surface of the leafstalk. As the leaves expand the leaf-
stalks develop a russeted or corky surface, which later becomes
hard and woody, more or less raised and misshapen, and exhibits
well-marked cracks. The unyielding nature of the leaves so
affected causes those which follow to be more and more severely
deformed, and in the later stages the leaflets either do not expand
or are almost entirely aborted, and the rachis itself is reduced to a
pointed stump.
Incidence.
Trees may be affected at 3 or 4 years old and at any later
stage. Natural recovery can take place and the affection be
thrown off, to recur again or not as the case may be. Most
frequently, it would seem, there is little or no natural interruption
to the course of the disease and the tree dies.
Causation.
Ashby reports that in Jamaica a small yeast occurs in the
early spots and on the advancing margin of the hardened areas,
and is always obtained, usually alone, in cultures. A similar
organism has been found by the writer in Grenada and Trinidad,
but inoculation experiments with pure cultures have so far had
negative results.
Control.
The disease has been found by several planters to be easily
treated, and if taken in reasonable time few cases fail to recover.
The first essential appears to be to relieve the constricted con-
dition of the heart by cutting through the strainers and opening
out the leaves, and then it is the practice to pour into the heart
milk of lime, Bordeaux mixture, copper sulphate solution, weak
Jeyes fluid, or tobacco extract. The last-mentioned substance,
obtained from the local tobacco factory and diluted is used with
complete success on an estate in Tobago.
Bitten-Leaf Disease.
S. F. Ashby has described a disease prevalent on the north-east
coast of Jamaica which affects the leaves while still folded in
T92 DISEASES OF CROP PLANTS
the bud. It is caused by the attacks of the " pineapple " fungus
familiar on sugar-cane, Thielaviopsis paradoxa, v. Hohn. The
disease has not so far been recognised in the Lesser Antilles.
Ashby's account of the symptoms is as follows : —
" The younger green unfolded and unfolding leaves appear
bitten off at the ends or show successive leaflets lower on the stalks
broken or reduced to stumps. Those nearest the heart show
spots on successive green leaflets which bend over or break and
hang down at these points.
" Leaves just pushing up can be detected with black tips ; if
the end of the heart leaf is cut out with the white leaflets over-
lapping and pressed on the stalk, characteristic discoloured
patches and spots will be seen. Infection usually starts as a
yellow discoloration with a brown edge along a narrow line beside
the ribs of the leaflets ; the infection runs across all the leaf-
lets overlapping at that point. These long, narrow spots widen
to wedge-shaped patches with dark brown broad edges and a pale
brown centre, with the skin raised into a blister so that it can
be torn off to expose the fibres, with the tissue between them
more or less decayed away.
' ' If infection is high up only the tips of the leaflets become
black. A heart leaf may show one or more wedge-like patches of
disease. Spots are also present on the leaflets, at first minute,
round or oval, yellowish with depressed centre, later with pale
yellow centre and brown edge ; by union these become patches ;
in bad cases the leaf stalk (rachis) beneath the diseased leaflets
is attacked by a dark brown rot so that when pushed out the end
of the limb may break off. Spots and patches show black specks
consisting of spore masses of the fungus. The frequency of affec-
tion beside the ribs of the leaflets is due to a thin line of cells there
which regulate the opening and closing of the mature leaflets
in dry and moist air, so as to control evaporation of water from
the leaf pores which are confined to the under sides of the leaflets.
" The disease, as already pointed out, starts on the heart leaves
while still white and tender before they push into the light.
Just what conditions favour this infection cannot be stated yet.
Check to growth during a period of drought may have started it
at some estates while the last two unusually wet years have made
it epidemic.
" The diseased patches attract the beetle Metamasius sericeus
(weevil borer of sugar-cane) which increases the extent of the
damage.
" Cutting out the visibly diseased parts will sometimes remove
the infestation, but to be certain of preventing its recurrence
it is found necessary to chop back the bud until no more dis-
coloured leaflets are met with. About a pint of a powdered
mixture of one part each of salt, lime and copper sulphate is
applied to the cut ends. In most cases the trees recover rapidly
from the treatment."
Photo by]
F. \V. inch
Fig. 73
Gumming of Coconut Stem
m
W/"'/^^-'^m
W\./^
WMl//^^- f/',. , '' vu. '.^^^^ *'^IB^1^V/'
■ ■' ■'■ "V■^s^^-^ v'
'^ J^!F w ^By ' i^ftfi
^ ■ ' ■' 'J
JmHbJBI ,^f^lii^9^^^^^^^^'^'^^^SlS^m^^^l9mKik
bI^hH^ / g^^ ''Wf^f^^^ ' wSSkStt^^KL
^Ki^'^^Mi
^^1
mk-mm^^:m
^ 1 ^^'vS^^i^HB^^H
Bi ' ■.i-.''SH?;vwnp9i^^P'-^1^^B^BBBi^^^^^B
Fig. 74
Little-leaf Disease of Coconut
[]. B. Rarer
DISEASES OF COCONUT 193
DiEBACK.
In Jamaica the pinnae of the older leaves die back succes-
sively from the tips and the crown of the tree may be reduced
slowly or rapidly to a few expanding leaves. The name dieback
has been given to this affection and a Diplodia is in most cases
found fruiting behind the advance margin of the dead zone.
This affection is regarded by Ashby as primarily a root deteriora-
tion, possibly of the same kind as those reported recently on cane
and other crops in Hawaii and on cane in Porto Rico,
Leaf Blight.
A condition of the leaves in which they prematurely take
on a more or less general yellow colour, while the individual
leaflets have yellow, brown, or grey patches scattered over them
and dead areas at their tips and along their margins, may be
designated by this name. Such a condition arises from un-
suitable cultural conditions, of which lack of drainage is perhaps
the most common, but packed soil, root competition with other
plants, and undue exposure under conditions of low rainfall and
absence of available subsoil water will produce it.
Usually the condition is accompanied, and in the immediate
sense caused, by infestation with scale insects and weakly para-
sitic fungi. It is on old and failing leaves, and on younger leaves
in a condition of debility that Pestalozzia palmarum Cke. has
been commonly observed in the West Indies. This fungus has
been reported from Java as being on occasion a serious leaf-
parasite on newly planted trees. Fetch has stated that in Ceylon
it occurs as a rule on old leaves and does no appreciable harm,
and that its appearance on younger leaves is a sign that something
is wrong with the conditions and should be met by manuring.
The fungus is found in yellow or brown oval spots which come
to have a diameter of a centimetre or more, and dry up in the
centre. The conidia are formed in pustules on both sides of the
leaf. Each has 5 cells, of which the 3 median ones are brown
with thicker walls. The terminal cells are hyaline, the superior
one bearing 3 threadlike appendages and the inferior one being
prolonged by a portion of the slender conidiophore.
A Diplodia, D. epicocos Cke. is also found occupying dried-out
spots on the leaves, and has been recorded from Jamaica, Trini-
dad and British Guiana. Its pathogenic relations are not
established.
CHAPTER XIX
DISEASES OF LIME AND OTHER CITRUS TREES
There are distinct differences in the resistance offered by the
various citrus species to dry conditions. In the lowland dis-
tricts of Barbados, for example, the lime, the shaddock, and the
rough lemon can exist, though they cannot be said to thrive except
under very sheltered conditions. The mandarin will grow and
flower but rarely sets fruit. The grapefruit and the orange only
exist, and that somewhat precariously, in a few sheltered spots in
the wetter uplands.
In Dominica and St. Lucia, on the other hand, the lime is
thoroughly at home, and other citrus fruits do proportionately
well. The sweet orange, however, is limited in duration unless
it is grafted on sour orange stock, by the development of gummo-
sis of the collar at the age of about 25 to 30 years. Two extremes
of habit can be recognised in the lime as grown in Dominica.
In exposed positions or on poor or water-logged soil, growth is
slow and " hard," the leaves are smaller and stiver, the foliage
is scanty and greyish green in colour. On rich and deep soil
with good drainage and shelter, growth is vigorous and rapid,
the leaves are large and thin, the foliage is heavy and very dark
green. There are corresponding differences in the size and
juiciness of the fruit. Under the latter circumstances the lime
may attain to the dimensions of a considerable tree.
The case of St. Vincent needs a special note. Although the
rainfall is more than sufficient and is fairly well distributed,
citrus trees are distinctly rare : there is not at the time of writing,
for example, a fruiting citrus tree of any kind, other than the
lime, in the Botanic Gardens or the Experiment Station. In spite
of many attempts at lime-growing there is not a field of bearing
limes in the island. Scattered lime trees may be met with and
oranges and grapefruit are produced in sheltered situations.
The form which the failure takes is that of heavy infestation with
scale insects and " black blight," but, as the pests found are
common to all the islands, these troubles can only be regarded
as due to the general inability of the plants to thrive. This,
there can be little doubt, arises primarily from the light and
porous nature of the soil, which, except under conditions of very
efficient shelter or heavy shade, does not hold a sufficiently
stable supply of water or contribute sufficiently to the humidity
of the atmosphere during spells of dry weather. The same
194
DISEASES OF LIME AND OTHER CITRUS TREES 195
disability is seen in cacao plantations in the island, but the
deeper root system of cacao enables the trees to maintain them-
selves, though not, in general, to flourish. It affords a curious
study in crop ecology to find such opposite types as cotton and
cacao growing in contiguous fields, while limes, which will grow
in many places too dry for cacao, fail to thrive. A contributory
reason is no doubt to be found in the more serious and lasting
effects produced by the scale insects which infest the lime at
periods of low vitality than by the thrips which affect cacao under
like circumstances.
The behaviour of the lime tree under cultivation suggests
that its natural habitat would be in bush or thicket growth on
ground unoccupied by large trees. It profits greatly from the
shelter of surrounding vegetation, but is somewhat intolerant of
shade. Even in most favourable circumstances, as in the climate
of Dominica and St. Lucia, the plants in their first few years
benefit enormously from being enclosed in any kind of sheltering
growth, and in poorish soils, even in wet districts in those islands,
may during the critical period which precedes their becoming
" established," die back several times or be killed outright by
scale insects unless some such protection is afforded them. Even
when the scale insects are kept down by regular spraying growth
in the open is nothing like so good. From this has arisen the
practice, which is desirable anywhere on previously cultivated
land, but especially where the rainfall is somewhat low, of
nursing young lime trees in sugar-cane, pigeon pea, or Tephrosia,
until they become well established. In fields where this has not
been provided for it may be seen how those plants which have
the advantage of some bit of natural shelter are the first to take
on the shape of trees and how their influence gradually brings on
in succession the rest of the field. Manuring is also of great
assistance.
Once well established and come to fruiting age the trees
generally do fairly well even under rather hard conditions for a
number of years. In Dominica unless the soil becomes unduly
impoverished or is allowed to get into bad condition they will
bear for 20 or 30 years and then be far from worn out. The
effect of neglect of the kind mentioned is the death of upper
branches, and the extension of the rot caused by the bracket
fungi which come to infest them down the stem of the tree,
which has not vigour enough to resist. This type of dieback,
common in some places in Dominica, can be arrested by appro-
priate measures as described below, and the tree restored to
another period of usefulness.
Such restoration is not possible, or at best is much more
temporary, in the dieback which sets in after some 10 to 15
years, without obvious neglect as a predisposing cause, in lime
fields established under conditions of smaller rainfall and more
marked dry seasons, as in Montserrat. The progressive decline
196 DISEASES OF CROP-PLANTS
which ensues, and is associated with Diplodia infestation, marks
the limitation of the Hfe of the tree under these circumstances.
A similar restriction in the citrus cultivations of some districts
in Queensland has been the subject of much investigation, without
any other conclusion being reached than that the decline is due
to a reduction of the natural duration of the tree. A comparable
reduction is recognised in the cacao tree when grown under
conditions near the margin of suitability in respect of depth of
soil, unequally distributed rainfall, and absence of shade or
shelter.
Lime.
Diplodia Dieback,
(The Montserrat Type.)
As indicated above, the result of experience in planting limes
in the Lesser Antilles has been to show that while the tree finds
permanently congenial conditions in the islands which are
mountainous in character and retain a large amount of forest,
in the islands which are more open and more completely cleared
its duration is restricted to the period during which it retains
its first vigour of growth. A comparison may be made with
the more familiar difference in the effective duration of sugar-
cane stools under similar circumstances.
The climatic conditions by which the difference is determined
are reduced rainfall, lower humidity, more exposure to wind,
and a more marked difference between the wet and dry seasons.
Montserrat being the only island of this type with a considerable
lime industry, affords the best illustrations of the effect, but
from such opportunities for observation as occur it is evident
that the same effect is produced in the other islands where
conditions are similar.
The sequence of events in its most regular form is as follows :
Young trees, after coming through the critical period elsewhere
described, may become established and do well for about ten
years, attaining to a good size and bearing normal crops. Then
in a uniform field of this nature the most exposed trees begin to
show signs of dieback, which sooner or later extends widely and
in two or three years may involve the whole field or may leave for a
time groups of less affected trees here and there.
To follow out the process of the dieback it is necessary to
know the growth form of the tree. This may be described as
of the fountain type, i.e., the main shoots, generally several
together with no pronounced leader after the first two or three
years, grow up long and slender, and sooner or later bend out-
wards from the top. As they become inclined new vertical
shoots arise from the upper side of the curve so formed, and
as the outer end of the original branch is bent lower and lower
DISEASES OF LIME AND OTHER CITRUS TREES 197
by the weight of foliage and fruit the secondary branches in turn
lose their upright position and are replaced by further vertical
shoots from themselves or from the original branch further back.
There is thus a continual tendency to the opening of the centre
of the tree and its filling with new vertical branches. Where
there is room for full development the mature tree is typically
dome-shaped, with its outer branch system supported by a num-
ber of more or less equal stems of the first or second order,
springing usually from points not far above the crown. As the
outer ends of the lower branches become bent into or below the
horizontal position the outward growth necessary to keep the
foliage in rank with that of the branches above it (it would
otherwise be overshadowed) is continued in the same way by
shoots arising from points situated on the curve of the branch.
This succession of the terminal shoot by a shoot produced further
back is in fact the principal characteristic of the growth of the
tree.
Its importance from the point of view of duration lies in the
fact that, whereas in a vigorous tree the later shoot supplements
the leader, in one which is lacking in vigour it tends to replace
it. The younger shoot seems to establish a pre-emption on the
ascending current of sap, and, when the supply does not suffice
for both, the terminal part of the older shoot beyond the junction
is stopped in its growth and during some spell of dry weather
drops its leaves and dies. By repetition of this process it is
not uncommon for the axis of a long basal branch to be made up
of successive sections representing ten or more orders of branching
and to come at last to bear an insignificant tuft of foliage.
For a time the growth about balances the losses and then
begins to fall behind. There is considerable recovery during
the wet season, which may be maintained in favourable years
or more than lost when severe droughts occur. On the whole
the decline, once it has set in, continues, and as the older branches
are reached there is a marked acceleration of the rate of loss.
The bearing of a heavy crop of fruit, especially in dry weather,
is frequently followed by a sudden loss of branches, and this
effect may to some extent account for the rapid failure which
has often been observed to follow upon heroic attempts to restore
the trees by cultivation and heavy manuring.
The dieback is closely accompanied by Diplodia infestation,
and when a branch of any considerable size fails, unless it is
promptly removed flush with the older branch from which it
springs, the sector of wood directly connected with it, and the
overlying bark, dry out and are found infested with the fungus,
so that long lesions the width of the dead branch above and
running out to a point below are produced on the older member
and are usually irreparable. Associated with their development
there is commonly a considerable flow of gum.
The inability of the fungus to attack wood and bark which
198 DISEASES OF CROP-PLANTS
are in connection with functioning leafy branches is regularly
shown by the persistence in a healthy condition of a sector so
connected embedded in a branch or stem otherwise dead and
infested. Such living sectors may produce considerable growths
of callus along their margins, but in the gradually waning tree
are not effective in making a permanent repair. The infestation
of the smaller branches becomes self-limited at their base.
In Montserrat, and probably elsewhere, a dieback comparable
with that of the branches takes place in the root system. The
two do not seem to be directly connected, but are presumably
related in their origin. Here again Diplodia, and probably
other fungi, infest the failing roots, and the same type of re-
placement by lateral branches and the same persistence side
by side of living and dead sectors are seen. The damage to the
roots is often complicated by the long groove-like wounds made
by the larvae of weevils of the genera Diaprepes and Exophthal-
mus. These are quickly healed in actively growing trees, but
in trees in failing vigour remain open and afford access to weakly
parasitic fungi which infest the wood. A similar slowing-down
of the process of repair is seen in the upper parts of the tree
where pruning wounds have been made.
A characteristic early symptom of the condition described
is the selective loss of shaded branches. This in its origin is a
natural process, seen in most trees however healthy, but in the
failing trees sensitiveness to shade is so much increased thcit it
leads to persistent reduction of the total amount of foliage.
Behind the whole process there appears to be a decline in
vigour such as we know to be universal in animals which have
passed their prime, but do not so readily recognise with regard to
plants. The conclusion reached is that, under the conditions
described at the outset, the natural duration of the tree is reduced
to the period to which the initial vigour extends, and this would
seem to be in general somewhere about 15 years. The ensuing
failure may be either relatively rapid or long drawn out, according
to local circumstances fairly easy to follow.
The history of lime-growing in Montserrat, where to maintain
production it has been regularly necessary to establish new fields
from time to time, accords well with this interpretation. There
is no evidence whatever of the occurrence there of any pest or
disease not existing in Dominica or St. Lucia.
Relation of Diplodia to Dieback.
The fungus accompanying this form of dieback is very
regularly a species of Diplodia. It is frequently regarded as
D. natalensis, but the writer has seen no evidence produced of
essential difference from D. Theobyomce, the ordinary dieback
fungus of cacao and many other trees. Infested lime wood is
recognisable by its slaty grey colour, and the fungus fruits freely
DISEASES OF LIME AND OTHER CITRUS TREES 199
in the dead bark. Evidence of its inability to attack tissue in a
state of activity is given above, but it is undoubtedly parasitic
when growth is checked or vigour is reduced. It would be possible
and legitimate to describe the disease as due to this fungus, as
has been done by F. S. Earle and J. M. Rogers, in treating of
grapefruit dieback in the Isle of Pines, but these authors agree
that " no tissue seems to be attacked until it is fully matured
or until enfeebled from unfavourable conditions." In the present
account the predisposing causes have been regarded as more
important, since it is upon the modification of these that control
must depend.
Relation of Scale Insects to Dieback.
Scale insects are so commonly associated with the dieback
of lime trees that they are very often credited with a very con-
siderable part or even the whole of its causation. While it is not
denied that the presence of the insects intensifies and accelerates
the failure of the trees, the writer is in agreement with that
section of entomological opinion which in this and similar cases
attributes the severity of the infestation to the impairment of the
resistance of. the tree, or, what comes to the same thing, the
special suitability of trees in this condition for the rapid increase
of the insects. Numerous cases have been seen in which the
dieback has followed its ordinary course in the absence of any
significant infestation, and it has always done the same in the
face of attempts at prevention by controlling the insects with
sprays.
Control.
The underlying cause of the decline appears to be one of
insufficiency and irregularity of water supply, and the duration
and completeness of the dry season seems to have more effect
than the intensity of the wet. Close shelter to conserve humidity
and hinder the soil from drying out has most influence in pro-
longing the life of the trees. The effect seems mainly to depend
on the protection given to the fibrous roots. It is admitted that
individual lime trees can and do exist under conditions of low
rainfall and considerable exposure, though the writer has no
evidence as to the age to which they attain. It is possible that
such trees continue to flourish in the open because they have
developed a type of growth suitable to their position, but it is
none the less certain that, in lime cultivations, the provision of
adequate shelter is necessary, first of aU to establish a regular
stand, and later to delay the incidence of dieback in the fully
matured trees. Between these periods it is advantageous but
not essential.
The provision of windbreaks in all exposed positions has
come to be regarded as a cardinal principle of lime-planting in
200 DISEASES OF CROP-PLANTS
Dominica and St. Lucia, and the importance attached to it
could not be more strongly emphasized than by stating the fact
that some planters have manured their wind-belts. If the
advantages are so definite in these wet islands the need is all the
clearer where the rainfall is much lower and less well distributed.
In this type of situation it is necessary that the wind-belts should
be well-grown and dense to the base, and, to secure the latter
condition, it is advisable to grow some suitable plant as secondary
cover for the gaps which develop between the stems of the main
line of trees. It must be remembered that the direct influence
of a wind-belt does not extend very far, but the protection to
the first rows of lime trees is passed on by them to the succeeding
rows. Conversely, where dieback begins opposite to some gap
which lets in the wind, the reduction of the nearest trees gives
access to those behind, as a result of which the disease may spread
through a field as if it were actively infectious.
The steady use of organic manures may be somewhat con-
fidently expected to increase duration ; there is little recorded
experience known to the writer to justify a more definite state-
ment. Spasmodic heavy applications of manure have been often
tried and seem invariably to make matters worse.
It would be interesting to learn the effect of a system of
close draining established with a view to keeping moisture in the
soil low during the rainy season, and thus discouraging the
"formation of roots at levels at which they are liable to be readily
injured by dry weather. Some amount of natural adaptation
in this respect is perceptible in the differences of root level in dry
and wet districts. It is in this connection that the use of surface
mulches in places liable to drought needs careful consideration.
They help in such circumstances to retain soil moisture, but some
experienced planters hold that this good effect is more than
counterbalanced by their tendency to encourage root formation
near the surface. The local experiments on which the advocacy
of mulching in respect of tree crops is based were carried out
under relatively wet conditions and need confirmation for dry
ones. If the mulch can be constantly maintained there should be
a decided advantage, but this involves large supplies of material.
In this, as in all matters relating to the management of limes in
dry districts, the maintenance of uniform conditions should be
the basis of policy from the time the field is established. Tinkering
with the conditions after the trees have begun to fail has always
given disappointing results.
The prompt removal of affected branches delays very con-
siderably the course of the dieback, but owing to the slowness of
the healing process and the tendency of disconnected wood to
dry out and become infested special care is needed to trim the
wounds flush with the stem.
DISEASES OF LIME AND OTHER CITRUS TREES 201
Deficiency Dieback.
A type of decline quite independent of water shortage, as
may be seen by its occurrence on favourably situated estates in
Dominica, becomes prominent where the trees are old and
cultivation and manuring have been neglected. It is also brought
about when growth and heavy bearing are stimulated year after
year by transient chemical manures without the condition of the
son being maintained by organic manures of a more lasting
nature. The resulting dieback can be very severe. The trees
lose their healthy colour and some or many of the upper branches
die. That lack of tilth and the exhaustion of the soil, rather
than age or parasites, are the cause of the condition, is shown
by the large degree of restoration possible by the addition of
organic matter to the soil and general good handling.
Wood Rot.
The dead and failing branches become conspicuously infested
with numerous species of fungi, many of them of the bracket type.
The bark is usually found to be infested with smaller fungi, of
which several species of Nectria and Stilbum are the most
common.
The damage done arises almost entirely from the destruction
of the woody framework of the tree, and not from any direct
attack on the active tissues of the bark and young wood. The
destruction is probably largely due to the mycelium of the
bracket fungi in question.
These fungi are able to establish themselves on any dead stub
left from the failure of a branch or from careless pruning, no matter
how healthy the tree may be, but the subsequent progress of the
infestation depends on the condition of the adjoining parts. In
vigorous trees the decay is arrested when the living branch is
reached, and the wood shows considerable powers of resistance.
It rots slowly where it is exposed, and , unless the wound is trimmed
so as to permit the bark to grow over it, there is produced in time an
unsightly cavity, but the process is no more than goes on in any
tree not carefully trimmed, and, if the wound is not a particularly
large one, it does not produce serious results for a long time.
The less vigorous the tree, the less resistant is its wood to
decay, and the extreme cases of fungus infestation are to be seen
in fields where, after the trees have attained to a good development
from the original humus content of the soil, or from the practice
of a good system of manuring, the fertility of the soil has then
been allowed to run out, and the trees are no longer sufficiently
nourished. The policy of substituting sulphate of ammonia,
without any backing of humus-building material, for pen manure,
mulch, and other organic manures is clearly seen in some cases to
have produced this effect. In other cases it has come about from
complete neglect of manuring, combined with careless treatment of
202 DISEASES OF CROP-PLANTS
the soil in the way of ignoring drainage, and, in some instances,
packing it by the constant pasturing of cattle. As starvation
takes effect one or more sections of the tree begin to die back
branch by branch, and not only the dead but the failing branches
become infested with fungi. The removal of the dead branches
does little to arrest the process ; the wood of the next larger
branch or stem is usually already infected, and offers little
resistance to the progress of decay. In the course of time the
base of the tree is reached by way of the stem or stems involved,
and permanent restoration is no longer possible. Growth is
usually continued by suckers put out around the base, which
may attain to a considerable size, and give the tree a renewed
lease of life. In time rotting of the central part of the
crown, due to the downward extension of decay from the original
stem, destroys the connection berween the suckers, depriving them
of mechanical support, and leaving them with exposed wood on
their inner sides in the neighbourhood of soil level. They are
rarely able to establish themselves as permanent stems, and sooner
or later, from the weight of the crop or the force of the wind,
they fall outward, or are broken off. The process of failure
thus outlined is a gradual one, commonly extending over some
years, and seems in consequence to be usually regarded with
indifference until the end result appears. There is, even in the
worst cases of this type seen in Dominica, no question of quick
destruction of trees such as is produced by an active parasite
like Rosellinia.
It has been further noted in cases where lime trees have been
grown on poorish land, and without manure, and have in con-
sequence grown slowly from the beginning, that, although they
may develop dead branches in the course of time, their wood
does not appear to be nearly so susceptible of decay as is the case
with trees which have grown more quickly.
Control.
The remedial and preventive measures which should be
adopted are clearly indicated. In fields where dieback is in
evidence, or where small and scanty foliage shows that the trees
are lacking in vigour, the first essential is to restore the fertility
of the soil. If it has become consolidated, or is covered with a
firm sod, it should be lightly forked ; as much organic material as
can be obtained in the shape of pen manure, mulch, cotton-seed
meal, lime seeds, and sheep manure should be applied ; the
weed growth, if rank, may be cutlassed and used as mulch
towards the end of the wet season. Leguminous cover crops
may be grown if there is light enough for their development.
At the same time the work of clearing the trees of dead and
sickly branches should begin. Men who have received instruction
in this work should be kept to it as much as possible, and it will
DISEASES OF LIME AND OTHER CITRUS TREES
203
be necessary that a responsible person who understands the lines of
action should direct it. The principles to be followed are set out
in Chapter XI.
As already indicated, when decay has invaded the base of the
tree a permanent repair is no longer possible. The life of the
remaining parts may, however, be much prolonged in many cases,
if it is considered worth while, by cleaning and smoothing off
the wood which remains sound, and coating it with a preserva-
tive. As the soil moisture hastens decay in this region, it is
usually advisable to draw away the earth from the crown, and
keep its neighbourhood clear of sheltering weeds.
Red Root Disease.
The red root disease of limes, due to an undetermined species
of Sphcerostilbe, was first described by F. W. South as occurring
on lime estates newly cleared from forest in Dominica. It has
been examined by the writer in similar situations, and in an older
cultivation situated in a valley subject to flooding by a river
carrying abundant forest debris. The disease is not known from
other West Indian islands, but material received from a lime
estate in British Guiana indicates the presence of a like disease
in that country.
RosELLiNiA Root Disease
(See General Diseases).
Fig. 75
Red Root Disease of Lime (Dominica^
T. Fetch has given an account of a root disease of a closely
similar type on Hevea and arrowroot in Ceylon due to Sphcero-
stilbe repens B. et Br., and S. F. Ashby a disease of banana in
204 DISEASES OF CROP PLANTS
Jamaica, also similar in character, due to 5. musarum Ashby.
The description of the perithecia of the Dominica species as given
by South suggests its identity with S. repens.
Symptoms.
The effects of the disease are usually visible for a longer
time in the foliage of the trees than is the case with Rosellinia
attack, so that it has the appearance of being slower in its action.
This comes from the fact that the root system is in many cases
destroyed gradually without the girdling of the collar wliich in
Rosellinia disease very often brings the tree to a sudden end.
Sometimes a soft rotted patch is seen extending on the bark of the
collar, while much of the root system is still sound, but generally
the foliage has become thin and yellow, and the tree is in some
cases nearly dead before any damage is visible in that region.
When the roots of a sickly looking tree are laid bare the
disease is seen advancing along them in the form of a soft rot of
the bark accompanied by a disagreeable smell. On the last 6
inches to a foot of the section adjoining the healthy part of a
root the surface of the bark shows a greenish black discoloration,
behind which the bark is rotted and easily comes away. Roots
that have been attacked for some time are devoid of bark, and
the wood is dry and blackened.
Between the layers of the bark, and on the surface of the wood
occur fiat branching rhizomorphic strands, which may be 6-7 mm.
broad in their older parts. In well-developed examples they
divide several times, the main branches keeping their ribbon-like
form and being mostly blunt and rounded at the tips. They
may, however, develop a secondary system of finer semi-flattened
branchlets along their margin, producing a sort of fishbone
pattern. The advancing tips of the strands are whitish and
papery ; further back the strands are brownish red and rather
fleshy in consistency ; still further back the colour becomes
dark brown. When old they lose their consistency and are with
difficulty recognisable as blackish streaks on the wood or in the
bark.
A light brown discoloration extending deeply into the wood
accompanies the advancing strands, and has been seen beginning
just in front of their white tips as they penetrated healthy
tissue. South describes the development near the collar of a
flat sheet, red on the surface and white within, of similar material
to that composing the rhizomorphs, and situated in the cortex
or in the place of the cambium layer.
The Fungus.
The conidial fructifications of the fungus are of the Stilbella
(Stilbum) type, and are produced on the margin of the rhizo-
morphs, of the mycehal sheet, or from tufts of white mycelium.
fe
/
.1!*^ / «;v *
\
^\'>'^':' .J .r^
^f;^/:^ .:;
V )
t
Fig. 76
Anthracnose of Lime
[W. Duthn
DISEASES OF LIME AND OTHER CITRUS TREES 205
They consist of pink slightly hairy stalks, usually 2-5 mm,
in length, but sometimes longer, rather irregular in form, and
surmounted by a white tip or by a white opaque cluster of
conidia.
The perithecia have been described as follows by South,
who found them on one occasion situated in groups on a rhizo-
morph : " The perithecia are minute, red, flask-shaped bodies,
the neck of the flask being often somewhat curved and about
as long as the diameter of the lower spherical portion of the organ.
Each contains asci and paraphyses. In the ascus eight, bicellular,
somewhat reddish-yellow transparent spores are formed, arranged
obliquely in a single row. The spores are bluntly pointed at
either end, constricted at the septum, somewhat thick-walled,
and measure approximately 17-20x8.5-10 microns.
" They germinated very readily in about four hours, and in
culture media gave rise to rhizomorphs and Stilbum fructifica-
tions resembling those found on the lime trees. In cultures a
third form of fructification appears, of the Cephalosporium type,
the spores formed on the short conidiophores being very similar
to those borne in the head at the summit of the stalk or stroma."
Conditions of Occurrence.
The incidence of the disease suggests its origin from the
forest. In new clearings it is probably directly transferred from
logs or the roots of stumps : in older cultivations from infested
material brought down by floods. This agrees with experience
of the Ceylon disease, and is suspected to be the mode of origin
of the attack on bananas in Jamaica. Another character in
which the Dominica and Ceylon diseases agree is in their occur-
rence being restricted as a rule to swampy or sour undrained
soil.
Once it is established in a lime field the spread of the disease
from tree to tree along the roots (it may possibly also occur
through the soil) is very definite and results in the death of a
group of trees around the original site of the infection. The
process of infection byjoot contact has several times been traced
out.
Control.
The preference of the fungus for undrained situations indicates
the point at which the attack upon this disease should be most
strongly pressed. A system of isolation trenches like that
recommended for Rosellinia disease should be established, and
care taken to have the trenches of good depth and linked up with
an effective drainage system. The disposition of dead and
dying trees should also follow the lines laid down for Rosellinia
disease.
2o6 DISEASES OF CROP-PLANTS
Collar Canker.
Lime trees up to four or five years of age, under conditions
where growth is slow and uneven, sometimes develop a rough
and swollen structure of the basal portion of the stem extending
for a few inches or up to a foot above the surface of the soil.
The cortex and bark are divided into irregular patches, and the
new wood is deposited in lumps and ridges. Open wounds
may extend to the wood, and frequently matters are complicated
by infestation with the lime bark beetle, Leptostylus prcemorsus,
which begins on the damaged spots but extends for some distance
under the healthy bark. The trees make a persistent struggle
against the condition, but not infrequently die from the girdling
of the stem. Wounds on the roots such as are described by
F. W. South in connection with this affection are caused by the
larvae of Diaprepes or Exophthalmus weevils and have no
relation to the disease.
The origin and nature of the disease are not known, and no
parasitic organism has so far been found to be at all constantly
associated with it. It appears to be connected with somewhat
impoverished or insufficiently drained soils, and is in the end
overcome by attention to these matters and by the trees which
survive outgrowing the trouble.
Branch Galls.
In one section of Jamaica there is a disease of lime and orange
trees, more severe on the first named, which was originally
reported in the following terms : " The lime trees are infested
with galls, which in some instances appear to do as much harm
as the scale insects. The galls are found on old and young trees,
but usually they grow first on fairly old limbs. These limbs
then send out strong young shoots for a time, but speedily get
exhausted and die, large numbers of limbs dying until many fine
trees are killed, or so nearly so as to be useless."
Causation.
The disease was investigated by L. S. Tenney and F. Hedges,
who found that a fungus Sphceropsis tumefaciens Hedges, could
be detected in the galls and that it reproduced the disease when
inoculated into healthy trees.
When present in quantity the mycelium imparts a black
appearance to the tissues, but frequently only small amounts are
present. The fructifications have been found by S. F. Ashby
in abundance on branches near the galls. They are in the form
of solitary or congregated sub-spherical pycnidia covered at first
by the epidermis, and emerging later, producing oblong or ovoid
spores, rounded at one or both apices, or sub-acute, sometimes
DISEASES OF LIME AND OTHER CITRUS TREES 207
curved, continuous or i-septate, 16-32 X 6-12 microns. The my-
celium is hyaline at first, darkening later.
The galls are approximately round, attached by a broad base,
and vary from the size of a pea to knobs 2 to 3 inches in diameter.
They are at first covered by smooth green bark ; later by a
modified corky bark, which may crumble away leaving the
wood bare.
Similar excrescences have been met with on lime trees m
Dominica and St. Lucia, and examined by F. W. South and by
the present writer. They have been strictly sporadic cases,
confined, in each instance seen by the writer, to an individual
branch of one tree in a cultivation. The branch is hypertrophied
in the sense of producing several abnormally long and thick
shoots, and the galls are usually associated with the buds along
the course of these. No fungus could be found suggestive of
the Jamaica species, nor any which would reproduce the disease
from inoculations. Galls occur in a similar way now and again
on cacao trees (see p. 176).
It would appear that the Jamaica disease is not present m
these islands, and that the galls which do occur are not of an
infectious nature.
Anthracnose of Lime.
(Wither-tip, Blossom Blight, and Fruit Canker.)
This is a very destructive disease of lime trees, in which the
ringing and tip-wither of new shoots, the infection and curling
of tender leaves, the blighting of buds and flowers, the loss of
newly set fruit, and the production of cankers on fruits which are
retained, are due to infestation with the fungus Glceosporium
limetticolum Clausen.
History and Distribution.
The identity of this fungus, and its specialised parasitism
on the lime and lemon have only in comparatively recent years
emerged from confusion with Colletotrichum glceosporioides Penzig,
which is very widely distributed on citrus trees generally, and
has been variously regarded as a dangerous parasite and as a
harmless saprophyte. The latter fungus (see separate account)
is common in these islands, doing Uttle recognizable harm,
whereas the affection under notice has been absent so far as is
known from the islands lying between Trinidad and Porto Rico
until its appearance and rapid extension in Dominica in 1922.
Recurring heavy losses of the nature indicated in the introductory
paragraph occur in British Guiana, where they have been attri-
buted to the direct effects of wet weather, but specimens received
in April, 1919, were found to be heavily infested with a fungus
indistinguishable from G- limetticolum An epidemic of the
2o8 DISEASES OF CROP-PLANTS
disease occurred in July and August, 1918, throughout the
colony of Trinidad, where it is now quite general and has put
a stop to the development of a lime-growing industry. An
attempt is being made to eradicate the disease from the one
district in Tobago in which it has appeared. The affection has
been longer known as a serious trouble of lime trees in Cuba
and Florida, and from accounts seen probably occurs in Jamaica.
A severe epidemic results in almost total loss of the current crop,
and even a mild prevalence has a marked effect on production.
Symptoms.
The fungus does not infect mature foliage or twigs, or fruit
after it has reached about three-fourths of an inch in diameter.
The attack is made on the young shoots while they are still
tender, and on the blossoms and young fruit. Infected shoots
commonly dry up at the tip for a distance of an inch or more,
or are ringed by an infection further back, and leaves which are
infected but not killed have dead areas at the tips or edges which
cause them to become distorted in their further development.
Young plants in nursery beds are similarly affected.
On the inflorescences a proportion of the buds have complete
petal infestation, turn brown and fail to open ; expanded flowers
become infested and fall without setting, and newly set fruit
is infested, most frequently by way of the stigma and style,
and shed. Infected fruits which are not shed show lesions
varying from rounded spots to rough deep cankers occupying
nearly half their surface.
The blossom blight and wither-tip may exist together, or either
one may be severe without the other being much in evidence,
the difference probably depending on the weather prevailing at
the time of greatest susceptibility. In the same way considerable
differences appear in fields or plantations not far apart.
There can be no doubt, from the variations in the incidence
of the disease, that it is greatly influenced by weather conditions,
but sufficient evidence has not been collected to enable an analysis
of the dependence to be made. No influence from cultural
conditions has been detected.
The Fungus.
The fungus is with difflculty distinguishable from the Gloeo-
sporium condition of Coll. glceosporioides, but setae have never
been observed. The distinctions indicated by R. E. Clausen
are as follows : the acervuli (pustules) in G. limetticolum are
flesh-coloured or salmon pink throughout, while those of the
other fungus are frequently dark, and on bleached dead spots
on leaves and twigs are black. The conidiophores are hyaline
throughout their length, and arise from a rather loose stroma of
hyaline hyphae, while those of C. glceosporioides are pale sooty
DISEASES OF LIME AND OTHER CITRUS TREES 209
at the base and are situated on a compact black stroma. In
cultures the latter species forms a black stromatic pellicle, a
feature absent from cultures of the parasite. The conidia are
more pointed than those of C. glceosporioides, with less tendency
to be constricted in the middle. The conidia when fresh are
markedly less granular, with a less distinct nucleus. The
measurements of the two fungi are about the same, the conidia of
the parasite being 14-18x6-7 microns, the conidiophores
20-30 X 5 microns.
Control.
A disease of this description could be readily controlled by
spraying, were it not that the flushes of new foliage and the
blossoming of the lime are liable to be scattered and erratic.
There are, however, fairly well-marked times of maximum
production, which vary from place to place, but are pretty
constant for any particular locality. By spraying at these
times it may prove possible to save a paying proportion of the
fruit, and it should be noted that it is just as important to save
the shoots as the blossom, since it is upon these that the next
crop of flowers is formed.
From information regarding field studies of control made on
behalf of the U. S. Department of Agriculture, kindly supplied
by H. P. Fulton, it is learned that a careful spray schedule
applied at the time of blooming and early setting can be used
with success, though whether the crop will justify the expense
is doubtful. An early cleaning-up spray before the blossom
comes is probably desirable. " The first fruit spray should be
made as soon as enough petals have dropped to allow for coating
a reasonable setting of fruit. This would be when the bloom is
perhaps one-half to one-third advanced. Later spra3^s will
depend upon the progress of the bloom and on weather conditions
which may favour the disease. In a general way two or three
later applications at intervals of one to two weeks are necessary."
Burgundy or Bordeaux mixtures are the most effective as
sprays, but have the disadvantage, where scale insects are con-
trolled by fungus parasites, of killing out the latter and so
inducing scale infestations. This action is much less marked
with lime-sulphur solution, which nevertheless gives satisfactory
control of the disease. The best results have been obtained
with commercial lime-sulphur diluted at the rate of about one
in 30 to one in 40, care being exercised not to use the greater
strength when the temperature is high. For this reason spraying
is best done in the cool of the morning or evening. The adoption
of a copper spray in the first instance, with the use of lime-sulphur
for succeeding sprayings, the combination found desirable for
citrus scab in Porto Rico, may be the most satisfactory system
to adopt. When no trouble is anticipated from scale insects
copper sprays may be used throughout.
210 DISEASES OF CROP-PLANTS
Grey Blight.
A fungus identified by G. Massee as Thelephora pedicellata,
Schwein, now placed in the genus Septobasidium, forms a dense
slightly raised layer of felted mycelium, slaty-grey when fresh,
changing to brownish grey, on leaves and twigs of lime in St.
Lucia, Dominica, and Montserrat. The same or a very similar
fungus has been seen in large patches on pigeon pea stems in
St. Vincent and Trinidad. S. Langloisii Pat., which may very
well be the species above referred to, is recorded by E. A. Burt
from Grenada. A brown species occurs in Dominica.
According to T. Petch, the genus Septobasidium, made up
of species all of similar encrusting habit, is parasitic on scale
insects, and this is so far borne out by local experience that the
fungus is found in association with colonies of purple scale on lime,
and mass infestations of a white scale (Chionaspis ?) on pigeon
pea. In the former case the mycelium has been found growing
under the scale and attacking the body of the insect.
An effect which may be accidental but is none the less real
is sometimes produced on the enveloped twigs and small branches
of lime. There is a free production of gum from raised swellings
where the bark is covered by the fungus, and this in a number of
cases seen has resulted in the death of the portion of the branch
lying beyond the invested part.
No evidence of penetration has been found, and the most
probable explanation of the effect is that it is due to suffocation.
It has no serious importance.
Leaf Spots,
Brown patches extending from the tip or margin of the leaf,
or smaller spots, occur occasionally, and are infested with Col-
letotrichum glceosporioides. (See Citrus Anthracnose).
Roundish raised spots, foxy red or black, caused by Cephal-
euros mycoidea are common in some localities. (See Algal
Disease, p. 152).
Citrus Trees in General.
Citrus Anthracnose.
A position of some uncertainty exists with regard to the extent
of the parasitism of the very generally distributed fungus Colleto-
trichtim gloeosporioides on citrus trees. It has been widely
regarded in the United States and some parts of the West Indies
as the cause of wither-tip, spotting and streaking of fruit, and
spotting of leaves on citrus trees generally. With the separation
from it of the active parasite Glaosporium limetticolum (see
Anthracnose of Limes) which has been shown to be responsible
for the blossom blight, wither-tip, and fruit canker of limes.
Fig. 79
Foot-Rot on Old Seedling Orange Tree
{a) Margin of Rot indicated by Chalk Line
Fig. 79a
{b) Treatment in Progress
Dull. 262. California Agri. Expt. Sta.
DISEASES OF LIME AND OTHER CITRUS TREES 211
and is probably the species producing these effects on the lemon,
there is a considerable weight of opinion at the present time
that the fungus in question is mainly saprophytic, and at most
a weak parasite of plants or parts otherwise injured or in poor
condition. This is the position taken up in recent accounts
from Cuba and Porto Rico, and agrees with experience in the
Lesser Antilles, where, owing to the absence of the more active
fungus, the affection has never been held to be of much account.
The fungus appears with great regularity on dead and dying
twigs or leaves of citrus trees, even when these have been re-
moved from the tree in a healthy condition. It is thus difficult
to secure evidence of parasitism, especially as infection experi-
ments on healthy material are reported in many cases to have
completely failed.
Symptoms.
Spots attributed to this fungus occur on leaves in the form of
dead brown areas extending often from the tip or margin, in
which develop numerous small dark pustules of the fungus, often
arranged concentrically. The inj ury appears usually on senescent
leaves or as a consequence of insect injuries.
On the fruits the fungus is found occupying hardened brown
spots or large brown to blackish blotches up to an inch or two in
diameter on the rind ; these are generally somewhat sunken, and
are situated in any position. As a rule only a few isolated
spots occur in the produce of any particular tree, and usually they
can be attributed to some predisposing injury. On trees which
are suffering from the general effects of some serious disease, how-
ever, a large percentage of fruit may be attacked by anthracnose.
Tear-streaking of the rind is attributed to the effect of the ger-
mination of spores washed down in quantity from dead twigs.
In the great majority of cases the fungus as it occurs on the
twigs appears to be entirely saprophytic ; in a few instances
which have come under the writer's observation it has assumed
the aspect of a parasite on twigs of which the resistance has been
lowered by external conditions.
Fungus.
The fungus in its common Gloeosporium condition is difficult
to distinguish from G. limetticolum, in comparison with which its
characters are discussed (page 208). On dead twigs fringes of
dark setae may be produced around the pustules, which then
appear as black dots to the naked eye. Setse have not been ob-
served in the lime parasite.
Control.
The position of the fungus as a weak parasite able to attack
only injured or debihtated parts indicates the nature of the
remedies to be applied where its effects are regarded as serious.
212 DISEASES OF CROP-PLANTS
GUMMOSIS.
The formation and exudation of gum in citrus trees is a general
reaction to injuries which may be caused by- mechanical means,
by chemicals, by physiological disturbances, or by infestations
with fungi. There are consequently various types of disease
of which it is a symptom. Two of these, foot-rot or mal-di-
goma and Diplodia disease are separately dealt with.
Symptoms closely resembling those of foot-rot, but not
confined to the base of the stem, have been regarded as evidence
of a distinct disease. On lemon trees in California, according
to H. S. Fawcett, gummosis of this type is very common, and has
been shown to be due in many cases to the fungus Pythiacystis
citrophthora of Sm. and Sm. In another variety of gummosis on
lemons in California the fungus found is Botrytis cinerea. The
first-named fungus inhabits the soil, infects the bark about
soil-level, and spreads indefinitely upwards. It is closely related
to Phytophthora. The conditions favouring the disease are the
same as in the case of foot-rot, and the same methods of pre-
vention and treatment are successful.
There are other forms of gummosis of stem and branches in
which, as in many cases of foot-rot, parasitic causation has not
been demonstrated. These affections begin with the degenera-
tion of a patch of the inner bark, resulting in the production of
gum or " sour sap " which oozes out if any crack in the bark is
available. The bark may die completely over this patch, or
the gum layer may be enclosed by new growth and form a charac-
teristic pocket.
The external development varies from dead sunken or bare
patches to rough bark broken up into scales owing to the irregu-
larity of its production by the cambium. The latter form of
affection closely resembles one aspect of the Florida scaly bark
disease, which differs in further attacking the twigs and fruit.
For practical purposes these types of bark affection on citrus
trees, including foot-rot, are best regarded as manifestations of
some unfavourable relation, which may be temporary or perman-
ent, between the plant and the physical conditions of its environ-
ment. Injuries due to temporary conditions may be met by
the treatment given for foot-rot. The subject of more general
incompatibility is discussed in the introduction of this section
on citrus diseases.
Foot-Rot or Mal-di-goma.
The type of disease to which these names are applied is
characterised by the decay of bark and wood on the crown and
collar of the tree, accompanied by the formation of gum and its
exudation from the affected parts. Disease of this general
type occurs in most if not all the citrus-growing countries of
Fig 80
Canker on Grapefruit
Bull, I2.|, Florida Agri. Expt. Sla.
Fig. 81
Canker on Leaves, showing Pale Zone around the Spots
Bull. 12 u Florida Agii. Exp!. Sla.
Fig. 8L>
Canker on Young Twig
DISEASES OF LIME AND OTHER CITRUS TREES 213
the world, and has been reported from time to time as the cause
of heavy losses.
It is not a prominent disease in the Lesser Antilles, as the
principal citrus tree, the lime, shows little susceptibility to it
under local conditions. Apparent instances have been met
with on this host in St. Lucia, Dominica, and Grenada, but only
under special circumstances. The case is very different with
seedling orange trees, which, in Dominica at least, invariably
develop this trouble as they begin to pass their prime. Similar
experience is recorded in the United States and Cuba. In a
district of Barbados which has lost most of its orange trees
the old surviving trees examined by the writer were affected
in this way.
In the heavy clay lands of British Guiana the lime is subject
to the disease.
Symptoms.
Foot-rot proper, as distinguished from other forms of gum-
mosis on citrus trees, is confined to the collar and crown roots.
It begins in a patch or patches, often in the hollows formed in the
neighbourhood of the crown ; there is often an exudation of
gum through cracks in the bark, the bark dries up or is rotted
by fungi and bacteria, and the wood is eventually left bare and
begins to decay. A sour smell usually accompanies the disease.
The patch spreads around the base of the stem and for some
distance along the main roots and the tree is slowly killed.
Usually, as in other bark diseases, a thinning and yellowing of
the foliage occurs, and, as the encircling of the stem approaches
completion, heavier crops of fruit are set.
Causation.
Various fungi have been found associated with this type of
disease in different parts of the world, but it is generally regarded
as of non-parasitic origin. Recently H. S. Fawcett has suggested
the complicity of the fungus Pythiacystis citrophthora in at
least part of the cases passing as examples of foot-rot in America.
The more recent investigations of H. E. Stevens in Florida
have shown that the disease there is caused by Phytophthora
parasitica Dast [Ph. terrestria Sherb.) and this fungus has also
been found in cases of foot-rot in Cuba, Isle of Pines and the
Argentine.
Apart from this question, there is general agreement as to
the conditions under which the disease is liable to appear.
Heavy or compacted soils, imperfect drainage, too deep
or too close planting are recognised as predisposing causes.
As noted above, it is also very liable to appear, even under
seemingly favourable conditions, on seedling trees of the suscept-
ible species after they reach a certain age. It seems probable
214 DISEASES OF CROP-PLANTS
that some of the so-called epidemics of foot-rot have been due
to this stage being reached by the trees of a plantation or of a
district.
The mandarin or tangerine varieties, and the sweet orange
are most susceptible, the lemon less so, while the sour orange
and the rough lemon are decidedly resistant. In these islands
the shaddock and grapefruit are regarded as resistant, but
according to H. S. Fawcett this is not the case with some strains
of the latter.
Prevention and Control.
The surest method of prevention is the use of sour orange
stocks. With seedling trees already established the ameliora-
tion of such conditions as are mentioned above should be carried
out so far as it is possible. Especially should the collar and
crown be kept free from soil. Treatment of existing cases is
possible on the following lines : permanently remove sufficient
soil to leave the cohar and crown roots exposed ; scrape and cut
away the dead bark until the patch presents a clean surface
surrounded by a margin of healthy bark, and paint the whole
place over with Bordeaux paste or a good wood preservative
such as is used for treating pruning cuts. Remove any low
branches which interfere with the free circulation of air around
the base of the tree.
Scaly Bark.
According to H. S. Fawcett, the term scaly bark is used for
two distinct affections in California and Florida respectively.
The first, psorosis, occurs in Cahfornia, Florida and Cuba. The
same or a closely similar affection occurs on lemon trees in the
Dominica Botanic Gardens. A patch of outer bark breaks up
into irregular scales, which are pushed up by new growth beneath.
The area expands slowly for years, and finally the bark may die
quite through and the wood begin to decay. There seems to be
little to distinguish the affection from a form of gummosis, and it
should be treated by the scraping and painting method described
under foot-rot.
The Florida scaly bark, or nail-head rust, is described as a
specific disease, mainly of the sweet orange, not known outside
that State, and believed to originate from infection with a
variety, ciiricolum, of the fungus Cladosporium herbanim Lk.
On the smooth bark of branches, of the age of 6 to 9 months
or more, raised rusty spots one-sixth to one-half inch broad,
with well-marked edges, are formed. The bark becomes glazed,
and then breaks into flakes and scales. On larger limbs the bark
may become rough and shaggy. On the fruit occur brown
sunken spots, some of them in the form of rings, one-fifth to one
half inch across. The spots are yellowish to reddish brown on
DISEASES OF LIME AND OTHER CITRUS TREES 215
the green fruit, finally becoming dark. Trees heavily infested
have been successfully treated by heading back and spraying
with Bordeaux mixture.
Scab.
Nature and Distribution.
Citrus scab is an affection which produces corky raised areas
and warts on young growing leaves, fruits and sometimes twigs,
resulting commonly also in some deformation. In the tropical
and sub-tropical regions of the New World it occurs with great
frequency, in some districts quite regularly, on sour orange and
rough lemon. From these hosts, in common use as stocks, it
appears to have developed strains virulent to grape-fruit, and it
is in connection with the important business developed in Florida,
Cuba and Porto Rico for the supply of the American market
with this fruit that scab has assumed great importance. The
lemon industry, so far as it exists in the places mentioned, has
also been seriously affected by it. The sweet orange and most
of the mandarin group show little susceptibility to the disease
The lime is reported to be attacked in the Isle of Pines (Cuba)
and in Ceylon. In Porto Rico and in the Lesser Antilles this
host has not been noted as susceptible.
The disease occurs on the sour orange in these islands, but
owing to the scattered distribution of its appropriate hosts and
their small commercial importance has received little attention.
In addition to the localities already mentioned, scab of the
type under notice is recorded from Japan, Australia, and South
Africa. It fails to develop any virulence in the dry climate of
California.
The disease is reported to have made its appearance in Florida
in 1884, and to have spread rapidly through that and the adjoining
States. It is conjectured to have been introduced from Japan.
Appearance and Effects.
It is advisable to describe the disease in some detail owing
to the liability to confusion between scab and canker. As
met with by the writer in these islands the affection has usually
consisted of small roundish raised corky spots on leaves and green
twigs. The following description quoted from J. A. Stevenson
refers to the more virulent form occurring in Porto Rico.
" The first signs of infection are circular, minute, trans-
lucent areas followed by a rapid production of the corky out-
growths so characteristic of the disease. On the fruit these
corky outgrowths, wartlike in appearance, are produced,
and vary much in size and shape, often running together
or occurring in such numbers as to cover a large percentage
of the surface o^ the infected fruit. At times plateau-like
2i6 DISEASES OF CROP-PLANTS
areas are produced, of an inch or more in diameter, irregular
in shape and marked by the dying of the epiderm and its
breaking up into silvery scales. Again it may take the form
of ridges, conical elevations, or other shapes. The corky
areas are dull brown in colour in some instances, but very com-
monly are a dull red with brown margins. This latter stage
occurs where the disease is especially virulent, the ridge and
plateau conditions being more often noted in instances where
but Httle disease is present. This latter is the type found on
oranges. Young fruits are frequently much distorted, assum-
ing triangular or other peculiar shapes. Those most dis-
torted fall to the ground soon after attack, It may be noted
that a larger percentage of scabbed fruits fall than of normal
ones. Infected fruits remaining tend to regain their normal
shape by subsequent growth, and as there is no increase in
size of the initial infections, the great increase of the surface
areas of the fruit gives all appearance of a partial recovery
or ' cleaning up ' from the disease. This, of course, is not
what takes place, but it merely means that there has been
no further spread of the scab areas.
" Leaves are frequently distorted, the same corky areas
appearing as on the fruit. Where infection is not general,
very marked conical projections, often as much as a centimetre
in height, appear.
" The disease areas are surface infections only, there being
practically no penetration in either leaves or fruit of the inner
host tissue. In fact, so effectively are the tissues protected
that scabbed fruits are not any more subject to decay than
normal ones ; less so, if anything. Many such have been
kept in the laboratory for long periods of time until they
finally become hard and mummified."
Causation.
There has been some dispute as to the causation of the disease,
but those mycologists most closely in touch with it are convinced
that it is due to infections, occurring only when external circum-
stances are favourable, with the fungus Cladosporium citri
Massee. The occurrence of the fungus in the scabbed tissue is
somewhat obscure and it is said to be masked, in Florida at least,
by a saprophytic species resembling the common Cladosporium
herbarum. The parasite is regarded as not typical of the genus
Cladosporium as ordinarily understood. The only reproductive
bodies obtained in pure cultures are thick-walled, dark-coloured
packet-like clusters or chains of cells, and these were also found
on old withered scabs.
The description of C. Citri Mass. is as follows : — Conidiophores
tufted, erect, branched, septate, brown, 30-75 X 2-4 microns,
conidia fusiform, dusky, continuous, or 1-3 septate, 8-9X2.5-4
microns.
DISEASES OF LIME AND OTHER CITRUS TREES 217
Influence of External Conditions.
Infection only takes place on young and tender parts of the
plant, and is very largely dependent on the prevalence of moist
and cloudy weather in periods when new growth is being rapidly
developed. Trees in poor condition are but little subject to
attack, presumably owing to the scarcity and slow development
of new shoots upon them.
Inherent Resistance.
According to Stevenson scab was generally prevalent on
sour orange and rough lemon stocks in Porto Rico by the year
1903, but grapefruit was almost immune until the occurrence
of an excessively wet season in 1911-12. Then the grapefruit
was attacked suddenly and in most virulent fashion in one
section, and since that date the disease has maintained its new
character and invaded most of the other citrus districts in turn.
This occurrence gives colour to the suggestion that a new and
virulent strain has come into existence in the island.
Prevention.
Outbreaks of scab are preventable by protective spra3/ing
with Bordeaux mixture, but this method has been found to have
the serious drawback of also repressing the fungi which keep
down scale insects, thus opening the way to serious infestations
with the latter. It is found that better general results are
obtained from the use of a lime-sulphur spray, which while
much less effective against scab is largely free from the objection
named.
Black Melanose.
The affection known as true melanose in Florida, and ascribed
to the fungus Phomopsis Citri does not appear to occur in the
West Indies. It is described as a superficial marking of the sur-
face of fruits, leaves and stems, consisting of small raised areas
with the appearance of wax or of small drops of melted sugar
burnt brown. These markings sometimes occur in patches and
circles.
The melanose met with in Cuba, Porto Rico, Jamaica and
the Lesser Antilles is a leaf affection distinguished as black
melanose or greasy spot. It occurs on various citrus species
in these islands, including rough lemon, grapefruit and orange
The back of the leaf is covered with slightly raised black and
brown markings, somewhat resembling, it has been suggested,
spots of dark grease under the epidermis. The affection is very
persistent, but does no apparent damage. Its nature is unknown.
2i8 DISEASES OF CROP-PLANTS
Citrus Canker.
History and Distribution.
The existence of the serious infectious disease now known as
citrus canker became known through its introduction into
Florida, though it had been present in some of the Western
States of the Union, notably Texas, for some years previously.
It appears to have come in from Japan on Citrus trifoliata stocks,
upon which the disease is not prominent. The date of first
introduction into Florida is authoritatively stated to be Novem-
ber, 1912, which is a matter of importance to those who have
received nursery stock or bud-wood from that State. In the
following year the disease began to cause alarm in the grapefruit
industry and a special organisation was created to deal with it.
The very drastic efforts which have since been made to entirely
eradicate the infection have had the effect of thoroughly sup-
pressing the disease, and their ultimate success remains to be
tested by time.
The distribution of citrus canker, as known at present, is as
follows : — United States (Texas, Louisiana, Missisippi, Alabama,
Florida), Japan, China, the Philippines, Java, Singapore, South
Africa and Australia.
Symptoms of the Disease.
The following description of the appearance of citrus canker
on leaves, twigs and fruit is quoted from H. S. Stevens : —
" The distinguishing feature of citrus canker as observed
in the field is the characteristic spotting produced on the
fruit and foliage. As usually seen the infection appears as
small light brown spots, from less than one-sixteenth to
one-quarter of an inch in diameter. The spots are usually
round, and may occur singly, or several may run together,
forming an irregular area. This last usually occurs on fruits.
The spots project above the surrounding healthy tissue, and
are composed of a spongy mass of dead cells, covered by a
thin white or greyish membrane. The membrane finally
ruptures and turns outward, forming a lacerated or ragged
margin around the spot.
" On the leaves, infections first appear as small, watery
dots, with raised convex surfaces. These dots are usually
of a darker green than the surrounding tissue. Sometimes,
however, the surfaces of the spots are broken as soon as they
appear. Spots may appear on either surface of the leaf, but
they do not at first penetrate through the leaf tissue. They
gradually increase in size, change to a light brown in colour,
and become visible on both sides of the leaf. In the older
spots one or both surfaces may be bulged or raised, and such
DISEASES OF LIME AND OTHER CITRUS TREES 219
spots are commonly surrounded by a narrow yellowish band
or zone. In the more advanced stages, the surface of the spots
becomes white or greyish, and finally ruptures, exposing a
light brown spongy central mass. Old spots soon become
overgrown by saprophytic fungi, and may appear pink or
black on account of these fungus growths.
" On the fruits the spots are very similar to those formed
on the leaves. They do not penetrate far into the rind. They
may be scattered over the surface, or several may occur
together forming an irregular mass. Gumming is sometimes
associated with the spots formed on the fruits. Canker,
apparently, does not cause a rot of the fruits directly, but opens
the way for other fungi to enter and cause infected fruits to
rot. The spots on young twigs are like those on the leaves
and fruit. On the older twigs they are more prominent, and
more or less irregular in shape. This is specially true of old
spots. They show the same spongy tissue that is found in
the spots on the leaves, but assume a cankerous appearance
and the surface membrane completely disappears. These
spots or cankers are formed in the outer layers of the bark
tissue, and do not penetrate to or kill the wood. The spots
once formed in the bark are persistent, and are not readily
sloughed off. They may remain for a long time and form
centres from which infections may readily spread. This was
confirmed by observations on infections produced on spotted
trees in the greenhouse, and in the grove by artificial infection.
Some of these spots have been under observation for over a
year, and show no tendency to slough off."
R. A. Jehle in a later account emphasizes the diagnostic
value of the translucent, glazed, oily appearance of the zone
immediately next to the ruptured portion of the spot. This is
lacking from scab, which moreover distorts the leaves and causes
warty projections on the fruit, characteristics which are absent
from canker. Scaly-bark spots are hard and glazed, those of
anthracnose are sunken.
SusceptihUity.
It has not been established that any citrus plant is quite
immune from canker, but wide gradations in susceptibility
occur. From the Philippines it is reported that the common
lime, which is presumably the commercial lime of the West
Indies, is highly susceptible, while a variety known as the Tahiti
lime seems practically immune.
Effects.
The direct losses due to the disease result from its effects on
the fruit. The rind is more or less covered with the canker
220 DISEASES OF CROP-PLANTS
spots, and, although this condition does not directly affect the
edible portion, the fruit is not only rendered so unsightly as to
be unsaleable, but is very liable to splitting and to the inroads
of secondary fungi, which cause rotting.
The very heavy monetary cost of the disease in Florida has
been due to the wholesale destruction of infested groves in the
attempt at complete eradication.
Causative Organism.
The disease is due to the action of a specific bacterium,
which has been described under the name of Pseudomonas Citri,
Hasse. The organism is yellow, one-flagellate, is variable in
shape and size, but as obtained from young cankered tissues is
usually a short rod with rounded ends, which measures
1.5-2.5 X 0.5-0.75 microns. No host-plants are known outside
of the order Rutacese.
Mode of Infection.
Infection occurs through natural openings and through
wounds. Drops of dew or rain on affected organs contain the
bacteria in large numbers, so that the disease is very readily
transferred by physical or animal agency from plant to plant.
The disease is, in fact, extremely infectious and has been seen
to spread with incredible rapidity.
Fallen leaves are believed to carry over the infection, and
inconspicuous lesions containing the parasite can occur on
mature wood. H. A. Lee has determined in an extensive series
of experiments that the organism quickly disappears from
unsterilized soil. Dried on cloth, according to Stevens, it was
still alive after five weeks.
Influence of External Conditions.
The disease is most prevalent and destructive in warm humid
weather, and is checked by cool dry periods. It is much more
active on thrifty growing trees than on those in poor condition,
and on the latter it may exist in a suppressed or dormant con-
dition for long periods. The frequent flushes of growth on
limes in Dominica and St. Lucia would, so far as can be foreseen,
afford very favourable conditions for the development of the
disease should it be introduced.
Control.
Reports as to the effects of spraying are contradictory. In
Florida cutting grapefruit trees down to stumps and coating
them with Bordeaux mixture did not prevent the new growth
from being infected. In South Africa an outbreak was success-
fully controlled by pruning and three sprayings with Bordeaux.
DISEASES OF LIME AND OTHER CITRUS TREES 221
Similar results have been claimed in Texas. Recent work by
H. A. Lee goes to show that only alkaline Bordeaux is effective,
and that the excess lime, not the copper, is the active agent.
Lime-sulphur, at ordinary strengths, is indicated as likely to be
effective.
Exclusion.
Citrus trees imported into the Lesser Antilles in recent
times, before the danger from citrus canker was known, have as
far as possible been examined, and no trace of the disease has
been seen. Thorough surveys of Cuba and Porto Rico have been
made with the same result.
In order to exclude the disease ordinances have been issued
in the British islands designed to exclude material that may
possibly be infected, and it is obviously desirable that these
should be carefully enforced.
Diseases, Injuries and Rots of Citrus Fruit.
Shipping trade in citrus fruits in the Lesser Antilles has
been developed on a large scale only in connection with the
New York market for green limes. Traffic in oranges and
grapefruit is confined to inter-island exchange carried out in an
unorganised fashion. Barbados especially, with its dense popula-
tion and lack of home-grown fruit, affords a market for low-
grade seedling oranges.
It is understood that green limes, selected and packed with
reasonable care, usually reach their destination in good condition.
Such complaints as are current appear to refer to poor material
carelessly packed by casual speculators. No examination of the
fruit for defects in condition as it reaches the market appears
ever to have been called for, or made by any representative of the
growers.
Under these circumstances there is but small interest in
these islands in a subject which in countries with a large trade in
citrus fruits has received a very large amount of attention. The
affections which have been met with are noted below.
It should be borne in mind that in addition to injuries and
infestations which take place while the fruit is growing or ripening
on the tree, often as part of the manifestation of diseases also
affecting other organs, there is a definite class of troubles which
are solely due to causes connected with picking and storage,
while the damage due to some of the affections of the former
class may be greatly extended by the manner of handling the
fruit during and after picking.
In this connection reference should be made to Chapter XIII
on the subject of handling fruit in general.
222 DISEASES OF CROP-PLANTS
Diplodia Rot.
The fungus associated with Diplodia dieback of citrus (which
see) is the commonest, in fact the only definitely recognised cause
of rotting of citrus fruits in the field in these islands, and reports
to the same effect have been made from Cuba, Porto Rico and
the Isle of Pines. It occurs to some extent on limes, and pre-
sumably other citrus fruits, while they are still on the tree, but
is most noticeable in the ripe limes lying on the ground.
The fruit becomes thoroughly infested and soured by the
fungus, the rind turns brown, gum is often exuded, and under
moist conditions pycnidia are developed which cover the surface
with a sooty deposit of spores ; under circumstances of greater
dryness or ventilation the fruit dries up and shrivels. Punctured
fruits are especially liable to infection.
Diplodia rot occurs on lemons in Natal, and on oranges and
grapefruit in the Greater Antilles and Florida. It is reported as
spreading mostly from the stem end or from an injury. The
fungus involved is usually named Diplodia natalensis, but no
sufficient reason seems to have been given for distinguishing it
from D. TheohromcB.
Scab.
Corky irregularities of the rind, not leading to rot or decay
or affecting the contents of the fruit. Refer to general descrip-
tion of the disease.
Russeiing.
Silvery or sometimes brownish scurf general on the rind,
giving the fruit a dull appearance. The epidermis is broken up
into minute scales. Sometimes very common on oranges and
limes for a period of a few weeks in a particular situation, then
going off. Here as in Florida both rust-mites and a fungus of
the genus Coniothecium are found on such fruits, and it is un-
certain how far each is responsible, alternatively or in com-
bination, for outbreaks. Dusting with sulphur should be adopted
as a remedy where appearance is of value, otherwise there is no
harm done.
Oil Spot.
One of the commonest of blemishes on local citrus fruits is
a small superficial green or brown spot, usually more or less
circular, which when closely examined is seen to consist of an
area in which the ground tissue is slightly sunken, leaving the
oil glands standing out as small green or yellowish papillae. In
old examples cork formation has taken place below the spots.
The prevalence of this condition caused some alarm on a Dominica
orange estate some years ago, as it was thought to facilitate
decay. Experiments showed that this belief was unfounded and
DISEASES OF LIME AND OTHER CITRUS TREES 223
that the defect is one of appearance only. It was found that
similar spots could be artificially caused by scratching the rind.
A convincing explanation of this condition, which is trouble-
some on lemons in California, was supplied by H. S. Fawcett
to the effect that it is caused by the action on the tissues of
drops of the essential oil of the fruit, when this is liberated by a
scratch or a slight bruise. It is found that the spots are only
produced when the fruits are moist with rain or dew, and that the
restriction of picking to dry fruit largely decreases the number of
spots which subsequently develop.
Splitting.
In Barbados the shaddock is especially liable to this trouble-
It is caused by failure of the rind to keep pace in growth with
the contents of the fruit. This may be caused by surface injuries
due to scale insects or fungi, and apart from these sometimes
appears to be brought about by irregularities in conditions.
Increased shelter, tillage, mulching, and avoidance of sudden
heavy waterings are measures calculated to prevent the occurrence
of splitting.
Blue Mould,
The soft rot of stored fruit due to blue and green moulds of
the genus Penicillium is universal and familiar. All experience
shows that these fungi are rarely or never able to obtain a footing
on sound uninjured fruit, but that any scratch, or a slight and
imperceptible bruise may suffice to afford an entry and cause
the rotting of the whole fruit. It is this fact which calls for
the most meticulous care in handling fruit which is to be stored
or shipped — a necessity which it would appear impossible for
the heavy-handed West Indian labourer ever to be made to
appreciate. Reference on this subject should be made to
Chapter XIII.
The course of this type of rot comprises, first, a progressive
softening around the point of infection ; second, the appearance of
white mycelium extending over the surface of the softened spot ;
and third, the production from this mycelium of conidia in dense
masses, forming a blue or green layer, and discharged as dust
clouds when disturbed.
Charcoal Rot.
An affection described by 0. T. Wilson as occurring on
Florida limes has been several times met with on West Indian
oranges bought in Barbados. In the fresh fruit it is recognisable
by the presence of surface areas, smooth in comparison with the
normal surface and more or less discoloured, the colour varying
from a deepened yellow to copper colour, with the margin of the
spot mostly brownish. Upon opening a diseased fruit an abnor-
224 DISEASES OF CROP-PLANTS
mal odour can be detected, and a distinctive fungus is always
present. It has been cultivated by Wilson, and the disease
reproduced in inoculated limes.
When the fruit is surface sterilised and left for developments
no soft rot takes place, but it gradually dries up and becomes
mummified, the exterior blackening and the contents being
converted into carbonaceous material mainly constituted of
dense black mycelium, in which condition it remains for months
if protected from scavenging insects. Attempts to induce spore
formation have uniformly failed.
Anthracnose Spotting.
See under Anthracnose of Limes and Citrus Anthracnose.
CHAPTER XX
DISEASES OF COFFEE
At the present day coffee is nowhere grown in more than a
scattered and casual fashion in the British islands of the Lesser
Antilles. From the French islands it is still an important
article of export, enjoying a protected market. Coffee-growing
was at one time the principal industry of Dominica, and was at
the height of its prosperity near the end of the eighteenth
century, when some 20,000 acres, mostly at low levels near the
coast, were occupied by coffee estates, and the export of beans
amounted to 4 to 5 millions of pounds. By 1823 this had declined
to 2 millions, and from 1833 the reduction was extremely rapid,
reaching an average of some 60,000 lb. in the years about 1850
and 15,000 lb. about 1870. At the present time the export is
only about a ton a year.
The almost precipitous decline of coffee production in
Dominica has been attributed to the prevalence of " blights,"
especially a leaf-mining moth. A report made to the President
in 1875 by H. Prestoe, Government Botanist of Trinidad, de-
finitely denies this, and attributes the failure to the neglect of
the most elementary principles of cultivation which followed
the disorganisation of estates after the abolition of slavery,
together with the effects of the reduction of humidity on the
coastal lands brought about by extensive clearing of forest.
At the same time sugar production offered a highly attractive
alternative industry, which was to a considerable extent adopted.
Apart from these causes one may trace the usual history
of planting on virgin soils. In the course of time, slowly with a
tree crop such as coffee, the original fertility declines, no means
of restoration are adopted, the trees grow old and lose their
vigour, while debility diseases and pests of little importance to
healthy trees become evident and receive the blame. A period
of acute depression ensues and it frequently happens, as in this
case, that a change of crop is made.
Alternatively a tradition of sound agriculture based on
cultivation and manuring may be gradually established, and
if it were commercially desirable to revive the coffee industry on
these lines there seems to be no sound reason to believe that the
present known diseases of the plant need cause any serious
interference.
225 Q
226 DISEASES OF CROP-PLANTS
ViRUELA, American Leaf Disease.
A fungus disease of coffee, second only in the attention it
has attracted to the Hemileia disease of the Old World, and
often known, to distinguish it from that affection, as the American
leaf disease, is widely distributed through the American tropics,
to which so far it appears to be confined. It is reported from
all the coffee-growing countries of Central and South America,
and in the Antilles from Jamaica, Porto Rico, and Dominica.
The writer has so far seen it only once in Trinidad. It has been
found on numerous herbs and shrubs of various natural orders,
and some trees, including orange and mango, and even occurs
on ferns growing in the neighbourhood of coffee. As is not
unusual with fungi attacking introduced plants, it has assumed
on coffee an especial virulence.
The presence and dissemination of the disease are closely
dependent on humid conditions and in these islands it accordingly
occurs most abundantly in the wetter hill districts.
Symptoms.
The disease consists in the production by the fungus of spots
dark at first, ultimately white or light grey, on leaves, young
stems, and fruits. On the leaves the spots are numerous and
conspicuous. They are approximately round, equally visible
on both sides of the leaf, and range from 6 to 12 mm. {\ to \ inch)
in diameter. As they become old the central tissue drops out
leaving a perforation. On the shoots the spots are more or less
elongated and from them the cortex eventually flakes away.
The spots on the fruit are similar to those on the leaves ; they
are rarely more than one or two in number on the same berry.
The beans obtained from the affected berries are to some extent
discoloured.
The effect of an infestation of the leaves is to cause premature
shedding, and entire defoliation may ensue from a heavy attack,
so that the ground is carpeted with leaves partly green. This
stops the development of any crop that may be on the trees, and
may lead, especially if soon repeated, to dieback of shoots and
to a condition of debility in the trees.
According to G. L. Fawcett, in Porto Rico the older planta-
tions in districts liable to the disease are well infested, while the
younger ones are as a rule free, and the spread of the disease is
quite slow, having been estimated in an observed instance to
take place at the rate of about 200 yards in a year.
Causative Fungus.
The nature of the disease was first detected by M. C. Cooke
in 1880. He described the fungus found on the spots as Stilhum
flavidum, a name later changed by Lindau in a taxonomic re-
arrangement to Stilbella flavida. G. Massee in 1909 found peri-
>i
Fig. S3
American Leaf Disease of Coffee
Bull. 17 Porto Rico A.E.S
DISEASES OF COFFEE
227
thecia on leaves which had been kept for some weeks in a moist
chamber, and, claiming to have produced typical spots by in-
fection with the ascospores, described the fungus as Sphcerostilhe
flavida. Fawcett in Porto Rico failed to secure infections with
the spores of a fungus corresponding to Massee's description.
More recently A. Maublanc and E. Rangel, working in Brazil,
have given a convincing account of experiments in which, by
keeping infested leaves in a very moist atmosphere, they obtained
an unbroken series of gradations between the form of fructi-
fication appearing in the field and a minute agaric (mushroom-
shaped fungus) with a bristle-like stalk and a membranaceous,
radially striated, hemispherical cap. They accordingly changed
the name of the parasite to Omphalia flavida. While it is de-
sirable that the observation should be repeated it would seem
that this name is at present entitled to stand.
On the spots as they occur on the coffee plant the fungus
always has the same peculiar form of fructification, which is
to be regarded according to the last-mentioned authors as an
abortive one. It is, however, quite effective as a means of
reproduction, and may be viewed as an adaptation in some re-
spects superior to the ordinary methods of reproduction by spores.
The bodies referred to appear on the spots as slender yellow
stalks 1.5-2 mm. in height, terminated by a rounded head of
the same colour, the whole somewhat resembling a tiny pin.
Some writers have described the formation of conidia on the
surface of these heads, but no infection has ever been known to
be produced by them. WTiat does happen is that the heads
themselves, which are very loosely attached and somewhat
mucilaginous, drop off and adhere to the leaves or berries on
which they fall. When the surface is sufficiently moist a head
so lodged sends out numerous hyphae, which infect the underlying
tissue and give rise to a new spot. The heads retain their
germinating power for some weeks even after having been
dried, and so can be distributed with the fallen leaves.
Control.
The most obvious way of reducing the prevalence of the
disease is by increasing the access of sun and air to the trees,
but as in the similar case of cacao pod-rot it may not be possible
to carry this measure far without producing equal injury from
exposure. Picking off the affected leaves has been found to
exercise some control but can only be profitable where labour is
cheap. Spraying with Bordeaux mixture is quite effective
against the spread of the fungus, provided that the fungicide
can be made to adhere for a considerable time to the leaves.
In this connection the information elsewhere given regarding
adhesives should be consulted. The protection of the upper
surface of the leaves is the more important. No adequate
account of the use of lime-sulphur sprays has been seen.
228 DISEASES OF CROP-PLANTS
Spraying is likely to have its greatest value when adopted
to preserve the freedom of young plantations from the disease.
Fawcett quotes an instance where the advent of the disease
in a plot of which the annual yield was known resulted in a
reduction of output of some 75 per cent.
Brown Eye-spot
A Cercospora spot disease of leaves and berries is common
throughout the Antilles, in Central and South America, and
has been reported from various situations in tropical Africa and
the East. It is the opposite of the disease last described in being
most abundant under conditions of exposure.
Symptoms,
The spots produced on the leaves are round, 6-10 mm.
{\-\ inch) in diameter, visible on both sides ; at first brown,
later becoming almost white in the centre, but retaining a reddish-
brown ring with a sharply defined outer margin. The central
part does not break away when dry, which serves as a rough
distinction from the previous disease. The leaf spots are pro-
duced somewhat scantily and are usually negligible in their
results. On the berries black blotches, which may cover as much
as half the surface, are produced, and are most fully developed
on the upper side where the sun causes earlier ripening. All
varieties of coffee are subject to attack.
Causative Fungus.
The cause of the disease is the fungus Cercospora coffeicola
Berk, et Cke., first described from Jamaica. It is visible on the
surface of the spots in the form of small greenish-olive tufts made
up of the simple conidiophores, on which are formed terminal
conidia. These are long and narrow, tapering to one end,
several-septate, colourless, with dimensions 75 X 3 microns at
the thicker end.
Fawcett has obtained in cultures a grey sterile mycelium
which produced spots with the typical spores on inoculation.
Losses.
The affection on the berry causes trouble in the preparation
of the produce owing to the adhesion of the flesh to the parchment
of the bean. It is also believed to be connected to some extent
with the poor development of some of the beans met with in the
crop.
Incidence and Control.
Although the disease is not entirely dependent on poor
condition in the trees it is most prevalent where shade is in-
Fig. 84
Brown Eye-spot of Coffee
Cook's Diseases of Tropical Plants, by permission of Macmillan & Co.
DISEASES OF COFFEE 229
sufficient, where the soil is poor, or where cultivation and manur-
ing are neglected. It is by improvements in these directions
that the disease should be combated. The question of spraying
is one that does not seem likely to arise, at any rate in the West
Indies. The disease has been reported to be more severe in its
effects in Africa.
Zonal Leaf Spot.
A leaf spot disease described from Porto Rico takes the form
of large spots characterised by concentric rings, sometimes
incomplete and frequently coalescing. On the under side of
the mature spots a thin white mould appears, which in places
becomes more condensed. This bears Cephalosporium fructi-
fications, and by inoculations from pure culture the fungus has
been shown to be parasitic and to be the cause of the spots.
The affection is most frequently found on the best grown trees,
young and well-shaded leaves being most susceptible. It has
been seen on all the varieties in cultivation, but is nowhere
abundant.
Bark Disease.
A disease of stems occurs to some extent in Porto Rico in
which local bark-rot occurs in situations ranging from near the
crown to some feet up the stem, and is believed to commence
from wounds of mechanical or insect origin. The disease develops
its effects slowh', finally causing the bark to dry up and fall away,
leaving the wood exposed and revealing the callosed margins
of the healthy bark beyond. The presence of the affection ia
recognisable by the smaller diameter of the stem where the death
of the cambium has prevented the deposition of new wood.
Under the diseased bark the wood is blackened. G. L. Fawcett,
who describes the disease, reports that a fungus of the genus
Fusarium is always present in the affected tissues, but that
inoculations with pure cultures have failed to produce the
disease. It is communicated readily to healthy stems by means
of small pieces of diseased material and when thus transferred
has shown itself to be rapidly destructive to the living tissues for
several inches above and below the point of inoculation.
Nematode Diseases.
The fibrous roots and the base of the stem of the coffee tree
are liable to infestation with the eelworm Heterodera radicicola,
a general and cosmopolitan parasite described in another section.
On the small roots gall-like swellings are produced and on the
stem the infested bark takes on a very rough and somewhat
swollen appearance, which may extend for a foot or so above the
soil.
230 DISEASES OF CROP-PLANTS
Alarming accounts have been published of the destruction
caused by this parasite on the Brazilian plantations, and it is
regarded as a serious pest of coffee in Guadeloupe and Martinique.
The accounts which have appeared from each of these locaHties
suggest very strongly that the effects described may be largely
due to fungus root diseases, with the symptoms of which they
are in close accord. This impression is to some extent borne out
by Fawcett's remarks concerning the status of the parasite on
coffee in Porto Rico. Although Heterodera is often active
there in trees suffering from root disease, no good evidence has
been found that the trees are injured by it to any extent, and its
presence is not a necessary accompaniment of root disease.
The effects of the same parasite on coffee in the German African
Colonies were reported to be trifling, and easily balanced by the
use of suitable manures.
Tylenchus coffece Zimm. is a pest of the local variety of
coffee in Java. Liberian coffee is immune, and on the newer
kinds the occurrence is sporadic and of small importance. The
larvae enter the tender roots, and occupy the root system of young
plants, causing wilting and death. The species is also reported
from Martinique and Sumatra.
ScLEROTiuM Disease of Liberian Coffee.
A preliminary account was given in 191 8 by G. Stahel of a
disease which had assumed importance on Liberian coffee in
Surinam in the previous year. An earlier mention of the affection
had been made by Kuyper under the name of Coremium disease.
Recently (1921) Stahel has published a full description and has
named the parasite Sclerotium coffeicohim.
On mature leaves the fungus produces brown dead spots,
with distinctly marked concentric rings. In continuously
humid weather these may expand over the entire leaf. The
underside of the spots bears white, spiny outgrowths, 2-4 mm.
long, which do not bear spores, but are easily broken off and
carried by the wind to other coffee trees, on which they serve
to originate new infestations.
The ripening fruits are similarly attacked, with the pro-
duction of concentric-zoned spots, and when dead they are
entirely covered with the white spines. On both leaves and
berries while still on the tree there may be developed in damp
weather brown or orange-brown sclerotia, and these occur in
profusion on the fallen berries. In very damp weather a feathery
mycelium, on which sclerotia also later appear, is produced
on the material lying on the ground.
Where a cluster of fruit is involved as a whole, the fungus
may invade the cushion, and thus inhibit further production.
The disease ceases to extend with the onset of dry weather.
It is supposed that it is again renewed from the sclerotia, but in
DISEASES OF COFFEE 231
what manner remains as yet unknown. Spore-formation of any
kind has not been observed, but from the presence of clamp-
connections in the hyphas the fungus is believed to be a Basidiomy-
cete.
Good results have followed the use of Bordeaux mixture.
The fallen leaves and berries should be collected in heaps and
burned. The disease is at present confined to plantations along
the Surinam River, and is especially severe on an estate on the
upper river. It has not been detected in the field on Robusta,
Uganda, or Surinam coffee, though infections were secured on
these by artificial means. Abeokuta and Excelsa are very
susceptible.
Stahel has also described another disease of Liberian Coffee
in Surinam showing a chronic or an acute phase of wilt. The
precise cause is not known, but degenerative changes in the
sieve-tubes of the inner bast have suggested to him that the
disease is in the nature of a phloem-necrosis like the well-known
leaf-roll of the Irish potato.
RosELLiNiA Root Disease.
Thread Blights. )■ See General Diseases.
Algal Disease.
CHAPTER XXI
DISEASES OF HEVEA.
The South American Leaf Disease.
A FUNGOID leaf-disease which occurs on the wild Hevea species
in the forests of Brazil, Peru, British Guiana and Surinam has
attacked the plantations of Hevea brasiliensis made in the two
countries last mentioned, and has assumed such severity that
there seems no likelihood of the rubber industry becoming
permanently established. The disease also occurs in Trinidad,
where although young plants are severely attacked the mature
trees in plantations have so far almost entirely escaped injury.
It has not appeared in the rubber cultivations of the Eastern
tropics.
The history of the disease is instructive as illustrating the
difference between the incidence of disease on plants scattered
in a natural way and collected in cultivations, and shows also
the advantage that may sometimes be gained by growing a
crop plant away from its native country and thus outside the
range of its specific parasites.
Nature of the Disease.
A detailed investigation of the disease has been made in
Surinam by G. Stahel, from whose paper the following account
is abstracted.
The attack on the leaves begins when they are very young,
and, as they develop, yellowish spots with a grey layer of conidia
on the under side become apparent. The central part of the
larger spots dries up and falls away, leaving a ragged perforation.
On full-grown leaves the surface becomes thickly sown with black
dots, and on old leaves these have developed into rounded black
spots interspersed with several or many ragged holes surrounded
by a black ring.
The fungus attacks the petioles and young internodes, but
much less frequently than the leaves. On the petioles the check
to growth on one side may cause curvature or spiral twisting ;
on the internodes swollen canker-like patches are produced.
The inflorescences and fruits may also be infested.
The Fungus.
The causative fungus Melanopsammopsis Ulei (Henn.) Stahel,
has three forms of fructification : free conidiophores, pycnidia,
232
DISEASES OF HEVEA 233
and perithecia. The conidial form of the fungus was named
Fusicladium macrosporum by Kuijper, Passalora Hevece by
Massee ; it was regarded as pertaining to the genus Scolecotri-
chum by Griffon and Maublanc, a conclusion with which Stahel
agrees. The conidiophores when young are short unicellular
brown hyphae thickened at the base, penetrating the epidermis
from subepidermal mycelium ; this form appears on the young
translucent leaves. On somewhat older leaves the conidiophores
are many-celled, elongated, and arise from a pseudo-parenchy-
matous base. The conidia are formed singly and terminally,
measure 20-65 X8-12 microns, normally are divided into a wider
basal cell and an elongated terminal cell, and are more or less
bent or spirally twisted.
The pycnidial form was first described as Aposphcsria Ulei,
Hennings. The pycnidia occur freely as small black dots on
the surface of leaves 1-2 months old, mostly on the upper side.
They are roughly spherical, with an apical pore and little or no
trace of a papilla. The pycnospores are 5-10 microns long,
2 microns wide at the ends, narrowing to i micron in the middle.
The perithecial form was described by Hennings as Dothidella
Ulei, but is placed by Stahel in the Sphaeriaceas in a new genus
Melanopsammopsis. The perithecia begin to take the place of
the pycnidia some two months after the leaves are full grown.
They occur in rings of 3-4 mm. or rounded groups of 1-2 mm.
diameter, are smooth, carbonaceous, and closely resemble the
pycnidia in form and size. The ascospores are hyaline, two-
celled (one larger than the other), constricted at the septum,
18-22 X 6-8 microns.
A species of Botrytis is common as a parasite of the fungus
in all its stages.
Infection.
The conidia germinate in water in 1^-2 hours and cannot
withstand more than 15-20 hours' exposure in a desiccator.
Stahel found viable ascospores to be rare, and unable to with-
stand more than four to six hours of desiccation. According
to his observations the pycnospores germinate very weakly and
appear to have no part whatever in the spreading of the disease,
nor do the ascospores exhibit more than weak powers of infection.
The conidia produce germ-tubes which penetrate the cuticle
and give rise to a sub-cuticular hypha, from which branches pass
between the cells of the epidermis and produce an intracellular
mycelium. Infection takes places only in the very young and
tender organs. Leaves are most susceptible in the first four days
after the opening of the buds, and lose their susceptibility after
seven days' growth. The first sign of infection is a yellowish
spot which appears in about five days, and one to two days later
the conidial stage appears, followed by pycnidia ten to twelve
days after infection. On the fully developed leaves the pycnidial
234 DISEASES OF CROP-PLANTS
fructifications at first predominate, succeeded later by the peri-
thecia.
Contrary to experience with most diseases, infections are
most abundant in dry clear weather, and least in rainy weather.
This is attributed by Stahel to the more favourable conditions
for penetration provided by an all-night coating of dew than by
intermittent wetting from rain, owing to the fact that the process
requires some ten hours for its completion.
Effects of the Disease.
Severe infestation of the young leaves causes defoliation of
the trees, and infestation less severe hinders the growth of the
leaves and reduces their efficiency. The production of rubber
is in consequence reduced, and should the defoliation, as fre-
quently happens, be repeated, a severe dieback sets in, and. the
process may eventually result in the death of the top or of the
whole tree.
Control.
No practicable means of control has been found. For spray-
ing to be effective the young leaves would need to be coated at
least twice during the first four days, which is more difficult to
arrange for since the trees in a plantation come into leaf in a
scattered way, and even different parts of the same tree do not
come into leaf all at the same time. Moreover, the way the
young leaves hang makes them difficult to cover.
Stahel has proposed keeping the trees artificially bare of
leaves for three to four weeks to prevent the formation and
dissemination of conidia, and has also suggested the use of
smoke clouds at night to prevent dew formation, a method
practised in vineyards against threatened frosts,'
CHAPTER XXII
DISEASES OF MINOR FRUIT TREES
Avocado.
This fruit-tree occurs throughout the islands, grown only in a
scattered way. The leaves very commonly bear roundish patches
of the alga of " red rust " Cephaleuros mycoidea. Withering
of the tips and edges of the leaves, sometimes involving a good
deal of leaf surface, occurs in dry situations. The tree is very
susceptible to Rosellinia root disease, and where, as in some
districts of St. Lucia, it has been planted as shade for cacao it
very often forms the originating centre of groups of affected
cacao trees. Apart from this disease certain other fungi have
been found on the roots and collars of dying trees, under con-
ditions suggesting their complicity in that condition. Diplodia
dieback (see Cacao) occurs on the twigs and branches.
Anthracnose.
Blossom blight, leaf injury, and fruit rot of avocado occur
which are due to the attacks of a fungus possibly identical with
that which produces similar effects on the mango, both being
regarded by some authorities as belonging to the species Colleto-
trichum glceosporioides found on citrus trees.
According to J. B. Rorer, the disease is very prevalent in
Trinidad, so that it is almost impossible to buy a fruit in the
market which does not show signs of infection. These are of
little importance on the fresh fruit, but result in complete decay
during shipment.
Quoting Rorer's account of the attack on the fruit : " The
disease appears first as a very small brown spot. If the attack
takes place when the fruit is green, the diseased spot ceases to
grow and becomes somewhat sunken, and the fruit cracks around
the margin ; small black pimples may be noticed in the centre
of the brown tissue. If the attack is bad, the young fruits fall
to the ground prematurely, otherwise the fruit may reach
maturity, but the skin is cracked at the various places adjacent
to the brown spots. When soft-skinned fruits are attacked, the
fungus penetrates into the flesh and forms a cone-shaped mass of
diseased tissue, which may extend all the way into the seed, and
gradually rots the whole fruit. When attack takes place on ripe
fruits, they rot very quickly, as shown in the figure. (Fig. 85 .)
235
336 DISEASES OF CROP-PLANTS
" As in the case of the mango, the fungus also attacks leaves,
and causes a certain amount of defoliation."
Date Palm.
Leaf Disease.
The date palm is everywhere subject to infestation with a
parasitic fungus Graphiola Phcenicis, Poit. of unknown affinities.
The fructifications are based on small yellow or brown spots
on both sides of the leaflets and on the rachis. They consist
of single or grouped black cups of carbonaceous texture, up to
I mm. in diameter, from the interior of which project con-
spicuous buff-coloured cottony tufts, thickly powdered with
light yellow pollen-like spores.
Each infection appears very narrowly localized, but their
number progresses with the age of the leaf, so that old leaves
are very heavily infested.
The writer is not acquainted with the results of any attempt
at control.
The same or similar species occur on certain other kinds of
palms.
Pomes Stem-Rot.
The fungus Fonies australis Fr. has occurred on the stems
of living date palms in St. Kitts, associated with a process of
failure ending in death. The same fungus has been seen on dead
palms in Grenada.
Grape-Vine.
Anthracnose.
Anthracnose of the grape-vine caused by the fungus Gloso-
sporium ampelophagum has been met with in Barbados. The
disease is well distributed throughout Europe and America.
Leaves, berries and shoots are attacked. The spots produced
are of the bird's-eye type, consisting of a pale centre with a brown
or reddish border. On the shoots they are elongated in the
direction of the axis, and sometimes by affecting the cambium
give rise to small cankers. The fungus closely resembles other
species of Gloeosporium ; the conidial measurements given are
5-6 X 2.5-3.5 rnicrons.
Repeated spraying with Bordeaux mixture is a fairly satis-
factory means of control, but needs to be aided by the pruning
out as far as possible of the diseased shoots and the removal
of infested leaves and fruit
Powdery Mildew.
The Oidium stage of an Erysiphaceous fungus, in all pro-
bability Uncimda necator (Schw.) Burr., a general pest of the
Anthracnose of Mango Fruii
Fig. 88
Anthracnose of ^NIango Leaves
DISEASES OF MINOR FRUIT TREES 237
vine, is common in Barbados and Trinidad. It affects leaves,
young shoots, flowers and fruit, appearing in the form of white
powdery patches, which may later cover the affected part.
The effect of the mycelium on the surface is to cause brown, more
or less fern-like markings, and the infestation of young and
tender organs stops their development
The disease is most prevalent in humid weather, and arrange-
ments to give access to sun and air reduce the tendency to its
development. The best means of control is dusting with sulphur
or with sulphur and lead arsenate mixture, which if carried out
under proper conditions (see p. 106) is highly effective.
Rust.
The older leaves of vines and, in conditions less suitable
for growth, the young leaves as well, are liable throughout
the West Indies to infestation with a rust fungus, which produces
large numbers of small pustules, giving off powdery yellow
urediniospores, mostly on the underside of the leaf. The fungus
from several West Indian localities has been determined by
J. C. Arthur as Phakopsora vitis (Thum.) Syd.
Probably the best means of dealing with it consists in giving
increased attention to cultivation, thus tending to confine its
attacks to old leaves which have largely passed their period of
usefulness. Spraying might also be tried.
Mango.
Mangoes are grown as scattered trees, for roadside shade,
or as windbreaks throughout the islands, much less commonly
in Barbados, however, than elsewhere. Trees producing good
quality fruit, propagated by grafting, are found only in gardens
or mixed orchards. Apart from the anthracnose described below
and one or two leaf- spots of no importance the tree is not known
to be subject to disease, except where it comes in contact, usually
in cacao plantations, vnth the fungus Rosellinia Pepo, to the at-
tacks of which it is, moreover, not very susceptible.
Anthracnose.
A fungus affection comprising a leaf-spot, blossom blight,
and fruit spot is fairly general in Florida and throughout the
West Indies, and is destructive in some seasons and situations
on the flowers and setting fruit.
The spotting of older fruit does not appear to be a very
notable character of the disease as it is met with in the smaller
islands. The description of this is derived from an account
by J. B. Rorer of the various phases of the affection as they
occur in Trinidad.
238 DISEASES OF CROP-PLANTS
Symptoms.
Blossom blight : Small black spots due to fungus infection
appear in the open flower panicle, involving the death of groups
of flowers either directly or b}^ drying up the flower stalks.
The setting fruits are also frequently affected. The loss may
be small or may result in the complete stripping of the panicle,
according to the conditions prevailing.
Leaf-spot : On the young leaves the fungus causes small
dark roundish or angular spots, usually i to 5 mm. in diameter.
They do not extend as the leaf grows older, but appear as
" shot-holes " in the otherwise healthy tissue. Care must be
taken to distinguish these from the injuries to both flower stalks
and young leaves due to infestations which may occur at this
stage with the larvae of a Cecidomyid fly. In the latter case
there are small swellings of the nature of galls.
Fruit effects : On older fruits black spots of varied form,
which may be cracked or sunken, appear and may coalesce to
cover large areas. They penetrate deeply, and either cause the
rotting of the fruit on the tree or form centres of decay after the
fruit is picked. Streaking and russeting, affecting only the skin
of the fruit, are attributed to the arrested germination of spores
washed down upon the fruit from an affected twig or flower
stalk.
Causative Fungus.
The fungus is usually known as Gloeosporium Mangiferce Henn.,
but it has been held to be identical with Colletotrichum glceo-
sporioides, best kno^vn in its connection with citrus tree diseases.
No description of the fungus adequate for its separation from
this and some other described members of the genus can be
given.
Influence of External Conditions.
The injury done by the disease is closely dependent on humi-
dity, the prevalence of rain or heavy dews at blossoming time
greatly increasing its incidence, and forming the principal factor
in determining the difference between a bad and a good mango
crop.
Control.
The disease can be controlled, and a heavier setting of fruit
secured even in normal seasons, by spraying with Bordeaux
mixture at frequent intervals during the blossoming period.
According to Rorer's experiments the fruit from sprayed trees
is cleaner and more attractive in appearance, and keeps much
better in storage or transit.
DISEASES OF MINOR FRUIT TREES 239
Nutmegs.
The cultivation of nutmegs on a commercial scale in these
islands is almost confined to Grenada, where it exists as a sub-
sidiary industry on cacao plantations suitably situated in the
wetter upland districts. The trees often attain to a height of
40 or 50 feet, and sometimes more, so that the conditions pre-
vailing closely resemble those of forest. The diseases which have
appeared are well marked, but have been the subject of no
close study.
Canker,
A somewhat serious disease has been met with from time to
time, the symptoms of which, according to the agricultural
officers reporting it, are identical with those of cacao canker.
R. D. Anstead described it as causing the trees to die back from
the head, or, in the case of individual branches affected, from the
tips of the twigs. In such cases the bark below is unhealthy
and in places bleeding occurs corresponding to that of cacao
canker. The sap in these places is of a rich dark red colour
and has a foul odour. In the late stages gum may be exuded
from the trunk near the base. The disease has been known
to cause the death of numbers of large trees.
Nothing definite can be said as to the causation of the disease,
but it has been found to be amenable to the same treatment
as cacao canker, to which reference should be made.
Perforated Leaves.
A common affection in Grenada is a leaf-spot, the final stage
of which is represented by clean-cut approximately circular
holes of 1.5 to 10 mm. diameter. The brown spot which pre-
cedes this condition contains the fructifications of an undeter-
mined species of Phyllosticta, which appears to be the cause of
the affection. Hundreds of species of this genus of fungi have
been described, from the leaves of almost as many species of
plants, often giving rise to affections of this class. How far
they are interchangeable has seldom been determined, but they
are generally regarded as specific within a narrow range.
In the absence of knowledge concerning the conditions
governing the disease on nutmegs no remedial measure can be
suggested. Spraying would be impracticable on trees of large
size on account of the low value of the product.
Red Rust.
The alga Cephaleuros mycoidea is very common on nutmeg
leaves. It has not been reported as affecting the twigs, in
which position it produces its principal ill-effects. (See Chapter
XVI.)
240 DISEASES OF CROP-PLANTS
RosELLiNiA Root Disease.
Nutmeg trees are among the numerous hosts of Rosellinia
spp. (See Chapter XV.)
Thread Blight,
The nutmeg is very subject to thread bhght caused by
Corticium Stevensii, described on page i6i. Humidity is the
chief predisposing cause of its prevalence and its effects are
commonly increased by close planting. The amount of loss is
probably always far too low to justify the expense of spraying,
which apart from reducing shelter is the only remedy.
CHAPTER XXIII
DISEASES OF BANANA
Banana Wilt, Panama Disease.
History and Distribution.
A wilt disease of bananas usually referred to as the Panama
Disease has during the last twenty years become widely pre-
valent on the Gros Michel banana in various parts of Central
and South America, and by its interference with the great banana
industry maintained or controlled by the United Fruit Company
has had very serious economic effects. Evidence has been
obtained which shows that the fungus giving rise to Panama
disease is specifically identical in the countries named and in
the West Indies, and the conclusions of reliable investigators
are in close agreement as to the nature of the affection and the
parasite concerned. Adequate descriptions of the disease have
been given by A. W. Drost (Surinam, 1912), S. F. Ashby (Jamaica,
1913) and S. C. Prescott (Costa Rica, 1917). Recently (1919)
E. W. Brandes has published a comprehensive account of this
disease as seen in Porto Rico, Cuba and Central America.
The first record of the disease relates to the recognition of its
effects in Panama in 1903, and within a few years it devastated
plantations over large areas in Panama and Costa Rica. At the
present time several areas of 15,000 to 20,000 acres and upwards
in Panama are abandoned or badly infested, and similar con-
ditions prevail in Costa Rica.
In Surinam, with the help of the United Fruit Company, a
large banana industry was established to replace the cacao
industry, which had been heavily reduced by the witch-broom
disease. The Panama disease was found in 1906 and in 1913 was
so widespread as to ensure the virtual ruin of what had become
a thriving business. The Gros Michel was largely replaced by
the Congo banana, which proved resistant, but its fruit was
found to be unsuitable in certain respects for commercial pur-
poses. In British Honduras plantations more recently estab-
Mshed by the United Fruit Company are at the present time
suffering severely from the effects of the disease.
Panama disease was recognised in Jamaica in 1911-12, when
several small outbreaks occurred in the Portland district. The
strenuous efforts which were made to eradicate it did not succeed,
but the latest ofiicial expression of opinion is to the effect that
241 R
242
DISEASES OF CROP-PLANTS
it can be controlled and prevented from destructive spread by
careful measures of quarantine. With Government assistance
the infected areas are being planted with the immune " Chinese "
banana to be prepared for export as a dried product.
The disease exists to a serious extent on the Manzana variety
but not the Chameluco in Cuba and on the Chameluco but not
the Manzana in Porto Rico (Brandes) : in each case the most
esteemed and most widely grown of the local kinds. An
affection believed to be Panama disease prevents the cultivation
of the Gros Michel on any large scale in Trinidad, but no adequate
description has been published. A reference by Brandes to
the existence of the disease in Barbados appears to be due to a
misapprehension. A disease which closely resembles and may
Fig. 89
Macroconidia of Fusarium vasinfectum. Exactly like those of
F. cuBENSE IN Size and Shape
From a drawing by C. W. Carpenter in Journal of Agricultural Research
be identical with Panama disease seriously affects the Gros
Michel in St. Lucia.
In Central America the disease is present in Panama, Costa
Rica, Nicaragua, Guatemala, Honduras, and British Honduras.
In South America it occurs in Surinam and British Guiana ;
there is no evidence of its existence in the large banana-growing
districts of Colombia, which are irrigated.
In India a disease which appears to correspond in all respects
with Panama disease is recorded as exterminating one kind of
banana in a district of Bengal, while a disease different in its
symptoms, but caused by a species of Fusarium has been described
from Pusa. Affections similar in their nature to Panama disease
occur in the Hawaiian Islands, Australia and the Dutch East
Indies, and in the first named the identity of the fungus with
Fusarium cubense has been established.
Fig. 90
Panama Disease : Early Symptoms
iBull. ^0, Depl. Agii., Suri
Fig. 91
,i:ull. ^u, Dipt. Agri., Sunnain
Panama Disease : Advanced Stage, showing most of the Leaves
Broken Down and Splitting of the Stem
DISEASES OF BANANA 243
Causative Fungus.
A fungus is associated with the disease from its initial stages,
the colourless intracellular mycelium developing primarily on
the inner sides of the walls of the vessels in the affected parts.
Micro-conidia are formed in this situation and in pure cultures
(the Cephalosporium stage) ; apparently they are carried for-
ward with the ascending sap and give rise to new points of
infection within the plant. On the external parts of the plant,
especially the failing leaves, and also in pure cultures of the
fungus, sickle-shaped, hyaline, mostly 3-celled macro-conidia
are developed (the Fusarium stage). Chlamydospores of more
or less spherical shape also occur in cultures, borne singly or
in pairs terminally on short branches, or singly and in rows
(intercalary) on the hyphse. Irregularly spherical sclerotia
about I mm. in diameter, yellow to deep indigo, also occur in
old cultures. The spore measurements are : Cephalosporium
form, 5-8x2.5-3 microns; Fusarium form, mature, 3-septate,
20-40x3.8-4.5 microns. Chlamydospores: oval 9x7; spheri-
cal 7-7.5 ; paired 13 X 7 microns. These are Ashby's particulars
of the fungus in Jamaica, and are in close agreement with those
more recently given by Brandes of cultures derived from various
sources in the West Indies and Central America. The latter
author has adopted for the fungus the name Fusarium cubense,
on the ground of its presumed identity with the fungus obtained
from banana in Cuba by E. F. Smith in 1900.
Nearly all pure cultures on sterilised media, especially on
potato and rice, develop a marked odour (of lilac ? ) which, as
Lathrop has shown, is most probably due to the production of
the volatile toxic propylic aldehyde ; odourless strains have
been isolated also from characteristic cases of the disease. Similar
odour-producing and odourless strains of the very closely related
Fusarium vasinfectum Atk., the cause of the vascular wilt disease
of cotton and okra, have proved to be equally pathogenic to
those crops so that in so far as the symptoms indicate a toxic
action of the parasite they are apparently not caused mainly by
a volatile aldehyde. Like most of the species of Fusarium
parasitic in the vascular systems of plants, F. cubense develops
a characteristic pink or crimson pigment when growing on
sterilised rice.
Drost in addition to the two forms above specified describes
hemispherical pycnidia with one-celled spores, 8-10 x 2-3 microns,
and perithecia with 2 -celled ascospores. On the basis of the last-
named form he proposed the name Leptospora Muses, but its
connection with the parasite lacks confirmation.
Successful infection experiments are recorded by Drost,
Prescott, and Brandes ; those of the last named carried out on
an elaborate scale, and conclusive in their results,
244 DISEASES OF CROP-PLANTS
Nature of the Attack.
The seat of the disease is the vascular system of the root-
stock. It does not appear that the vessels are actually plugged
as in many Fusarium diseases, rather it is their walls which are
disintegrated. The discoloration of the vascular bundles which
ensues is most marked in the rootstock and in the lower part of
the pseudo-stem, but frequently may be traced upwards into the
leaf-stalks and even into the midribs of the blades. In advanced
stages of the disease the fungus is accompanied by bacteria which
may cause a more or less general decay of tissue, especially in
the rootstock.
Mode of Infection.
The fungus can enter from the soil either by means of the
infection of roots or by way of the cut surfaces produced in the
removal of suckers. Attempts to infect parts above ground do
not lead to general infection of the plant. In diseased stools
the mycelium extends through the connecting vessels to the newly
formed suckers, and the severance and distribution of these as
planting material is one of the principal factors in the spread of
the disease. Mulching with infested trash has also played a
perceptible part. The Fusarium spores can be conveyed to a
distance by wind, by drainage water, and in earth adhering to
implements and to the feet of men and animals, and in this
way give rise to new centres of infection in the soil. Various
parasitic species of Fusarium are known to be capable of existing
for many years in the soil, and in respect of this disease Ashby
records a case where re-infection occurred three years after the
field had been thoroughly cleared and limed.
Symptoms.
The external syrnptoms are in general such as follow the
stoppage of conduction in the vessels affected by the fungus,
i.e., they are the characters of wilting. Typically the leaves,
usually but not invariably in order of age, turn yellow from the
margin or develop yellow spots and then dry up in succession.
The last leaf often stands erect and green for a time before it
falls. Eventually the whole column falls and rots. In young
and actively growing suckers the leaves may droop and break
down while still green. The symptoms vary in this respect with
the season and the age of the plants. A further, but not wholly
characteristic symptom is the splitting of the outer leaf-sheaths.
The bunch of fruit, where growth proceeds so far, may show
signs of arrested development and some or all of the fingers turn
black and fail to ripen. On badly infested land the stools are
often severely dwarfed, and most of the shoots succumb before
they reach the fruiting stage. The external symptoms cannot
in every case be depended on to distinguish the Panama disease
Fig.
[Dcpt. Agri., Jamaica
Panama Disease: Longitudinal Section of a Bulb and
Base of the Stem
Photo]
Fig. 93
Agri., Jamaica
Panama Disease : Young Sucker infested from Parent
Plant through the Neck
DISEASES OF BANANA 245
from others which operate in a similar manner. They occur in
a very similar way, for example, in the nematode disease later
described.
The internal sjnnptoms are more specific. A section near the
base of the pseudo-stem of a diseased plant shows the cut surface,
which in healthy plants is a uniform white, with rows of yellow,
orange, red, or red-brown spots marking the position of the
diseased vascular bundles. The discoloration thus revealed can
be traced into the rootstock, where it is exhibited very plainly,
especially towards the centre. It may often be followed further
into the central strand of the roots. In advanced cases the
rootstock is frequently rotted and may be putrid. The occurrence
of a central rot associated with the massed vascular bundles in
that region is of value in distinguishing Panama disease from
diseases in which the rot extends from the periphery. There
may however, even in Panama disease, be no development
beyond a diffused yellowing and softening of the upper part of
the bulb.
Influence of External Conditions.
The disease is destructive on all types of land, though there
are indications that physical differences may to some extent
influence the existence of the fungus in the soil. Favourable
conditions for the plants, as produced by rich soil, good cultiva-
tion and manuring, are stated to produce a greater capacity for
disease endurance, but there is no evidence of increased resistance
being obtained in this way. It would appear from the general
evidence that the destructiveness of the disease is mainly governed
in extent by the accumulation of the fungus in the planting
material and in the soil, so that when the disease once appears
in a cultivation it is only a matter of time for it to render the
growing of susceptible varieties impossible. The time required
depends on the natural factors influencing infection and on the
sanitary precautions taken. Wet weather has considerable
influence in the rapid dissemination of the disease, and it has
been frequently observed that extension is particularly liable to
occur in the direction of the flow of drainage water. In arid
regions or situations where banana fields are watered by irrigation
the disease is unknown.
Inherent Resistance.
The Gros Michel as stated is highly susceptible. The red or
claret banana is somewhat more resistant, but does not maintain
itself for more than a few years on infected ground. The
" Chinese" {Musa cavendishii) and " Congo " bananas are highly
resistant, the former at least, it would appear, to an extent
approaching immunity. The plantain [M Varadisaica) is believed
to be quite immune.
246 DISEASES OF CROP-PLANTS
Control.
The most successful form of control against Fusarium diseases
has been found to be the adoption of resistant varieties, and it is
in the search for a resistant variety with the necessary commercial
qualities that most hope for the control of Panama disease
appears to lie. The breeding of such a variety by the usual means
is made difficult by the partial or entire sterility of the edible
species of banana, but in a group with such a wealth of forms
it seems obvious that some type of variation must be liable to
occur. It should be noted that the seriousness of the disease is
mainly in connection with production for export. Varieties
adequate for local use, grown in a less systematic way, generally
exist undamaged.
An enormous range of materials has been tried for the dis-
infection of the soil, but in this, as in other soil infections, no
commercially applicable method has been found.
Rigid quarantine should be maintained against infected areas,
and some other crop substituted on infected land. There is no
guarantee that even an extended rotation will free the soil from
the fungus. Whether it will lessen its amount sufficiently to
allow for paying crops to be again obtained can only be deter-
mined by trial.
Marasmius Root Disease and Stem-Rot.
A root disease of banana, accompanied to some extent by
rotting of the leaf-sheaths which combine to form the pseudo-
stem, associated with Marasmius spp., usually, it would appear,
with M. semiustus Berk. & Curt. {M. stenophyllus Mont.), seems
to occur throughout the Antilles on bananas when grown under
conditions which are in some respect unfavourable. It closely
resembles in its nature the Marasmius root disease of sugar-cane,
and the same species has been recorded from the latter host.
The disease was investigated to some extent by F. W. South,
to whom the following account is mainly due, in Barbados and
St. Lucia, and has been the subject of notes from Jamaica (S. F.
Ashby) and from Trinidad.
Symptoms.
The outer leaves dry up more rapidly than they are replaced
by new growth, so that the amount of green top is reduced ;
in extreme cases to only two or three leaves. The drying of the
leaves extends to the outer layers of the pseudo-stem, which
turn brown or grey, and adhere closely to the layers beneath,
so that they are difficult to strip, or are so rotted that
they come away piecemeal. On and between the dead
leaf-sheaths are layers or patches of white Marasmius myceliiun.
The inner living leaf-sheaths when exposed exhibit at various
DISEASES OF BANANA 247
points oval patches, up to several inches in diameter, of a dark
brown colour, and watersoaked in appearance. The spots
usually penetrate two or more leaf-sheaths, becoming smaller
as they progress inwards. In extreme cases they reach the
centre of the trunk. When the discoloration passes from one
leaf-sheath to the next, superficial white mycelium may be
developed between them.
The surfaces of the leaf-scales below the soil as well as the
bases of the leaf-sheaths are frequently covered with white
mycelium, and the leaf-scales are discoloured brown.
Many of the roots are shrivelled and dead for part of their
length. The cortex becomes brown and soft and eventually the
vascular cylinder darkens and decays. Infested plants are
easily uprooted.
On cutting across the trunk the inner leaves are healthy
except where the brown patches have penetrated. The vascular
bundles are not discoloured in either trunk or rootstock.
The disease spreads from the outside and appears to begin
on the outer leaf-sheaths, usually near soil level. In this situation
the Marasmius fructifications are freely produced during wet
weather. The caps are pale yellowish white to pale reddish tan,
the stalks white at the apex, pale reddish below, the spores
ellipsoid, smooth, 7-9 x 5-6 microns.
Conditions of Occurrence.
As to the nature of this disease a good deal of the discussion
regarding root disease of sugar-cane will apply. It occurs as
the effect of drought, poverty of soil, or defective soil aeration
due to lack of drainage or of tilth. It is of small account on good
soil adequately watered, but may be severe on gravelly or sandy
soil and poor land generally. Drainage, manuring, cultivation,
and the use of healthy suckers for planting are the measures of
control indicated.
The Moko Disease.
A disease of banana and plantains in Trinidad was investigated
in 1909-10, by J. B. Rorer, and found to be due to a bacterium,
not yet described, for which the name Bacilhis Musce was
suggested. The disease was first found on the " moko," a coarse
banana at one time largely used for shading young cacao in
Trinidad, but reported to have been almost exterminated by an
epidemic somewhere about 1890. The disease attacks other
varieties of banana, including the dwarf Chinese or Canary type
resistant to Panama disease. The Gros Michel, which is very
susceptible to Panama disease, is highly resistant to the moko
disease.
248 DISEASES OF CROP-PLANTS
Symptoms.
The presence of the disease is first shown by a slight drooping
and yellowing of the lower leaves, similar to the effect of drought.
A little later the petiole of one of the leaves gives way just at
the base of the leaf-blade, and all the other leaves quickly fail
in the same way. Eventually the terminal leaf bends over and
the plant dies and rots to the ground. The longitudinal sphtting
of the leaf-sheaths characteristic of Panama disease is absent
from this affection.
" Transverse sections of the pseudo-stem show that
practically all the vessels are discoloured, the colour ranging
from pale yellow to dark brown or bluish black, and filled
with bacteria. The discoloured bundles run back into the
true stem and thence into the young suckers and buds. Some-
times in badly diseased plants the tissues of the leaf-stalks
and stems are broken down completely so that fairly large
bacterial cavities are formed.
" If transverse sections of leaf or stem are allowed to stand
for a short time the cut surfaces soon become covered with
bacterial drops which have been forced out from the ends of
the bundles. If the sections when freshly cut are put in large
covered dishes away from the air, pure cultures of the organism
may be obtained directly from these drops. If the disease
is not severe, or a plant does not become infected until it has
formed a bunch of fruit, it may remain perfectly healthy
looking, but many of the young fruits, or ' fingers ' do not
properly mature ; they remain small and eventually become
black and rotten. In such cases it is found that there are some
discoloured bundles filled with bacteria in the leaves, stem,
fruit-stalk, or fruits. When diseased suckers are planted the
terminal leaf frequently turns black and dries up, so that the
plant dies " (Rorer).
Causation.
The organism was isolated in pure culture and was found in a
series of experiments to reproduce the disease in typical form
from inoculations made on the petioles of young leaves. In
several cases the whole stool was infested and killed.
The bacterium was distinguished in cultures by the blackening
produced on potato cylinders. It is short lived and loses its
pathogenic power quickly. Control has been effected by sanitary
measures : digging up and destroying diseased plants as soon as
they are found and sterilizing by fire the implements used.
Eelworm Black-Rot,
In Grenada the banana known as the bluggoe, a cooking
variety extensively grown as temporary shade for cacao, is
Fig. 94
MoKO Disease of Baxaxa
DISEASES OF BANANA
249
subject to attacks on a large scale by a disease believed to be due
to the infestation of the roots and rootstock with a nematode
worm, Tylenchus musicola, recently described as a new species
from Grenada material by N. A. Cobb. The bluggoe is probably
identical with the moko of Trinidad, but the disease under
description is quite distinct from the bacterial disease of the moko
described by J. B. Rorer The closely related eelworm, Tylenchus
Fig. 95 Tylenchus Similis
Afler N. A. Cobb
similis Cobb, was described as the cause of a serious outbreak
of a disease of bananas in Fiji, in 1890-gi, by N. A. Cobb, and has
been found by S. F. Ashby in Jamaica in association with a
disease similar to the one in Grenada.
Symptoms.
The disease affects the development of shoots of all ages,
but is most evident on those that are approaching or have
250 DISEASES OF CROP-PLANTS
actually reached the fruiting stage. The effect on the plant is
that of shortage of water, and the view of an infested field conveys
the impression that all the outer leaves have been scorched.
The leaves wither in the order of their age, their tissue drying
up from the margin towards the midrib. Bearing plants fail
properly to mature their fruit, which commonly dries hard and
black while only partly grown.
Examination of the base of the plant shows many roots partly
or entirely blackened, and extensive black patches, or a general
blackening of the external tissues, present on the rootstock.
Slices of the diseased rootstock show peripheral blackening
penetrating to a depth of about an inch in places, and blackened
areas isolated in section but connecting with the exterior at a
higher or lower point. The thick roots have short swoUen cracks,
connecting with extensive discoloured patches spreading upwards
and downwards in the tissue of the cortex, reaching in many
cases to the central vascular cylinder but not penetrating it.
The discoloured tissue is in various stages of disorganisation, but
in much of it the cell-walls are not visibly decayed. The nema-
todes have been found to be regularly present in all the material
examined ; their eggs occur in the least altered and deepest
seated of the discoloured tissues, and the worms themselves
may be seen in the cells of undecayed tissue close to the central
cylinder of the roots.
Resistance and Control.
The effects of the disease show up most plainly in dry weather.
There is evidence of a considerable measure of recovery during
the more favourable conditions of the wet season, presumably
because more rapid root development has gained on the progress
of the infestation. This points to the usefulness of good cul-
tivation in the control of the disease. The use of suckers from
affected clumps as planting material should be carefully avoided,
and where land is badly infested the bluggoe should be replaced
by some other plant. The Chinese banana and the " silk fig "
when seen growing amongst infested bluggoes have been much
less severely affected. The former at least seemed to be resistant
to a degree which might permit of its successful cultivation in
infested soil.
The worms are too small to be seen with the naked eye,
their length being only a little more than half a millimetre.
Diseases described from Jamaica.
Several diseases have been described from Jamaica by S. F.
Ashby which have not been met with so far in the Lesser Antilles.
This may be due to the fact that bananas are grown only in a
scattered way, so that there is less opportunity for disease to
become noticeable. It seems desirable to include brief accounts
DISEASES OFl^BANANA
251
of these diseases as it is quite probable that they will be met with
sooner or later.
Black Spot of Leaves.
This leaf disease has been reported from several localities
in Jamaica, where it is confined to good soils containing a high
proportion of marl. It begins with tiny black spots on the
main veins of the leaf-blade, which increase in size and become
Fig. 96
Cercospora Musarum
Bull. 6, Dept. Agri., Jamaica
lenticular and are then often surrounded by a narrow bright
yellow border. As the disease advances dry pale brown areas
extend from the largest spots fanwise to the edge of the blade.
Later the patches so formed unite and may give rise to a broad
zone of dry tissue extending round the outer half of the blade.
No fungus has been found in very young spots, but in the later
stages hyphae are found in the tissue round them and the coni-
diophores of a Cercospora break through the epidermis in large
numbers. This fungus regularly accompanies the black spot
disease in its occurrence on healthy dark-green leaves and has
252 DISEASES OF CROP-PLANTS
not been found on the sickly leaves resulting from disease of more
remote parts of the plant.
Associated Fungus.
The fungus is described as a new species by Ashby under the
name Cercospora musarum. The conidiophores appear mainly
on the under side of the leaf. They are septate, simple or once
branched, and often elbowed. They terminate in an inflated
head bearing one conidium, and may continue to grow producing
a succession of such heads. The conidia are smoky olive in
colour, pear-shaped, widest above the base, and narrowing
thence to a blunt point ; the base shows a dark pad or disc
marking the point of attachment ; they are thick-walled with thick
septa, occurring as long forms with 6-q cells of average dimen-
sions 50-60 X 16-17 microns, and stumpy forms with 4-5 cells,
similar in breadth and not exceeding 30-40 microns in length.
Should treatment become necessary the indications are that
it would be best effected by spraying with Bordeaux mixture
at an early stage.
BONNYGATE DISEASE.
The Bonnygate disease, named from the place of its discovery,
has been found at several scattered points in Jamaica. It is
attributed to the infestation of the rootstock with a fungus
described as a new species by Ashby under the name Sphcerostilbe
musarum.
Symptoms.
The leaves show a narrow zone of pale brown dry tissue
extending along the margin of the blade, bordered on the inner
side by a narrow but sharply defined bright yellow band. This
appears on one or several among the older and outer leaves.
The younger inner leaves usually bear broader dry and yellow
bands. This condition may continue stationary for weeks,
the rest of the blade remaining green. Suckers are backward
in growth and if they produce a bunch it often fails to reach
full size. The lower part of the trunk often turns black for a
few inches above the base and the plants may be broken across
at the bulb by a moderate breeze.
In the cortical region of the rootstock occur rounded water-
soaked areas bounded by a narrow red line. In parts where the
affection is more advanced the tissue may be discoloured in dark
brown bands which enclose the root-like strands, black without
and white within, of the causative fungus.
Causative Fungus.
As in other Sphasrostilbe diseases the fungus produces broad
flat branching rhizomorphs, which give rise to the strands
Pholo\
Fig. 97
[Dept. of Agri., Jamaica
BONNYGATE DISEASE OF BANANA
Inset 1. Conidial Fructifications on Head of Bulb
Inset 2. Conidial Fructifications in Pure Culture
ON Sterilized Potato
DISEASES OF BANANA
253
described as penetrating the tissue of the rootstock, and these
in turn give out hyphae which grow among the cells.
The fructifications are borne at or just above ground level.
Fig. 98 SpHAEROSTILBE MUSARUM, CONIDIA, ASCUS AND ASCOSPORES
Bull. 6, Dept. Agri., Jamaica
The conidial stage occurs on small yellow or orange cushions
up to 2 mm. diameter, bearing one or more slender white stalks
furnished with a brown or brownish red spherical head or ending
in a point.
254 DISEASES OF CROP-PLANTS
The perithecia are borne either on independent cushions or
in succession to the conidiophores ; they are yellowish brown,
sessile, and crowded closely over the whole surface. The asco-
spores are brown, two-celled, fusiform, often slightly constricted
at the septum and longitudinally striate, with blunt apices.
The conidia are bluntly spindle-shaped and four-celled.
The two equal-sized median cells are larger, thicker walled and
crowded with oil drops of an orange colour. The terminal cells
are hyaline of equal size, inflated, and rounded. The long
stalks of the conidia are hyaline, septate, and enlarge rapidly
towards their attachment to the conidia. Paraphyses are
numerous in the heads, exceeding the stalked conidia in length.
The bark of cacao trees near the base of the stem has been
found penetrated by the rhizomorphs of the fungus and covered
by its conidial fructifications ; the attack was confined to cacao
in mixed cultivations with affected bananas on land subject
to periodic flooding.
Treatment.
If the diseased plants are present sporadically Ashby recom-
mends that the stools be dug out, cut up and charred, or cut up
and mixed with freshly slaked lime, and returned to the holes,
these being then filled in with soil and left for a few months
before replanting. The parasite is not present above the black-
ened region If diseased stools are numerous the cheapest
method is to cut off the unaffected parts, then chop the stump
down to the ground, cover this with freshly slaked lime, earth
up and leave for three months. It is desirable in view of possible
spread through the soil to isolate the stools with trenches.
The disease appears to originate most commonly from
infested material carried by floods. Thorough cultivation is
said to enable the suckers to resist attack.
Blackhead Disease.
A disease of the rootstock, which produces brown or dark
water-soaked patches either near to the surface or working
inwards from it, was found to be due to the invasion of the tissues
by the fungus Thielaviopsis paradoxa, the cause of stem diseases
of sugar-cane, pineapple and coconut.
Conidia or bits of mycelium brought into any slight cut or
into a slit in the bulb of a young sucker rapidly invade the
tissues with the production of rusty discoloration and formation
of spore chains in the cells. The tissues though water-soaked
remain firm and united.
As in the case of cane cuttings the disease is most frequently
found on planted suckers which are growing with abnormal
slowness owing to dry weather, and they may grow away from
the attack when rains occur. The cut surfaces of the suckers
DISEASES OF BANANA 255
or any wound expose them to infection, and where the disease
is prevalent a protective coating of Bordeaux mixture should
be applied as soon as possible after the severance is made.
Otlier forms of blackhead disease are caused by the invasion
of the rootstock by Diplodia or b}^ a species of Pythium, in
association with a rot of the deeper roots in undrained or imper-
fectly drained heavy soUs, but apparently the most frequent form
of ail the blackheads is caused by the nematode, Tylenchus
similis, Cobb. (See Eelworm Black-rot.)
Heart Leaf Disease.
An affection occurs in Jamaica in which one or more of the
younger leaves are pale or yellow, and the youngest or heart leaf,
often almost white in hue, may be pushed up while still folded,
and fail to expand before beginning to wilt, turn brown in patches,
and rot. The condition of chlorosis may endure for a time, but
is frequently followed by a rapid fall of all the leaves at their
junction with the trunk, and a speedy brown rot of the latter
moving downwards from the top.
The cause of the disease is uncertain. It is considered that
the condition of chlorosis may be induced by the depletion in the
sandy soil of one or more essential mineral elements through
washing with heavy and frequent rains, and that the rapid sappy
growth which ensues permits the rotting of the tissues by feeble
parasites.
CHAPTER XXIV
DISEASES OF CORN AND SORGHUMS
Corn.
Brown Rust.
The most widely distributed leaf-rust of maize is caused by the
fungus Puccinia Sorghi, which in spite of its name does not
attack sorghum. It causes comparatively little discoloration
of the leaf, but the brown pustules in which the spores are
developed are more or less conspicuous, according to the degree
of infestation.
Leaves heavily infested with this fungus were received from
Montserrat in February 1915, accompanied by a report that it
was very prevalent on a large field planted some ten weeks
previously, and had been common from the time the plants
flowered. It has since been reported from St. Vincent, St. Lucia
and Antigua.
In the United States this rust is not regarded as having much
economic importance. Usually it becomes common too late in
the season to do much damage. When favoured by exceptional
climatic conditions it may appear earlier and have a considerable
influence on the yield. In South Africa serious outbreaks are
reported as having occurred in certain districts.
Causative Fungus.
Puccinia Sorghi, Schw. (P. Maydis Bereng.)
(i) Aecial stage {Aecidium oxalidis) on Oxalis spp. Peridia
usually on the under side of the leaves, crowded, concentric ;
epispore smooth, 24-28 micr.
(2) Uredinial stage, on maize. Sori on both sides of the leaf,
numerous, often confluent ; spores brown, globose to ovate,
23-30x22-26 micr., slightly verrucose.
(3) Teliospores black, ovate-oblong or clavate-obtuse, con-
stricted ; epispore thick ; 28-45 x 12-17 micr. ; smooth ;
pedicel long ; 5 micr., persistent.
Red Rust.
The fungus Puccinia purpurea Cke. (see p. 261), which is very
common on sorghums in these islands, is reported to attack
maize in Natal. It may be distinguished on that plant by the
deep purplish-red discoloration produced on the leaves, like that
familiar on sorghum.
256
DISEASES OF CORN AND SORGHUMS 257
Smut.
General Characters.
The corn smut, UsUlago ZecB (Beck) Ung. {JJ . zece-Mays,
U. Maydis) is widely distributed in the West Indies as in North
America, without assuming any serious proportions. It is dis-
tinguished by the swelhngs to which it gives rise on the leaves,
the ears, or the tassels, which may attain on the ears to an
enormous size. The pustules are covered at first with a greyish
envelope, which later bursts and reveals the masses of spores.
Any young and growing plant is liable to infection The leaves
are usually first attacked and the disease may be recognized
at an early stage by the whitish swellings there arising.
Mode of Infection.
When the spores are released a certain amount of direct
infection may take place, but the capacity of the spores to germi-
nate apart from the host plant on any moist nutritive material
very greatly increases the possibilities of infection. In rich soil,
for instance, or in manure heaps, a yeast-like growth takes place,
with the result that m}T-iads of conidia are formed, which are
easily distributed by the wind, and are able to start the disease
on any growing part of the corn plant they happen to reach. A
moist atmosphere facilitates infection, so also does the soft
growth induced by high fertility. Wide spacing reduces the
incidence of the disease. When the original spores do not attain
to a situation which favours their growth they remain dormant,
and are capable of germinating at any time, should the conditions
become favourable, during a period usually to be measured in
years.
Prevention and Control.
It will be seen that the deposition of smutted material in
cattle pens or manure heaps, or, since the spores can pass un-
injured through the alimentary canal, the feeding of diseased
plants to cattle, is a sure way of increasing and distributing the
disease.
The idea that the smutted ears, of which cattle are very fond,
are unwholesome as food, has not been borne out by experiments
which have been made.
Certain smut diseases, in which infection is confined to the
young seedlings, are capable of effective control by seed disinfec-
tion, but this is obviously not the case with the disease under
consideration. Still, spores may very well be present on the seed,
and where land is free from smut, and seed from an outside source
is being used, disinfection may be worth while, and may be carried
out as described on pp. 11 1-3.
Where this species of smut already exists, the best means of
S
258 DISEASES OF CROP-PLANTS
control lies in the cutting out and absolute destruction of diseased
plants, or their affected parts, before the spores are ripe.
Head Smut.
Another smut disease which sometimes occurs on maize is
due to the fungus Sorosporium reilianum (Kiihn) McAlp., which
causes the head smut of sorghum. It attacks the cobs and
tassels, and is usually confined to them, but may exceptionally
occur on the leaves or bracts. It reduces the cob or tassel to a
formless sooty mass, and can be distinguished by the absence
of the monstrous swellings characteristic of the common corn
smut. The spores of the species under discussion are minutely
but abundantly warty, those of the latter are prominently though
rather bluntly spiny.
Head smut is uncommon in North America, and has not, to
the writer's knowledge, been recorded for the West Indies. In
Victoria, Australia, it has been reported as occurring to the
extent of one per cent in some districts. It occurs in South
Africa
It does not yield to seed treatment, and can only be con-
trolled by the collection and destruction of diseased material.
Leaf Scorch.
A disease variously known as leaf-scorch, leaf-blight, and
helminthosporiose occurs on maize in N. America, S. Africa, and
various localities in Europe and Asia. Two instances of its
occurrence are reported from Porto Rico. It does not appear to
have been noticed in the Lesser Antilles ; possibly the local types
are resistant. The affection is capable of causing serious loss
of crop.
Nature of the Disease.
The disease begins on the leaves with the production of pale
blotches, which spread and may extend to nearly the length of
the blade. Wilting takes place, and the leaf becomes dry and
brittle. The conidiophores are then developed and are visible
as a dark hairy mould. Infection is said to take place b\' way of
the stomata.
There is definite varietal resistance to the disease, amounting
in some cases to almost complete immunity.
The Causative Fungus.
Numerous species of Helminthosporium cause leaf diseases,
and biological races are believed to occur on specific plants.
The fungus attacking maize in America has usually been referred
to H. inconspicuum C. & Ell., but this is probably a synonym of
H. turcicum Pass., described at an earlier date from S. Europe.
"mm-'
^/<fr Nat. Pflanzen
Fig. 99
Corn Smut
DISEASES OF CORN AND SORGHUMS 259
The conidia are spindle-shaped, pale olive, 5-8 septate,
80-140x20-26 micr.
Prevention.
Protection against the disease is to be secured by rotation
of crops, avoiding the use of manure made from diseased material,
or by the adoption of resistant varieties.
Dry Rot.
Several fungi are capable of infesting the ears of growing
maize, causing the adhesion of the husks and a development of
mould on the grain. In North America a very large proportion
of this type of injury is due to the fungus Di^lodia Zece. In some
years the disease results in a very serious decrease in yield.
Notable losses are also reported from South Africa in connection
with the same fungus, and there is strong suspicion that paralysis
and death of stock in that country results from feeding on the
diseased cobs. The idea that pellagra is similarly caused is now
discredited.
So far as the writer is aware, corn in the West Indies is subject
to but little loss from the type of disease under discussion.
Appearance of the Disease.
In the early stages of infection there is little external appear-
ance of disease. The first sign is the premature fading of the
bright green of the outer husks to a yellowish colour, while the
inner husks may be more or less browned. In the ear the
fungus is first visible around the tips of the kernels if these are
removed. Later the ear loses much of its weight, becomes
mouldy with the development of grey mycelium around the
kernels, and these appear dull and dry. The most distinctive
sign of the presence of Diplodia is the early development of the
minute black pycnidia on the scales which surround the inner
ends of the kernels. Pycnidia can also be found on the husks,
and in advanced cases on the grains themselves. On old stalks
they can be seen as black specks under the rind near the nodes,
or breaking through and discharging their spores.
Mode of Infection.
It has been claimed that the common method of infection is
from the soil, the fungus entering by way of the roots, working
up the stem, and entering the ears and kernels from within.
On the other hand, typical cases can be readily produced by
placing spores on the silks or under the outer husk at the base of
the ears. Little or no infection takes place from ear to ear in the
same season, the period of susceptibility being apparently a short
one, confined, so far as external infections occur, to the period
when the silks are exposed.
26o DISEASES OF CROP-PLANTS
Causative fungus.
Diplodia Zece (Schw.) Lev. {D. maydis Sacc.) Pycnidia black,
spherical to pyriform, obtuse or free ; conidia dark brown,
cylindrical to elliptical, obtuse, usually slightly curved, one-
septate, 20-33 X 5-6 micr. Specific, so far as is known, to corn.
Two or three species of Fusarium have also been described as
producing similar dry rots in the United States.
Prevention.
From the nature of the disease, control must depend on the
elimination of infected material which can carry over the disease
or infect the soil. Rotation of crops is indicated where soil
infestation is suspected.
Root Disease.
Varying annual losses, sometimes extending to half the
crop in certain districts, are attributed to root disease in St.
Kitts, Antigua and St. Vincent. The plants show signs of
attack when i to 2 feet high, and eventually wilt completely,
usually at the time of flowering. If cobs are produced at all they
are generally small and misshapen, with little grain.
The disease or diseases are much in need of investigation.
F. W. South reports the presence in St. Kitts material of a
Fusarium and also of mycelium resembling that of Marasmius
Sacchari on the sugar-cane. The only example seen by the
writer, said to be typical of the St. Vincent disease, had the
leaves cemented around the base of the stem by mycelium of the
latter type.
Recently T. F. Manns and J. F. Adams have shown that root
disease is widely distributed in the United States and that it may
be caused by four species of fungi working singly or in com-
bination— Diplodia Zece, Fusarium moniliforme, Gibberella sauhi-
netii and Cephalosporium Sacchari (the cause of a wilt disease of
sugar cane in India and the West Indies). These fungi infect the
kernels and are therefore planted in the seed.
Minor Diseases of Maize.
The rust Uredo pallida Diet. & Holw, previously known only
from Mexico and Central America on the grass Tripsacum lanceo-
latum was found on maize in Porto Rico in 1916. It is reported
by J. A. Stevenson to attack the older and lower leaves generally
at a time when the ears are so nearly mature that little damage
can be done. It appears as numerous inconspicuous pale-brown
pustules on the under side of the leaves.
The parasitic Ascomycete Phyllachora graminis (Pers.) Fkl.,
common on various grasses and sedges, giving rise to rather
conspicuous black raised spots on the leaves and leaf-sheaths, is
DISEASES OF CORN AND SORGHUMS 261
reported to be common on maize in Porto Rico, hastening the
death of old or basal leaves. It has no economic importance.
Sorghums.
Rust.
The most conspicuous rust disease in these islands is that
caused on the cultivated sorghums by Puccinia purpurea. Cke.
It appears to be general on Guinea corn, imphee, Sudan grass,
and Johnson grass. It also occurs in the southern United States,
in Cuba, Jamaica, Porto Rico and Bermuda.
Appearance of the Disease.
Elongated purplish-red blotches are produced on the leaves,
and in these the sori are developed. In severe infestations the
leaves are in this way very much discoloured, and there would
seem to be no question that the growth of the plant is con-
siderably affected. The crop is nowhere in these islands of any
great importance, and the disease receives no attention.
Causative fungus.
Puccinia purpurea Cke. [Uredo Sorghi, Fckl. not P. Sorghi
Schw.) Urediniospores ovate, 35x25-30 micr. smooth, brown.
Teliospores elongate ovate, brown, long-pedicellate, 40-45 x 22-25
micr. Occurs also on maize, which see.
Control.
Would be best attempted by selection of resistant types.
Kernel Smut.
The common sorghum smut, Sphacelotheca Sorghi, occurs from
time to time on imphee and Guinea corn in the West Indies. It
is of general occurrence about the world, and occasionally is
severe in its effects. In these islands, where the crop is more
usually grown as fodder than for grain, the disease has not
attracted much attention.
The spores of the fungus are usually sown with the seed,
germinate in the soil at the same time, and infect the seedlings
at an early stage. The mycelium of the fungus keeps pace with
the extension of the plant in the form of thin threads which grow
up through the softer tissues, especially the pith. No particular
damage to the plant is caused and no sign of the disease is visible
until the period of flowering, when the fungus develops rapidly
in the anthers and ovaries, and there produces masses of black
powdery spores which replace the pollen and the seeds. The
outward appearance of the panicles is not much altered until the
membrane which at first encloses the spore masses bursts, when
they appear as if coated with powdery soot.
262 DISEASES OF QROP-PLANTS
The spores are very resistant, and the occurrence of a small
quantity of sniutted material among the seed corn is capable
of producing widespread infection in the following crop. The
disease is thus cumulative, and its occurrence even in small
proportions should therefore not be considered negligible.
Causative Fungus.
Sphacelotheca (Ustilago) Sorghi (Lk.) CI. Spores single, sub-
spherical to spherical, smooth, contents often granular, 5.5-8.5
micr. diameter ; covered at first by a membrane of fungus origin.
On various Andropogon (Sorghum) species, including Johnson
grass.
Control.
Treatment is rendered easy by the absence of the fungus from
the interior of the seed, and owing to the fact that only very
young plants can be infected. Any method which secures the
sterilisation of the seed without destroying its germinative power
is effective. Steeping the seed in copper sulphate solution,
according to the instructions on page loi is the simplest remedy ;
the formalin treatment is also applicable.
Head Smut.
The head smut differs from the preceding disease in that
instead of the grains being affected separately, the whole head
or, rarely, a section of it is conglomerated into a smutted miss.
The same fungus occurs on corn. It has a wide distribution
where sorghums are grown, including North America, but is not
common in that country. It has not been noticed in these
islands. The disease is not amenable to seed treatment, as,
although the infestation is systemic and seedlings are commonly
infected, this appears to arise from material originating from
wind-blown spores in the soil.
The variety known as milo is immune to this as to other
smuts, and kaffir and broom corn are resistant to it.
Causative fungus.
Sorosporium reilianum McAlp. {Ustilago reiliana Kiihn,
sphacelotheca reiliana Clint) Spores loosely aggregated into
groups, minutely roughened, 9-14 micr.
CHAPTER XXV
DISEASES OF COTTON
Internal Boll Disease
The internal boll disease of cotton is a special case, and was the
first to be studied, of a type of parasitic disease (stigmatomycosis),
in which certain specific fungi are conveyed by plant-feeding
bugs, and gain access to the interior of the fruit by means of the
punctures which the insect makes in feeding. Similar affections
have lately been found to be of very wide distribution in the West
Indies and to affect the fruits or seeds of a great variety of
plants. (See p. 29.)
In the cotton boll the fungus so introduced develops freely on
the immature lint, which may be thus completely rotted or more
or less severely discoloured, according to the stage of development
reached when infection takes place. The seeds may or may not
be infested.
There is in addition, a bacterial form of the disease, in which
infection similarly takes place through bug punctures, but in
regard to this it has not been demonstrated whether the invading
organism is actually conveyed by the bug or only enters from the
surface of the boll by way of the passage provided by the puncture.
This form of infection has not been noted on plants other than
cotton.
History and Distribution.
The nature of the disease was not discovered until 1914, but
effects undoubtedly attributable to it have been reported from
time to time since the revival of cotton-growing in these islands
in 1902-3. The greater part of the damage done, especially the
recurrent heavy loss in St. Vincent, was attributed to other
causes.
Since 1914, the disease has been verified by the writer as
existing in Tortola, St. Kitts, Nevis, Antigua, Montserrat, St.
Vincent, Barbados, Grenada and Trinidad. It was indepen-
dently discovered by S. F. Ashby, in Jamaica, in 19 15, and a
reference to fungi found in cotton bolls by A. W. Bartlett shows
unmistakably that it occurred in British Guiana in 1907. Its
prevalence is closely associated with that of its principal carriers,
the cotton stainers [Dysdercus spp.) and the green bug {Nezara
viridula), of which the former are by far the more important.
263
264 DISEASES OF CROP-PLANTS
Their absence from Barbados, and the comparative scarcity of the
green bug, make the disease of very Httle account in that island.
The fungoid form of the disease has not so far been reported from
other territories.
Symptoms.
There are no external symptoms on the infected bolls, save
in some cases a hardening perceptible to pressure ; the appear-
ance is that of a sound boll even when the contents are com-
pletely ruined. The shedding of young bolls is frequently due to
infection, but may also be induced by bug punctures alone, and
frequently occurs as a reaction of the plant to certain physical
conditions. Shedding from bug injury or infection extends to
bolls up to three weeks old and 20 mm. in diameter, whereas the
limits in natural shedding have been found to be about 8 or 9
days and a diameter of about 14 mm.
In describing the symptoms displayed when green bolls are
cut open it is necessary to discriminate between the direct effects
of bug punctures (stigmonose) and of added infection (stigma-
tomycosis). Damage of the former nature is very heavy when
the bugs are present in large numbers and has been found in a
particular case spoiling 60 per cent, of the locks in a representa-
tive sample of bolls. Some or all of the seeds in a lock may be
killed outright or the embryo arrested in its development, the
immature lint soaking down into a discoloured film. In severe
cases the boll, if not more than half-grown, is usually shed. In
older bolls a large proportion of the uninfected punctures do not
reach the seed, and either have no recognisable effects, or the
effect is limited to a superficial patch of discoloration on the
lint beneath the puncture. The common pea chink {Edessa
medittabunda), which has relatively short mouth-parts, appears
to be limited on bolls of any age to the production of the latter
type of injury. Very little staining of the lint which reaches the
pickers' hands is attributable to direct bug injury. Much of the
loss from this cause goes disguised as shedding, and for the rest
the contents of heavily damaged locks when dry shrivel into a
hard mass which the pickers leave. When only part of the seeds
in a lock are attacked the remainder bear good unstained lint.
The result of infection accompanying the punctures is greatly
to increase the amount and widen the range of damage. For a
lock to be destroyed by direct injury many punctures are re-
quired, for the same result from infection one early puncture
may suffice, since the fungus can spread from the lint of one seed
to that of all the rest. The effects vary with the age of the boU
attacked. Among young boUs a great deal of shedding results,
similar to that caused by direct injury. In bolls that reach
maturity the locks infected young are usually quite ruined. The
seeds are reduced to a pasty mass, with a black and yellow cover-
ing of rotting lint. Infections made later each produce a patch of
DISEASES OF COTTON 265
yellow stained lint, variable in size owing to the decrease of
susceptibility and the shorter time available as the boll develops.
The staining is not superficial, as in the parallel case of bug
injury, but extends to the seed, nor is it confined to the lint of
one seed, but is indiscriminate.
To this kind of infection a large part of the staining of seed-
cotton which has occurred in these islands has been due. Staining
indistinguishable in appearance is also caused by the ordinary
bacterial boll disease, the organism of which gains access to the
lint through the spots caused by external infection of the boll.
This disease is largely dependent on wet weather ; dry weather
staining, which has occurred abundantly towards the end of the
season, as well as a variable proportion of that which occurs in
wet weather, is the effect of the disease under description.
The statement regarding staining which has been usually
repeated by entomologists, that it is due to the excrement voided
by Dysdercus and to the crushing of that insect in the gins, has
so far as the West Indies are concerned no basis whatever of
observation, experiment or probabihty The staining is present
days or weeks before the boll is due to open.
The Parasites.
A general description of the invading fungi is given in
Chapter I, pp. 29-32. Four species, evidently of general distribu-
tion through the islands, have been met with in cotton. Species
A, not yet named, has been predominant in cotton stainer
infestations in St. Vincent ; Species B {Eremoihecium cymhalarice,
Borzi) has been found most abundantly in bolls from Nevis and
Tortola, again in connection with stainers ; Species C has been
predominant in stainer infestations in Montserrat ; Species D,
which is Nematospora Coryli Peglion, or very near that species,
occurs in close association with the green bug, a fairly general
feeder, which presumably accounts for this fungus having been
found over much the widest range of hosts, and especially in
leguminous species, which the green bug prefers.
The first three species are hyphal, the fourth, though it
produces hj^hse in starved cultures and sometimes in the cotton
boll, is normally yeast-like. All four develop freely on the im-
mature lint, and frequently enter and occupy for some distance
the lumen of a fibre. Spores are produced in great abundance
before the boll opens. Their after-history is unknown, but in
cultures they germinate freely. In what form the infection is
conveyed to other bolls has not yet been discovered, but bugs
taken from infected plants and bagged on previously protected
bolls almost invariably cause infection, while with bugs from
iminfected plants, though hundreds of punctures may be made,
no infection occurs, showing that the insect not merely provides
the means of entry but conveys the fungus from plant to plant.
The bacterial form of disease has not been investigated suf-
266 DISEASES OF CROP-PLANTS
ficiently for the species to be recognized, but comparison of
cultures has led to the tentative conclusion that Bacterium
nialvacearmn is frequently, but by no means invariably, the
organism involved.
Incidence. '
The accompanying curve illustrates the incidence of the
disease where cotton stainers become generally abundant about
December, as was the case in Montserrat and St. Vincent before
controlling measures were taken. Nevis has the same distribu-
tion. In St. Kitts, Antigua, and the Virgin Islands the insect
is not usually abundant before February, and the damage has
consequently been much less in amount. In Montserrat it was
possible to avoid the worst effects by early planting, but in St.
Vincent the heavy autumn rainfall made late planting a necessity,
and the precarious chances of the crop depended on whether
the first picking escaped the effects of this weather, since the
second picking, produced under more suitable climatic conditions,
had come to be regularly and completely destroyed by stainer
injury and internal boll disease. (See Fig. loo.)
The effects of green bug outbreaks are less severe. Infection
as a rule is much less general and the production of stained
lint consequently small. (See Fig. loi.) The outbreaks are
usually controlled in the course of a few weeks by e^^ parasites.
Control.
Realization of the fact that the constant heavy losses in the
second half of the crop in St. Vincent were due to disease borne
by the cotton stainer led to more careful study of the habits of
that insect on the part of the resident agricultural officers. The
previous vagueness as to its connection with staining or other
injury had been reflected in the attitude taken towards it as
a pest.
It was found that although stainers in the absence of cotton
could subsist for a considerable time on other foods they were
only capable of breeding to any extent on two trees, the silk
cotton {Eriodendrun anfractiiosum) and the seaside mahoe or
John Bull {Thespesia popidnea). This observation provided the
key to the situation. The trees were eradicated at the cost of
much labour but at a relatively small expense. In the following
year the appearance of stainers in significant numbers was so
far delayed that the picking of clean cotton continued into April,
and conditions in other respects being also favourable the crop
was double that of the previous year. In the untreated neigh-
bouring island of Bequia the crop was destroyed as usual, but
equal success was attained there in the following year by the same
method. It is now being tried in the Leeward Islands.
Where Malvaceous weeds, such as Malachra and Sida species
DISEASES OF COTTON
267
are abundant, as isjreported to be the case in Tobago, the problem
of eradication is different and is likely to be more difficult. The
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Fig. 100
Incidence of Internal Boll Disease
most promising method to adopt would seem to be the establish-
ment of hardy grasses in the place of roadside weeds, the com-
pulsory cleaning of neglected arable land, and the afforestation
of waste ground with shrubs or trees. As cotton stainers readily
268 DISEASES OF CROP-PLANTS
migrate in swarms from one district to another these measures
would need to be general over an island.
Natural control of the green bug may be assisted by the
transfer of parasitized egg clusters to affected localities.
Angular Spot.
History and Distribution.
The affection produced by Bacterium (Pseudomonas) mal-
vacearum E.F.Sm. on the leaves and bracts of cotton was first
described, under the name of angular spot, by G. F. Atkinson in
Louisiana in 1891. Some ten years later E. F. Smith isolated
and described the causative organism and made successful
infection experiments.
The disease is common in the United States on both Upland
and Sea Island cotton. In the West Indies it is general on Sea
Island in most districts. Some of the perennial types are still
more susceptible, others are highly resistant. Its distribution in
other parts of the world is imperfectly recorded.
In the active stage of the disease the spots are not conspicuous.
They appear as small water-soaked areas thickly scattered on the
undersides of the leaves and on the bracts, irregular in shape,
generally angular through being bounded by the smaller veinlets,
and not usually more than 3-5 mm. in diameter. A succession
of them, more or less in contact, often runs along by the main
veins. They quickly penetrate to the upper surface of the leaf,
and then undergo no further development. At this stage a slightly
yellowish varnish or thin crust of bacteria can often be found on
their surface. The tissue involved soon dies and turns brown,
and then first becomes very noticeable. As this condition of the
spots naturally persists for the remainder of the life of the leaf,
a good deal of misapprehension is caused as to the actual time of
occurrence and duration of attacks.
Recent observations in St. Vincent in the month of November
showed that the disease occurred in fresh condition in periodic
bursts which appeared to be connected, at an interval of about a
week, with a previous showery day. Studies by R. C. Faulwetter
point to the conclusion that the predominant factor in the
distribution of the disease is rain splashed from the leaves and
blown as spray by the wind. This accords with the known ability
of the bacterium to produce infection on healthy leaves provided
a iilm of water is present. The organism is demonstrably present
in the film consequent upon heavy dew. Infection appears to
take place as a riile by way of the stomata.
Effects.
The disease in itself is not usually considered to be of much
importance, though severe attacks, which involve the loss of a
good deal of effective leaf-area, cannot be regarded as neghgible.
DISEASES OF COTTON
lot''
26g
3ot-^rLic Ot.72,-
;4::3:tiw/ss:«»4j
Bsl
a^i-r
BcLtCL-l^ia^
1
I
/^tteYsHo/b^-
^E
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rErspcyrrrsrrr
Alon.i^se.rrcLt
m fn/ection a/i/y, CombleU. StoLineii LiTht.
'^IrLfe.otiorLOTily ) /yartia.1, Partly stajjizd lint
^ Com/^Ute. direct iTiJury . Total loss.
H ComjbltU direct Injitry^ in/ectU. Total loss
^Toyttod dhtci- injury, So/nedecun lint.
Fig. 101 Internal Injuries to Cotton Bolls. St. Vincent, Etc.
1-6 : Green ^BuG, etc. 7-9 : Stainers.
270
DISEASES OF CROP-PLANTS
If the affection on the leaves can be held to be the main source
of material for infection producing black-arm and bacterial boll
disease, which is a natural conclusion, its presence becomes a
matter of serious concern. Frequently, however, there is no
obvious evidence of correlation in the prevalence of these
affections.
Influence of External Conditions.
The affection is so common in the cotton fields of the islands
that with little qualification it could be described as universal,
but there are wide variations in the degree of its prevalence and in
the period at which it becomes abundant. Isolated plots some-
times remain practically free of it throughout the season. In
agreement with the theory of its spread noted above, its occur-
rence is closely associated with rainy weather. When frequent
showers fall during the growing period it is usually well distributed
through the fields by the time the bolls begin to mature. It has
been seen to be abundant on or absent from the leaves of the
secondary growth according to the weather in which they are
developed.
Inherent Resistance.
There are undoubtedly degrees of susceptibility and resistance
to this affection in various types of the cotton plant, and even
what appears to be complete immunity. Attempts carried on
over several years to develop resistant strains from the Rivers
type, which is the basis of most of the Sea Island strains in
cultivation in the West Indies, have not been successful, but in
St. Vincent strains evolved from the Superfine type, which is in
general less susceptible than the Rivers, are reported to be re-
sistant to a considerable degree.
In hybrids the character of resistance is irregularly distributed
and its method of segregation has not yet been worked out.
Control by Spraying.
F. M. Rolfs has reported that in South Carolina angular spot
has been found to be controllable by six sprayings with Bordeaux
mixture. Experiments with Burgundy mixture have been made
in St. Vincent and reported on by S. C. Harland. Spray fluid
containing 2 per cent, of copper sulphate was used in 1917-18,
but caused very obvious injury to the plants. In the following
year a one per cent, mixture was used in the first two applications,
and a further reduction was then made to 0.8 per cent. The
plants were sprayed 13 times in 5 months. The control of angular
spot was almost complete, but there was no significant increase
in yield.
The injury caused by spraying chiefly affects the small buds,
many of which shrivel and turn black. The amount of this was
Fig. 102 Angular Leak-spot of Cotto>
Fig. ]u:5 Bacterial Boll Disease of Coi
Fig. 104 Anthracncse of Cotton Bolls
Fig. 105
Curly-leaf Cotton, Top View
Fig. 106
Curly-leaf Cotton, Side View
DISEASES OF COTTON 271
not large with the 0.8 per cent, mixture, and it is compensated
for by the lengthening of the flowering period which follows
shedding. This point is of importance since spraying may still be
found desirable on account of soft rot.
Black Arm.
Infections with Bacterium malvacearum occur on stems, twigs
and petioles of the cotton plant, giving rise to the affection known
as black arm. It is characterised by the formation of dead patches
in the cortex, which may be small or may extend for several
inches, girdling the organ on which they occur. On vigorous
plants the effect is small, but on plants which have made poor
growth owing to drought or unfertile soil it can be severe, leading
to the drying up of the fruiting branches. The v/eak spots pro-
duced by this disease in stems and branches often lead to break-
ages when the plants are struck by a gust of wind.
Bacterial Boll Disease.
The disease of cotton bolls arising from infection with
Bacterium malvacearum is frequently the cause of very heavy
losses in these islands. In the experience of the writer, at least
a large part of the damage generally attributed to anthracnose
has had its origin in such infections, the spots produced having
afforded a foothold to one or other of several fungi, of which
that of anthracnose is under some conditions the most frequent.
History and Distribution.
There is very little literature dealing with this disease. It
appears to have been first noticed by E. F. Smith in the United
States in 1901, it was referred to as a disease of Sea Island cotton
by W. A. Orton in 1907, and is discussed at some length, with
records of inoculations, by C. W. Edgerton in two papers pub-
lished in 1912. As described by that author the position in
Louisiana with regard to the disease and its relation to other boll
rots closely resembles the local one.
Regarding the disease in the West Indies there is a doubtful
reference by L. Lewton Brain in 1904 to a bacterial boll rot
attacking the inner tissues and contents of the boll, which seems
rather to refer to what was afterwards described by the same
writer as black boll and is now called internal boll disease. In
1909-10, F. W. South recognized the disease and published an
adequate description of its typical appearance. He records it as
apparently prevalent throughout the cotton-growing islands,
causing losses estimated to vary from 2 to 20 per cent. This is
about its present status, the variation occurring under the
influence of external conditions as discussed below.
The disease has been recorded (1910) as destructive to Egyp-
tian cotton grown in low-lying wet districts in Nyasaland.
272 DISEASES OF CROP-PLANTS
Nature and Effects.
The first stage of the disease exhibits a small rounded water-
soaked green spot on the surface of the boll. This extends at its
margin, while the tissues at the centre dry up and turn brown or
black. If the development of the lesion continues the carpel
is penetrated and the lint becomes thoroughly infested with
bacteria, including species which themselves are unable to attack
the boll, very commonly accompanied by one or other of the fungi
which readily occupy the original spot. A good deal of lint may
in this way be stained bright yellow, as in the internal boll disease,
or in severer cases completely blackened and rotted. Infections
at the base of the boll often involve the central column, while
those near the tip cause premature separation of the points of
the carpels and prevent complete separation when it is naturally
due. In both base and point infections more than one carpel
maj'' be affected, whereas spots on the face of the bolls usually
only affect the lock on which they occur.
The development of the lesion may become arrested at any
stage, and under some circumstances this occurs before the lint
is reached ; the damage due to the disease is thus very variable
in amount. The appearance of the developed spots on the green
boll similarly varies. If they are infested with the pink anthrac-
nose fungus or the common similarly coloured Fusarium it is
identical with that familiar in figures and descriptions of anthrac-
nose injury ; if not so infected a somewhat sunken, dry, rough,
dark spot or patch, often with a water-soaked green margin, is
produced.
In addition to the obvious injuries on developed boUs heavy
losses also occur from the shedding of flower buds and young
bolls infected at the base.
Causation.
From the very earliest appearances of the water-soaked spots
bacteria are present in the tissue in great numbers, and often
form a pale yellow transparent film on the surface of the spot.
The organism {Bacterium malvacearum E. F. Sm.) is identical
with that which gives rise to angular spot on the leaves.
Mode of Infection.
The spots are most often located either at the apex of the boll,
extending for a short distance down one side, or at the base near
to or under the calyx. They occur quite commonly however on
other parts of the surface, both between and on the suture lines.
Attempts to correlate the spots with bug punctures have revealed
no evidence of connection.
The organism is commonly present on the plants in great
amount in connection with angular spot of the leaves and bracts.
When a plant so infected is thoroughly wetted by rain the bacteria
DISEASES OF COTTON
273
cannot fail to be well distributed over its various parts. Since
the number of spots arising on the bolls is comparatively few in
the circumstances it is a matter of great interest to know what
determines the occurrence of an infection
Edgerton reports that infections similar to those of natural
occurrence and situated mostly near the apex of the boll are
readily produced by dropping a bacterial suspension into the
open flower. He obtained 45 per cent, of infections appearing in
12-14 days in field trials of this method, the controls remaining
healthy.
It is possible that many of the natural infections have their
origin in the flower, and it is suggested that transfer of the
bacteria by insects may play a part in the process. At what
stage the basal infections are produced is not clear ; they do not
appear to be correlated with infestations of angular leaf-spot
on the adjacent bracts. Nor is it evident whether the occasional
occurrence of spots with a fresh appearance on full-sized bolls
is due to late or to delayed infection.
Influence of External Conditions.
Like other subjects in connection with the disease, this re-
quires a good deal of further study ; losses are heavy and general
in wet seasons and are usually very small in dry weather. Under
certain conditions, however, which are not understood, severe
local damage may occur in what may be regarded as a normal
or good season. Thus in St. Vincent in November, 1917, a
single field on an estate had a general infestation of the bolls of
the first picking, and was giving 10 per cent, of stained lint due
to this disease, while adjacent fields differing only in being
planted a few weeks later were practically free. In early January,
igi8, in dry weather, a field of cotton in Barbados in the stage of
full secondary growth was seen heavily infested with bacterial
boll disease and black arm There was little or no angular
leaf-spot in the fohage present, which was all new.
In such dry weather infestations the bacterium itself is more
directly responsible for the damage done than in wet weather,
when the occupation of the spots by secondary organisms is
the rule.
Resistance and Control.
It is uncertain at present what relations exist between the
prevalence of bacteriad boll disease and of angular leaf-spot.
From direct or indirect causes certain types of smooth-boiled
perennial cottons occurring in these islands are certainly much
more susceptible to the boU disease than Sea Island, an awkward
fact for those who claim an all-embracing immunity for the
so-called native cottons.
Attempts to reduce the amount of the disease, whether by the
T
274 DISEASES OF CROP-PLANTS
selection of resistant varieties or by spraying, have been made
indirectly by adopting these measures for the control of angular
leaf-spot, in the St. Vincent spraying experiments a 3 per cent,
increase in the percentage of bolls to flowers was obtained in
1917-18, which is too small to be regarded as significant. The
subject is one which needs further study.
Soft Rot of Bolls.
History and Distribution.
The soft rot of bolls, which is caused by one or more species
of Phytophthora, does not appear to have been met with outside
the West Indies. It was reported first from Montserrat by W.
Robson, in 1910. From specimens then forwarded the nature of
the disease was tentatively determined by F. W. South. In
1916 the writer was present in St. Vincent towards the end of a
period of extremely wet weather, and found the disease to be
very destructive so long as this prevailed. In 1917 the course of a
small outbreak was followed in the same island and cultures of
the fungus were obtained, but its specific identity has not yet
been determined. The presence of the disease in the other
islands is suspected.
Recent observations indicate that boll rot is not always
caused by the same species of Phytophthora,. A form isolated
from bolls in Montserrat was apparently Ph. parasitica Dast,
while a species isolated from bolls in St. Vincent on which
it had developed sporangia luxuriantly proved to be closely
related to the Phytophthoras of cacao pod-rot and coconut
bud-rot.
Symptoms.
The first appearance of the disease is shown by a darkening of
the colour of the boll, which proceeds from a green darker than
the normal to blue-black or black. By the time this is completed
the boll and its contents are more or less soft-rotted and com-
pletely spoiled. The fungus appears externally on the boll as a
white cottony envelope of mycelium, or as a white mealy covering
of conidia. No other part of the plant has been seen to be at-
tacked.
Influence of External Conditions.
The development of this disease is closely dependent on a high
degree of humidity. It makes its first appearance on bolls near
the ground, especially amongst crowded bushy plants after
heavy rain. Only in very wet, cloudy periods does it attack bolls
on the higher branches. At such times it is responsible for a very
large part of the heavy losses which occur. Severe outbreaks
pass off very quickly with the succession of dry sunny weather.
Fig. 10 7
Loggerhead Cotton, Young Plants
FiS. lOS
Loggerhead Cotton, Old Plant
Fig. 109
West Indian Leaf Mildew of Cotton
DISEASES OF COTTON 275
though the indications are that when once the disease has made
its appearance it is hable to recur on the lower bolls under cir-
cumstances less extreme than are needed to induce the iirst
attacks.
Mode of Infection.
Nothing is known as to the carrying over of the fungus
from year to year, but the assumption is made that, as in other
members of the group, resting spores are retained in the soil.
The means by which infection is conveyed from boll to boll
have not yet been studied, but may be assumed to be the same as
in other Phytophthora diseases (p. 21).
Resistance.
S. C. Harland reports that the Montserrat strain H23, when
introduced in 1919 into the wetter climate of St. Vincent, proved
so highly susceptible that practically all the bolls were destroyed.
Another Montserrat strain D.i. was more resistant under the
same conditions, approaching nearer in this respect to the ordinary
St. Vincent strains, and gave a fair crop. The St. Vincent
strains A.B. and A.N., developed from the Superfine type, show
considerable resistance to soft-rot as well as to the angular spot
disease.
No type has been found sufficiently resistant to avoid loss of
the lower bolls in wet weather, the difference being expressed in
the survival of the bolls on the upper parts of the plant.
The control of soft-rot by spraying with Bordeaux or Burgundy
mixture has not yet been sufficiently tested. No cases occurred in
the sprayed plots in St. Vincent in 1918-19, while a fair number
appeared on the unsprayed plots. It may be possible to show,
when an opportunity occurs, that spraying at the outset of a
threatened epidemic can arrest the progress of the disease,
which at such times is liable to destroy most of the visible crop
of bolls.
DiPLODiA Boll Rot.
History and Distribution.
The existence of a species of Diplodia on cotton bolls has been
several times recorded in the United States since M. C. Cooke
described Diplodia gossypina from Indian material in 1879. C.
W. Edgerton, in 1912, seems to have been the first to attribute a
definite boll disease to its agency. He reports that considerable
loss from this cause occurs throughout the State of Louisiana,
averaging perhaps 2 per cent, on the whole crop, but occasionally
in small areas reaching 10 per cent. The present writer has long
been familiar with the envelopment of rotted West Indian Sea
Island bolls with masses of black Diplodia spores, but only in
«276 DISEASES OF CROP-PLANTS
1916 became acquainted with the fungus as the apparent cause
of a brown rot of bolls, to which his attention was called by S. C.
Harland at the St. Vincent Experiment Station. Some further
notes were made at the same place under the drier conditions
of 1917, when the affection was almost confined to one perennial
plant. Certain perennial types appear to be specially susceptible.
In 1919 more Sea Islands bolls than usual were seen to be affected.
These are so far the only records of Diplodia disease in the
Lesser Antilles.
Nature of the Attack.
The earliest external sign of attack as seen on the St. Vincent
bolls is a brown patch of discoloration of somewhat indefinite
form which rapidly extends over a great part or the whole of the
boll. The surface soon becomes dry and hard, and is roughened
by the appearance all over it of slight projections marking the
developing pycnidia. The contents are by this time fully in-
fested with the dark slaty-grey mycelium. The pycnidia dis-
charge at first whitish tendrils of hyaline one-celled spores, and
later the more mature black two-celled spores are produced in
such quantity as to cover the whole boll in a soot-like layer.
The boll is completely ruined.
These notes agree with Edgerton's account of the disease in
Louisiana.
Mode of Infection.
According to Edgerton, the fungus gains entrance through an
insect puncture or other wound, and infests the contents of the
boll before it is able to attack the outer wall. The fungus is
found on dead stems and other parts of the cotton plant, but is
regarded as saprophytic in these situations.
The most significant function of the fungus in these islands
is judged to be that of the infection and complete destruction of
bolls already attacked by other diseases.
Control.
The inability of the fungus to penetrate the uninjured epider-
mis suggests that measures taken to preserve the bolls from injury
offer the best means of protection against this disease. An
additional reason, if one were needed; is thus given for efforts
to keep down the numbers of bugs infesting the cotton
plant.
Prevention of the damage resulting from its function as a
secondary parasite likewise depends on the control of other ex-
ternal boll diseases which cause the formation of spots on the boll
and thus give it access to the lint and seed.
DISEASES OF COTTON 277
Anthracnose.
The most destructive boll rot occurring in the United States
cotton belt is agreed to be the anthracnose produced by Glomerella «•
{Golletotrichum) gossypii (Southw.) which on occasion may destroy
50 per cent, or more of the crop in some localities.
Occurrence in the Lesser Antilles,
A disease which in general appearance closely resembles
anthracnose was noticed in the West Indies soon after the re-
vival of the Sea Island industry. Bolls from Barbados submitted
to W. A. Orton were found to bear a fungus which agreed with
the characters of the American species, except for the smaller
size of the conidia. This was described by L. Lewton Brain
as 0. gossypii, var. Barbadense, and it has been generally accepted
that boll rot due to anthracnose existed to an extent sometimes
serious in all the cotton-growing islands. It was regarded as
particularly destructive in St. Vincent, where practically the
whole of the heavy losses occurring in the wet periods to which
that island is subject were attributed to anthracnose. Investi-
gations made by the writer in St. Vincent on various occasions
since 1915, and the maintenance of constant observation on boll
diseases there by S. C. Harland, have shown that at least a very
large part of the loss is due to internal boll disease, Phytophthora
rot, and bacterial boll disease. Lesions closely resembling the
anthracnose of American writers and infested with a similar
pink-spored Golletotrichum or with a pink Fusarium are fairly
common, but in all the examples seen by the writer they appear
to develop on the spots initiated by Bacterium malvacearuni.
The status of Golletotrichum gossypii on Sea Island cotton in these
islands is therefore at the present time a matter of doubt, and re-
quires to be completely re-investigated. The description which
follows refers to the American type of disease.
Symptoms.
The fungus attacks the boll, according to the account given by
C. W, Edgerton, through the pistil from conidia reaching the
flowers, by penetrating the uninjured epidermis of young tender
bolls, and by way of insect punctures or bacterial spots on older
bolls. Beginning with a small spot the lesion extends to a vari-
able extent and may cover half the boll. The affected patch
is sunken and brovmish, with usually a red border.
The lint and seed are affected to an extent varying from
slight damage to complete destruction, depending on the age of
the boll and also on the previous or concurrent presence of other
organisms.
278 DISEASES OF CROP-PLANTS
Course of the Disease.
The seeds are liable to be infected in the boll without necessarily
impairing their power of germination, and when they are subse-
quently planted the fungus develops and produces spots or
marginal injury upon the cotyledons. The conidia are also
carried on the surface of the seed. A certain amount of damping-
off, and of spotting on young stems are also produced.
The fungus continues its existence in dead spots on the
leaves and stems of mature plants, but does not spread to any
marked extent in the living tissues apart from the bolls.
Carried over as above described the fungus only assumes
prominence when the bolls are formed. The first infections on
these produce enormous numbers of conidia, which are released
when wetted, and the combined effects of rain and wind, probably
supplemented by insect carriage, serve for their distribution,
and enable them to give rise in suitable weather to epidemics
of the disease.
The Fungus.
The manner of production of the conidia is that typical of the
anthracnoses (p. 22). The conidia are hyaline, one-celled,
cylindrical, generally straight but sometimes slightly curved,
with rounded ends, and usually slightly narrower towards the
middle. They measiire about 3.5-7 X 12-25 microns. The
measurements of the variety Barbadense are given by Lewton
Brain as 4-5 x 11-14 micr.
The ascogenous (Glomerella) stage has beaked perithecia,
dark brown to black in colour, subglobose to p3'riform, 80-120
X 100-160 microns, usually buried in the host tissue with only the
beaks protruding ; ascospores nearly elliptical, hyaline, 12-20
X5-8 micr. ; paraphyses abundant.
Control.
As pointed out above, the amount of injury effected by
anthracnose in these islands, alone and in association with
bacterial boll disease, requires to be determined, Should the
need for control arise the most promising measures are selection
for resistance and the use of clean seed. In view of internal
infections the latter can only be secured by picking from uninjured
bolls. It is desirable on general grounds that all cotton seed for
planting should be externally disinfected.
Curly Leaf, Chibble Leaf.
Under certain conditions which appear to be closely con-
nected with the prevailing weather Sea Island cotton during its
vegetative period may take on an abnormal form of growth to
which the names above have been applied. It most frequently
DISEASES OF COTTON 279
supervenes after some amount of normal growth has been made,
and sooner or later it is thrown off and normal growth resumed.
Meanwhile the form of the plant is strikingly changed and bearing
is almost entirely inhibited.
The affection was widespread and long-continued in St.
Kitts and Nevis in June- July, 1914. It had been noticed in
previous years affecting fields here and there in St. Kitts, and
more regularly in particular localities in Nevis. It has also
occurred from time to time in Antigua, Montserrat, and St.
Vincent. An affection with symptoms the same in kind but
developed in somewhat different proportions has been trouble-
some in St. Croix.
Description.
The general character of the form of growth in curly leaf
disease is shown in Figs. 105-6. It is most noticeable towards
the upper extremity of the main shoot, but occurs also on the
laterals. The internodes of stems and branches, and usually the
leaf stalks as well, grow out to be abnormally long and slender.
The productive branches commonly have only two or three
nodes, which makes the number of possible flowers small. The
leaves from their earliest appearance are strongly crimped all
round their margin ; they are thin in substance, and pale green,
tending more or less to yellow, in colour. In some cases there is a
diffusion of still paler colour bordering the larger veins. In what
have seemed to be the more extreme cases examined numerous
small irregular holes occur torn in the body of the leaf and on
its edges. They arise from the inability of small brown spots
formed in the very young leaf to expand with the rest of the
tissue.
The effect on bearing of the onset of this condition is profound.
There is a strong tendency for the flower buds to dry up, turn
black, and drop at a very early stage in their development ;
very many are lost when the bracteoles are but 2 or 3 mm. across,
others at various later stages. Some plants are rendered com-
pletely barren even of flowers in this way. Of the flower buds
which do succeed in developing many fail to open ; the outside
of the petals becomes slightly discoloured pink and takes on a
characteristic rather woolly appearance, and as the corolla
becomes mature the margins of the overlapping petals towards
the tip of the bud lack their normal firmness and cling together,
failing to expand. Complete withering of the petals follows.
Flowers examined in the first visible stages of the process have
been found to have the anthers already brown and withered.
The effect is possibly produced by gradual wilting of the petals
from the margin inwards, or it may be that they lack from the
first the consistency necessary for the expansion of the corolla.
All experience has shown that so long as the curly-leaf con-
dition is prevalent the yield is exceedingly small.
28o DISEASES OF CROP-PLANTS
Comparison of notes and specimens with those of Longfield
Smith in St. Croix shows that in that island the crimping of the
leaves is not developed to the same extent, but that the occur-
rence of small irregular holes, as mentioned above, is so abundant
as to make this the most prominent feature of the disease. The
symptoms on the flower-buds and the consequent effect on bearing
are the same.
Nature of the Affection.
Careful investigation has revealed no evidence of association
with any insect, fungus, or other organism. The manner of
occurrence does not suggest a parasitic origin and the affection
does not appear to be communicable. It has no connection with
seed of any particular origin and can occur in old or newly
imported strains. It must therefore be attributed to some
functional disturbance connected with external conditions.
Conditions of Occurrence.
It is the firm opinion of local observers that the appearance
of ciirly-leaf is connected with dull rainy weather, and it is
especially liable to occur in shaded and sheltered situations. In
St. Croix the disease is always associated with vigorous growth
in the first two months after planting and is especially prevalent
on the eastern side of the island where the soil is deep. Cotton
planted on new land is generally worst affected. The stunting of
plants through water-logging has been noticed to cause their
escape from the disease, and the outermost rows of affected
fields commonly remain normal.
The affection is by no means regular in its occurrence in the
same district at the same time. It is usually generally distributed
through a field in which it occurs, but in other fields may be
absent or only slightly developed.
Accounts agree that normal growth is always resumed when a
period of dry sunny weather ensues.
Causation.
The conclusion indicated is that the curly-leaf disease is
definitely connected with rapid growth brought about under
certain soil conditions, including depth and easy permeability,
when plants in the active vegetative stage receive an abundant
supply of water and the checks to growth ordinarily exercised by
sun and wind are removed by the prevalence of still cloudy
weather, the shading of the situation, or the crowding of the plants
themselves.
Loggerhead.
In the same season (1914) that the curly-leaf affection was
most abundant in St. Kitts and Nevis, a second form of abnormal
DISEASES OF COTTON 281
growth, locally named loggerhead, occurred to about the same
extent. This appears to be closely similar in nature, but produces
changes in the form of the plant which are in many respects the
exact reverse of those seen in curly-leaf.
The most striking feature of the loggerhead form of growth is
the shortening, to a greater or lesser degree, of the internodes of
both stems and branches. A plant which is affected at an early
stage of growth assumes the form of a low dense bush, with the
primary leaves, and the shortened reproductive branches with
their leaves and flowers, all crowded together on the shortened
vegetative axes. If the plant becomes affected only after normal
growth has been made, then the tops of the main stem and of the
vegetative branches show similar close bunchy masses. The
flowering branches have numerous internodes, so that large
numbers of flower buds are produced.
The leaves of the loggerhead form are very distinctive. They
are normal in thickness ; the general colour is a dark green,
usually distinctly darker than that of the leaves of normal
plants ; there is no tendency to the crimping at the edges which
is the most conspicuous feature of the other form, but fre-
quently the leaf substance is puckered along the main veins
owing to their insufficient elongation ; the secondary veins,
instead of diverging at the usual wide angle may be tied at a
narrow angle to the primary veins for some distance, apparently
from the failure of the intervening ground tissue to expand.
Such undeveloped spots are almost transparent and of a light
yellow colour. In many cases there is not this interference
with the form of the leaf, but in all cases there is a definite
mottled effect produced by the presence of lighter green or
yellowish areas. In all typical cases the mottling is quite difl'erent
in appearance from the diffused yellowish discoloration along the
course of the veins which has been already described as occurring
in some cases of curly-leaf. A few examples have been noticed,
however, where in plants of the loggerhead form the light areas
have followed the course of the veins. In such cases the yellow
colour has usually been more definite than in the curly-leaf
examples, but occasionally the approach is very close. This
was seen in plants in which the suppression of the internodes was
hardly noticeable.
As in the case of curly-leaf, large numbers of flower buds
turn black and drop in their earliest stages. This symptom
cannot, however, be regarded as a specific character of these
affections, since the same thing has been observed on cotton
plants of normal form when exposed to unfavourable conditions.
The abnormal condition of the flowers described in connection
with curly-leaf also occurs in plants of the loggerhead form.
The loggerhead form of growth can be assumed at any stage
at which active growth is proceeding, including plants only a
few inches high. Most commonly its occurrence is distinctly
282 DISEASES OF CROP-PLANTS
sporadic, much more so than with curly-leaf. A few cases have
been seen in which its incidence was fairly general over the whole
or part of a field, but more often the majority of the plants have
been normal.
Sometimes a single abnormal plant, sometimes two or three,
have occurred in a large field.
History and Distribution.
Attention seems to have been first attracted to the logger-
head affection in 1910, in which year it appeared to an alarming
extent on an estate situated towards the western extremity of
the island of St. Kitts. Then, as later, it was not regular or
universal in an}- field. The seed was from the local Experiment
Station at La Guerite. It has since been noticed to some extent
in the same district every year. In 1913 it was severe on two
estates 2 or 3 miles to the north-west. Again the cotton happened
to be from local seed, and in consequence a change was made in
1914, and seed of a Barbados strain, grown three years in St.
Kitts, was used. Early plantings from this seed were so badly
affected that a fairly large area was cleared about June and
replanted.
In the neighbourhood of Basseterre, sporadic examples have
occurred each year since the affection was recognised, and have
been usually rogued out. In July, 1914, the affection appeared
to a much greater extent, but still with a scattered distribution,
in various fields situated on the La Guerite lands. This cotton
was planted, owing to the occurrence of early rains, some two
months earlier than is customary.
In Nevis, though curly-leaf has been common on some areas
since 1911, the loggerhead affection has only occurred sporadi-
cally, and has not been regarded as of serious importance. In
1914 it was somewhat more common, and in two or three places
seriously affected small areas. One of these was a plot at the
Experiment Station planted with St. Vincent seed.
Loggerhead has not been recorded from any other island.
Conditions of Occurrence.
As in the case of curly-leaf there is nothing to indicate a
parasitic origin for the loggerhead disease, or its transmission
by infection or by seed. The evidence as to the conditions
inducing its appearance is less definite than that regarding
curly-leaf. A plant recovering from the affection can occur
within a few yards of one of the same age just developing it.
Whole plots of similar age separated only by a road have shown
the same contrast. Frequently two plants, one healthy and one
badly deformed, have been seen growing in close contact from
the same " hole."
The one definite piece of evidence bearing on the origin
DISEASES OF COTTON 283
of loggerhead is that its appearances have mostly been made in
fields planted out of the normal season.
There is no evidence of direct relationship between curly-leaf
and loggerhead. They may occur in the same field, or be entirely
dissociated. But it is suggestive of the similarity of their nature
that both reached their fullest development in St. Kitts and
Nevis in the same season, that both ceased to be developed when
fine sunny weather set in, and that neither has since been noticed
to any significant extent.
Rust.
A species of rust fungus is common on the leaves of perennial
cottons throughout all the islands. The variety known as Nevis
Purple and some others are specially susceptible to it. Sea
Island is usually only attacked on old leaves toward the end of
the season, but earlier infestation occurs on unthrifty plants.
The fungus in all cases appears to be Kuehneola Gossypii
(Lager.) Arth., first described as Ure.do Gossypii by Lagerheim,
in 1891, from specimens collected in Ecuador. It is recorded
from British Guiana, the Lesser Antilles, Cuba, Porto Rico, and
Florida. The uredospores commonly met with are borne mostly
on the underside of the leaves in numerous small round sori,
which are generally enclosed by a purple ring and marked on the
upper side of the leaf by a purple spot.
Red Leaf Blight.
Cotton grown in poor or deficient soil is liable to show a
special symptom of ill-health which is known as red leaf-blight or
as " rust." The latter name is unfortunate as the affection has
no relation with the actual rust fungus occurring on cotton.
The condition referred to is marked by a discoloration of the
leaves, which become conspicuously suffused with red or reddish
brown. It is commonly associated with poor growth but may
occur on large plants in the middle or later stages of their exis-
tence. The affected leaves tend to fall prematurely.
In St. Vincent the affection is very common from an early
stage on the undersized plants produced by land inadequately
manured, especially on slopes subject to loss by washing. The
manurial experiments conducted in that island appear definite-
ly to connect the affection with shortage of potash, a conclusion
reached many years ago in connection with the closely com-
parable affection occurring in the United States,
Yellow Leaf Blight.
A condition which in its general characters resembles the
affection described under this name by G. F, Atkinson, and also
known in the United States as " mosaic disease," or in its later
284 DISEASES OF CROP-PLANTS
stages as " black rust," is sometimes met with in these islands,
on plants suffering from drought or poor nutrition, usually on
shallow or sandy soils.
According to Atkinson, in the normal and usual progress of
the disease there first appears a peculiar yellowing of the leaf,
which gives it a checkered or mosaic appearance. The yellow
colour appears in small areas and bears a definite relation to the
venation of the leaf, being bounded by veinlets which subtend
areas more or less rectangular in outline. The green colour is
found along the larger and intermediate veins.
Up to this stage the affection is non-parasitic, and due to
imperfect nutrition. Later the weakened leaves are usually
infested with weakly parasitic fungi (Alternaria, Macrosporium,
and Cercospora ; see below) which give rise to the condition
known as black rust, in which leaf-spots are heavily developed.
The treatment found to be successful in preventing the
development of yellow leaf blight on light soils in Alabama is the
manurial application of potash salts.
Macrosporium Leaf-Spot.
In the condition known as yellow leaf blight one of the fungi
commonly found on the browned margins and in zoned spots on
the failing leaves is a Macrosporium sp. In the United States,
G. F. Atkinson described the species thus occurring as M. nigri-
cantium, as follows :
" Hyphse amphigenous, subfasciculate, or scattered, 50-140 x
6-7 micr., nodulose, septate, olive brown. Conidia 18-22x36-50
micr., strongly constricted about the middle, stoutly rostrate
at one side of the apex, smooth, transversely, longitudinally, and
obliquely septate, olive brown."
Whether the Antillean form or forms are identical with this
has not been determined ; in some cases the agreement with the
description above is not very close.
Alternaria Leaf-Spot.
A species of Alternaria is common in these islands on leaves
which are failing or are damaged by disease. Similar records
have been made from time to time in the United States and the
fungus is generally regarded as probably assignable to Alternaria
tennis Nees., a species described from cotton in Italy.
R. C. Faulwetter has recently made a study of Alternaria
leaf-spot as it occurs in S. Carolina, and his conclusions are as
follows :
" The lesions are characterised by a pale green, then straw-
yellow and finally rusty brown colour, brittle papery texture and
irregular concentric ridged zonations.
" The identity of the fungus causing the disease cannot be
DISEASES OF COTTON 285
determined with certainty, but it is very similar to Alternaria
tenuis, Nees.
" Artificial inoculations show the fungus to be a weak parasite
and able to infect healthy tissue only under most favourable
conditions."
Leaf Mildew.
Under the name of West Indian leaf mildew a leaf affection
of Sea Island cotton has been referred to in all accounts of the
diseases met with on Sea Island cotton in the Lesser Antilles
since the revival of cotton-growing. Perennial cottons are also
attacked. The disease is universally distributed through the
islands and occurs in abundance in most fields towards the end
of the crop. The fungus was described in 1920 from material
collected by the writer as Ovulariopsis Gossypii E. M. Wakefield.
Symptoms.
The affection is caused by the growth of the fungus, in patches
or with a general distribution, on the backs of the leaves. The
dark patches produced are covered with a more or less dense
white web on which abundant large conidia are produced. On
the upper surface of the leaves the infested areas are marked by
irregular reddish-purple or purplish-black blotches or by a more
general suffusion of the same colours.
The Fungus.
The mycelium is almost entirely superficial, hyaline, and
freely septate. A few hyphae penetrate by the stomata into the
mesophyll. The conidiophores are upright and simple, septate,
bearing terminally in succession the hyaline, reticulate conidia,
which are oblong or broadly elliptical, with one end rounded or
slightly produced, the other flattened at the point of attachment,
and measure 50-60 x 16-22 microns. Not more than two, one
immature, have been seen attached to the conidiophore at the
same time.
Incidence.
The disease has been usually reported from the various
islands as attracting attention only after periods of wet weather.
Its occurrence on the foliage borne by the plants towards the end
of the crop season is not limited in this way, for it can be general
even in very dry weather, but it appears that excessive moisture
induces an earlier prevalence, and in such an event it is probably
responsible for some indirect loss of crop. That it is not neces-
sarily confined to old or even to mature plants has been shown by
its occurrence on the fresh leaves of quite young plants grown in
pots, but under normal conditions the plant is resistant up to a
286 DISEASES OF CROP-PLANTS
time when the prevalence of the affection is of no practical
importance.
Areolate Mildew.
The cotton leaf disease known in the U.S.A. as areolate
mildew, due to Ramularia areola, has recently been recognised
to be common in Trinidad. It gives rise to small angular
whitish spots on the undersides of the leaves.
Crinkled Dwarf, Man Cotton, Mottled Hybrids.
Certain types of cotton plant abnormal in form owing to their
genetic constitution are met with in the cotton-growing islands
and are liable to be regarded as diseased. In the first two cases
to be described they occur as very occasional individuals in a
normal Sea Island population, in the third as a result of accidental
or deliberate crossing of Sea Island with local perennial cottons.
Crinkled Dwarf.
This type has been noticed for many years in Barbados, and
has been studied from a genetic point of view by S. C. Harland
in St. Vincent.
The plant as a whole is not more than 2 feet 6 inches in height
and is generally less than 2 feet. There is a general reduction of
all the vegetative and reproductive parts. The leaves are small
and characteristically crinkled, with torn and ragged edges, and
are uneven in colour from the presence of patches of yellowish
green. Vegetative branches are seldom produced and flower
production is early and profuse. Shedding takes place to such
an extent, however, that only an occasional boll matures and
many plants are quite sterile.
Crosses with normal Sea Island result in a first generation
indistinguishable from the normal parent. The second and
subsequent generations reveal segregation of the simple Mendelian
type into pure and heterozygous Sea Island and pure dwarf.
It would appear that the crinkled dwarf is a mutation resulting
from the loss of a single factor.
Man Cotton.
The definite morphological type known as " man cotton "
occurs in Barbados, St. Kitts and St. Vincent, and probably in
the other islands. Its genetics have been studied by S. C.
Harland.
Man cotton differs from normal Sea Island in the reduction in
size of its parts and in a degree of sterility which is complete as
regards the absence of viable pollen from the anthers and almost
complete in respect of the ovules, even when pollinated from
normal plants. Many of the fruiting branches abort after pro-
ducing one or two nodes. The effect of these factors on habit
DISEASES OF COTTON 287
is to produce a plant which is at once recognisable by its wiry
upright stem without spreading branches.
The plants grown from the only three seeds which have been
found, which must have developed from pollination with normal
Sea Island, were themselves normal in appearance. The abnormal
type has occurred in small proportions in apparently pure strains
of Sea Island which have been selfed for four successive years.
Mottled Hybrids.
In crossing Sea Island with native perennial cottons a pro-
portion of the progeny not seldom shows a form of growth some-
what similar to that seen in the loggerhead affection already
described. It is reported that 100 per cent, of such plants
regularly occur in crosses with the St. Croix native. The writer
has met with 50 per cent, on one occasion in Barbados, and 20
per cent, is recorded in Nevis. Some years ago such plants were
fairly common in Barbados fields as the result of accidental
crossing.
The internodes of the plants in question are very much
shortened so that a low and bushy habit is developed. The
leaves are always strongly mottled with yellowish green. The
stems exhibit a precocious formation of cork which gives even the
youngest internodes and the petioles a rough scaly appearance.
Bearing is not affected, and the plants may give a very high
yield. The abnormal characters usually but not invariably
persist throughout the life of the plant.
CHAPTER XXVI
DISEASES OF SUGAR-CANE
The sugar-cane is by origin a coarse and vigorous grass, which
retains in the forms developed by cultivation the general habit
of grasses, and is therefore particularly suited to field conditions.
The principal diseases to which it is subject are not such as are
encouraged by heavy rainfall, and in good soil it is well able,
especially while the shoots are young, to withstand exposure to
sun and drying winds. It produces a crop which is dependent
only on vegetative growth, and not as in the cereal grasses on the
successful ripening of grain. For these reasons it is adaptable
to a very wide range of conditions of soil and rainfall.
Succeeding on an original diversity of crops, the cultivation
of sugar-cane was for a period general throughout the settled
parts of the British West Indies ; it grew in light and heavy soils,
under copious and scanty rainfall, in humid and in exposed and
wind-swept situations.
The disastrous depression of the cane-sugar industry in the
second half of the nineteenth century led to replacement of sugar
cane by other crops wherever this was seen to be possible, and
the nature of the alternatives available meant for the most part
that the well-watered and more humid situations were planted
in cacao and limes. This applies to Dominica, much of St.
Lucia, Grenada, Tobago, and a considerable part of Trinidad.
On the other hand, St. Kitts, Antigua, Barbados and certain
areas of Trinidad, unsuited to orchard crops, maintained a much
impoverished sugar industry ; while the remaining islands be-
came almost dereUct until the revival of cotton cultivation.
Though the central factory system increased stabilitj^ and the
Great War brought temporary prosperity, this remains, with a
few survivals in other localities the distribution of the sugar
industry in these islands. An account of the conditions prevail-
ing in each island is given in another section. In general the
present position, as the outcome of the history outlined above,
is that sugar-cane cultivation is mainly restricted to the drier
islands or in the case of Trinidad the drier or more exposed
districts, a result of considerable ecological importance.
The outstanding factors favouring the growth of sugar-cane'
are rainfall, atmospheric humidity, soil aeration, and nitrogen
supply The combination of optimum conditions in these respects
288
Fig. 110
Marasmius Mycelium on Sugar-Cane
Fis. Ill
FRucxrFiCATioNS OF Marasmius associated with Acute Root
Disease of Sugar-Cane, Trinidad
Fig. 112 Acute Type of Root Disease
Infestation of Young Plant Cane with Marasmius
\
Fig. Ii:? Acute Type of Koot Disease
Buds of Young Plant Cane infested with Marasmius
Fig, 114
Fructification of Odontia Sp. on Sugar-Cani
DISEASES OF SUGAR CANE 289
is rare, and the deficiencies encountered in different situations and
circumstances lead to wide diversities in agricultural practice.
The primitive method of growing sugar-cane, still practised
where good forest soil is available, is to plant up the cleared land
and take off a long succession of ratoon crops, then perhaps letting
the land relapse for a time to bush. Sooner or later this method
has to be replaced by a more permanent system of agriculture,
with more frequent replanting, more careful cultivation, the
use of manures, and eventually the interposition of rotation crops
or fallow. The extent to which these measures have to be adopted
to get an average crop with average ability of management is a
measure of the general suitability of the conditions for cane
production.
On good soils in Trinidad, where heavy dews and humid air
supplement the effects of the actual rainfall, it is still possible
in some places to pursue a system of continuous cropping and
long ratooning. The opposite extreme is reached in the arid sea-
board districts of Barbados, where plant canes only can be grown
and these have to be alternated every third or even in some cases
every second period with a rotation crop, in addition to the land
being thoroughly prepared well ahead of each planting season
and each crop heavily manured.
Between these extremes there is a variety of practice, with a
general tendency in recent years to the further reduction of
ratoons, which is variously interpreted as the result of less efficient
cultivation owing to shortage and increased expense of labour,
the substitution of chemical for organic manures, and the demand
for increased tonnage under the system of central factories.
It is more than usually necessary in the case of sugar-cane
that an understanding of agricultural practice should precede
the study of its diseases, for the manner in which any defect in
the adaptation of agricultural methods to conditions of soil and
climate finds expression is in the prevalence of the general condi-
tion known as root disease. It is thus permissible from one
point of view to regard all the modifications of practice referred
to above as measures for the avoidance of this condition.
Excepting injuries of insect origin, root disease is at the
present time the only serious affection of sugar-cane in these
islands, and the effects of insect injury are often so entangled
with those of root disease that they cannot be studied apart.
Root Disease : General Type.
The Nature of Root Disease.
In its application to sugar-cane the term root disease in its
general sense is an expression for the form which is taken by
failure due in the first instance to adverse conditions of soil,
climate, or insect infestation. It owes its recognition as a disease
u
290 DISEASES OF CROP-PLANTS
to the fact of the invariable association with it of the myceHa of
certain fungi which frequently become conspicuously developed
on the affected stools. The presence of these fungi is not in
itself sufficient to produce any notable effect on the health of
the plant, for with the system of cultivation followed in localities
favourable to the cane they occur in some abundance in fields
bearing good crops, and it is certain that they are present to
some extent, though it ma}^ be in an inconspicuous condition,
in all cane fields.
It is usually assumed, and the evidence as a whole supports
the idea, that while the root fungi normally exist on dead or
senescent material they are able to attack and injure the plant
when its vigour is for any reason depressed. The amount of
this injury cannot be exactly determined, for it must in the
nature of the case be exceedingly variable. The adverse condi-
tions which enable such injury to take place produce their own
direct effects, and it is in all cases a matter of judgment to what
extent the action of the fungus has increased them.
In accordance with these characters root disease may be
regarded in either of two ways :
(a) as a condition of ill-health non-parasitic in its origin,
increased to a varying extent by secondary infestation with semi-
parasitic fungi ; or
(6) as a fungus disease for the development of which a
weakened condition of the plant is necessary as a predisposing
cause.
The difference is mainly one of phrases, and one or the other
conception may be the more appropriate in a given case.
Distribution.
The association of the fungus Marasmius Sacchari with a root
disease of sugar-cane in Java was described by J. H. Wakker in
1895. Root disease in which this fungus among others commonly
appears is generally distributed through the West Indian region
from Louisiana to British Guiana. It occurs in the Hawaiian
Islands and in Queensland and has recently been reported from
South Africa.
In the West Indies the fungus was first recognised and its
relation to root disease described by A. Howard. The affection
has been discussed by every mycologist who has had to do with
sugar-cane in these regions, but almost entirely in general terms.
There is need for detailed investigation of the several fungi
concerned and their relationship to the plant, but from the
nature of the case this study to be complete would need to include
the whole ecology of the sugar-cane.
The Associated Fungi.
Marasmius spp. The fungus most usually associated with
DISEASES OF SUGAR CANE
291
root disease is Marasmhis Sacchari Wakker, which is very
common in cane fields generally, its mycelium developing freely
in the thrown-out parts of cane-stools after cutting, on dead
shoots, and on adhering trash ; it can be shown to occur in old
roots. The extent to which it appears above soil level is largely
determined by the amount of moisture locally present. The
mycelium is recognisable by the way in which, by developing a
gelatinous consistency, it sticks the leaf sheaths together, appear-
ing when they are separated like a layer of dried flour paste.
It is further distinguishable from that of the fungus next to be
described by the absence of either feathery structure or of a
uniform granular layer formed on the external leaf sheaths of
the infested stools.
Fig. 115
Odontia sacchari, Cystidium and Basidium with Single Spore
From Bull. Porto Rico, Ins. Expt. Sta.
The fructifications of Marasmius are small white papery
toadstools, usually less than half an inch across. They may be
formed in abundance on infested material, but only under very
moist conditions, generally after heavy rain. They quickly dry
up and disappear on a sunny day. Their connection with the
white mycelium described has been established by cultures, and
they develop on infested material, including dead roots, kept in
a moist chamber.
There are other species of Marasmius occurring on sugar-cane,
seen especially in Trinidad, but to some extent in Barbados
Their mode of occurrence appears to be essentially the same as
that of M. Sacchari.
292 DISEASES OF CROP-PLANTS
Odontia spp. A white m5^celium, easily distinguishable from
that of Marasmius by its feathery pattern of growth and the
production of abundant stellate crystals of calcium oxalate, has
long been known as apparently capable of producing effects
similar to those of Marasmius. It has been commonly referred
to as the stellate crystal fungus, and, as a mycelium of unknown
affinities, was given the name Himantia stellifera by J. R. John-
ston. More recently E. A. Burt has described the fructifications
of Odontia Sacchari and 0. Saccharicola from Porto Rico material :
two closely related fungi which produce their spores on cane trash
in a close-lying, granular, buff-coloured layer hardly distinguish-
able without the aid of a lens from the surface of the dry trash
itself.
The constant association and apparent organic connection of
Odontia fructifications with the typical stellate crystal mycelium
of Himantia suggests strongly that the two belong to the same
fungus, to which the name Odontia would in that case apply.
The relationship is highly probable but has not to the writer's
knowledge been proved by pure cultures. Both Odontia and
Himantia are generally distributed in these islands.
Other fungi :— In Hawaii the stinkhorn fungus Ithyphallus
coralloides has been regarded as responsible for some amount of
root disease and in British Guiana the mycelium of the related
Phallus aurantiaciis is commonly vaet with. Forms of infesta-
tion with distinctive mycelia have occurred in Antigua but
without identification of the fungus concerned.
C. W. Carpenter has shown recently that root decay, par-
ticularly of the Lahaina cane, in the Hawaiian Islands is caused by
Pythium hitler i Subram, which he considers to be responsible
also for wilt disease of the pineapple and a root-rot of rice.
Symptoms.
The prevalence in growing canes of the ordinary type of root
disease results in a backward and stunted condition of the stools.
The general effect is that of shortage of water brought about by
interference with absorption through reduction of the roots.
The leaves of affected shoots have their edges rolled inwards
when those of healthy plants are expanded ; they eventually
turn brown from their edges inward, and in order from below
upwards, until the shoot is withered to the top. The trash,
where it has not been stripped, is cemented firmly around the
lower joints of the canes, and under it matted roots occur up to a
foot or more from the ground, sent out in an endeavour to find the
necessary water. A clump infested early has a characteristic
stunted appearance and several or many of the shoots ma}^ be
dead and dry. Infested stools are easily turned over and exhibit
roots which are mostly either dead or lacking in fibrous branches
It is not usual in this type of the disease for the stem bases
to be attacked, and the amount of injury to the roots is often
Fig. 116
HiMANTIA StELLIFERA
Mycelium on Sugar- cane
DISEASES OF SUGAR CANE 293
difficult to estimate. Senescence and death of the older roots is to
some extent normal, and changes in aeration brought about by
the saturation or packing of the soil frequently cause the gradual
replacement of the deeper roots by others nearer the surface,
without necessarily producing much effect on growth so long as
the weather is favourable. The most that one can usually say in
moderate cases is that the number of functioning roots is less
than one would expect to find in the situation, or than is found in
normal plants at the same stage. It is also possible to trace a
certain amount of invasion of the young roots by the hyphae of
the fungus ; th 'r tips are frequently brown and dead, the cortex
Fig. 117
HiMANTIA STELLIFERA, HyPHA WITH STELLATE CRYSTAL AND SWOLLEN
Bodies
From Bull. Porto Rico Ins. Expt. Sta.
is red or brown in spots, and considerable distortion may be
present as a result of interference with the growing tips.
In the acute type of root disease later described, the ability
of Marasmius to become parasitic on roots, buds, shoots and
stem bases is convincingly demonstrated, and it is reasonable to
assume that, in the general type now under discussion, this
ability is exercised to some extent.
Conditions of Occurrence.
The condition of infestation described is very rarely
general and uniform over a whole field. Typically it
occurs in its pronounced form in patches related to the
294 DISEASES OF CROP-PLANTS
contour of the ground or the nature of the soil. Where its
origin is due to drought the windward edges of the field are often
worst affected. The predisposing causes of its prevalence may be
summed up under the headings : dry weather, poverty of soil,
defective aeration, and insect injuries.
Soil poverty and drought are much the same in their results,
and root disease as the effect of the former most frequently
becomes evident as rain fails. On the other hand, it is possible
by thorough cultivation, by heavy manuring, by reducing ratoons
and by rotation of crops, to so increase the resistance of the cane
and reduce the quantity of fungus that the effects of a low rain-
fall are largely offset. It is from this point of view that the
intensive agricultural methods characteristic of Barbados must
be regarded.
Defective aeration, which confines the roots to the upper
layers of the soil, reducing their feeding range and leaving them
susceptible to the first influence of dry weather, may be due either
to want of tilth in the soil or to the saturation of the soil with
water. Both causes operate to induce root disease. The retention
of tilth depends, (a) on the physical consistency of the soil, so
that some soils are for this reason less suited to ratooning than
others, (6) on its content of organic matter, (r) on the frequency
and thoroughness of cultivation, and ((/) on restriction of the
operations of implemental tillage to suitable weather. The watP'^
logging of the soil is usually due to imperfect provision 1
drainage, but in some cases is attributable to the highly retenti;
nature of certain soils, which in wet weather refuse to drain at l
adequate rate even on the margin of the trenches. At the same
time that the drying out of water-logged soil causes an improve-
ment in canes suffering for that reason, dry weather takes effect
on those in light or compacted soils and makes their condition
worse.
Insect injuries and root disease may be intimately associated,
as in the froghopper bhght of Trinidad. The fields attacked by
froghopper are usually those which by their age or situation are
most liable to root disease, and after they are damaged by the
insect the fungi get a hold on them which prevents recovery when
the insect infestation has passed. In this way an end result of
whole fields or tracts appearing completely crippled by root
disease is produced in the situations where the froghopper has
been prevalent. The infestation of stools with root grubs also
produces or increases susceptibility to root disease.
So far the discussion has emphasized the harmfulness of root
fungi, but an account of root disease would give a wrong im-
pression which did not indicate the ability of plants growing under
good conditions to make excellent growth in spite of heavy
infestation of the stools. This is best seen when canes in good
soil, infested as a result of drought, resume growth with the return
of rains. Stools which appeared dead will send out vigorous
DISEASES OF SUGAR CANE 295
shoots and abundant new roots and continue to thrive so long
as the conditions are favourable. In the same way fields of
ratoons which have been infested in a dry year and have given
next to no return have produced good crops when the following
year provided suitable weather. This is in strict conformity with
the nature of the disease as set out above, and no such recovery
can be expected where the soil is in a defective condition.
Infection.
It is not at all uncommon for cuttings and especially for top
plants to have root fungus present upon them in a living con-
dition when they are put into the ground. It is in fact the
custom of some planters, from motives of economy, to take their
plants from their worst fields, which in many cases are running out
owing to infestation with this disease. Such material may give
quite good results in a favourable season, but the fungus is present
from the beginning, ready to take advantage of any unfavourable
circumstance that may occur. Infection may also take place from
the old stools left in the ground, from imperfectly rotted trash
and, where there has been no suitable rotation of crops, from
mycelium present in the soil. Ratoons are especially liable to
infestation owing to the fungus which has gained a footing on the
plant canes attaining to a vigorous development on the material
thrown out of action when the first crop is cut, at the same time
that the effect on the plant of the progressive deterioration of
the unworked soil affords it increasing opportunities for harm-
fulness. The spores of the fungus afford another means for
its dissemination, but of this nothing is directly known.
Varietal Resistance.
In a given locality there are stronelv marked differences in
the susceptibility of sugar-cane varieties to root disease. It is
questionable, however, how far resistance or susceptibiUtv can be
regarded as constant varietal characters. It is the writer's
opinion that canes which are well or ill-suited to a particular
district, especially in regard to soil, show differences in vigour
which are reflected in their relative liability to root disease. A
cane which has an excellent reputation in one place may have a
very poor one in another, and various degrees of such differences
give rise to the strong and varied local preferences which
exist.
Under present conditions the Bourbon and B 208 appear to
be the most widely susceptible of standard kinds ; both require
soils in good tilth to succeed. To these C. K. Bancroft added
D 625 and Green Transparent from British Guiana experience,
with D 109 rather susceptible, D 145 and D 118 more resistant,
and D 216 and D 159 not affected. White Transparent shows an
increase of susceptibility over most other varieties in the dry
296 DISEASES OF CROP-PLANTS
conditions of Barbados and on heavy soils in Antigua. In
Antigua Sealy Seedling is highly valued for its resistance on
heavy clay soils and D 95 is fairly satisfactory under the same
circumstances. The most strongly resistant of all canes grown
in the West Indies is the Uba, which is used for planting in poor
soils and on thin patches in Trinidad and Antigua. Even this
becomes severely infested and can even be killed outright by
root disease in combination with froghopper attack.
Control.
It is clear from the foregoing that it is to the general efficiency
of his agricultural practice rather than to direct action against
the fungus that the planter must Igok for the control of root
disease. The greatest of all safeguards against it lies in the rota-
tion of crops. Where this is not practicable the more thoroughly
the ground is worked in the necessary interval between reaping
and planting the better, since exposure leads to the rapid decay of
infective material, and tends to the sterilization of the soil.
Under such circumstances deep ploughing is advisable at an early
time. The practice of leaving the stools in " resting " or thrown-
out lands is a very bad one from this point of view. With regard
to the ratooning of infested fields, although as has been mentioned
above it may be possible in a good year to ^ei satisfactory re-
sults, such conditions are not so frequent in most of the cane-
growing islands as to make the chances worth taking. The
advantages of selecting healthy material for planting have been
pointed out above. The soaking of the sets in Bordeaux mixture,
originally recommended as a precaution against pineapple
disease, is of very little value against the mycelium of
Marasmius.
Where the disease appears in single stools or patches some
local cause such as soil grubs, sucking insects, bad drainage or
shallow soil should be first looked for. The effects of exposure
may cause the disease to appear on the outside rows of wind-
swept fields.
The digging of isolation trenches sometimes recommended is
not regarded as having any practical value. Another recom-
mended measure, the application of lime, must be considered
with due regard to the general condition of the soil and the
indications found of the causes of the infestation. Quicklime
has a directly sterilizing effect, slaked lime hastens the decay of
material that would support the fungus in the soil. Exposure
of the soil by repeated turnings is probably quite effective enough
for practical purposes.
It should be noted that chemical manures applied to diseased
stools are wasted unless the formation of new roots has first
been stimulated by moisture and cultivation.
DISEASES OF SUGAR CANE 297
Root Disease : Acute Type.
Instances have on several distinct occasions come under the
observation of the writer in young plant canes in Barbados, and
recently m fields of first ratoons in Trinidad, in which plants
growing in good well-tHled soH and previously healthy and
vigorous have rapidly failed, and have been found to be heavily
infested with Marasmius, not only on the roots and leaf-sheaths,
but in the tissues of the basal joints of the cane. In such cases
the fructifications of the fungus have been produced with un-
usual readiness and in considerable quantity.
The attacks on plant canes have occurred in somewhat
scattered stools during the dry season. On one occasion numbers
of stools of Ba. 6032 were quite killed out in this way, while
plants of B 6450, in the same field, which were not nearly so
forward, were unaffected. The basal joints, and the sprouting
buds in all stages were internally reddened and filled with Maras-
mius mycelium. This type of disease agrees with the effects of
Marasmius Sacchan as first described by Wakker in Java, where
the ordinary West Indian type, presumably owing to the scarcity
of ratoons, does not seem to be famihar. In Barbados M.
Sacchan was the species met with in the cases described.
The most striking instance seen in Trinidad was in a field of
Hill s Seedlings 6 and 12, unusually well-grown first ratoons in
deep and fairly heavy loam, sufficiently drained. Very many of
the large canes were badly infested or completely rotted for
several joints at the base, the parts above remaining sound until
dried up by the cutting off of their supply of water. The young
leafy shoots were also dying upwards owing to infestation in their
base. The stools were exceedingly loose in the soil, and many
were turned out by the weight of their own canes. An un-
identified species of Marasmius, with bluish black stalks, was
fruiting abundantly from the roots, the root " eyes " on the stem
and the young shoots. Other fungi were not conspicuous.
_ While no proof can be offered, the cases described, and others
sunilar, present the appearance of active parasitism by Marasmius
species. The Barbados examples were attributed to the weaken-
mg of resistance by drought, and stools not completely killed
recovered after rain. The sudden failure of the Trinidad field
described could only be attributed to the effect of a second
dressing of sulphate of ammonia on a soil already almost depleted
of its small supply of lime.
The Epidemic on the Bourbon Cane
The Bourbon cane was introduced into the West Indies in the
i8th century from the island of Otaheite, where it is believed to
have originated, and up to the years about 1895 was the variety
m general cultivation in aU the islands. About 1890 the fields in
298 DISEASES OF CROP-PLANTS
some places began to return large quantities of " rotten cane,"
and the epidemic of disease of which this was the first sign
developed with great suddenness and intensity throughout the
islands.
Losses variously estimated at 25 to 50 per cent, are reported
from Trinidad, Grenada, St. Vincent, Barbados, Antigua and St.
Kitts in the years from 1892 to 1894. The disease is reported
to have appeared in British Guiana in the latter year and sub-
sequently to have made rapid progress in the colony.
The diseased canes showed infestation with a " shot-borer "
beetle {Xyleborus perforans) and the affection was at first attri-
buted to this agency. Material was sent for examination to the
Royal Gardens, Kew, and from the examination of this and on
the strength of cultures and inoculation experiments made at
Kew, G. Massee, in 1893, reported that the " vexed question "
of the nature of the disease had been " conclusively settled," and
that it was caused by a fungus named by him Trichosphcsria
Sacchari, which formed perithecia on dead canes and had as
active conidial stages a Melanconium form {M. Sacchari, Mass.),
and a form producing micro and macro-conidia (the Thielaviopsis
ethaceticus of Went). Later he added as probably another form
of the same fungus Colhtotrichum falcatum Went, to which, from
its presence in certain stools examined, he attributed root disease
in Barbados. From the general occurrence on the surface of the
shrivelling canes of the acervuli and conspicuous sooty conidia of
Melanconium Sacchari the affection was commonly known as
rind disease.
Later research has produced no support of Massee's pro-
nouncements : Trichosphaeria does not seem to have been met
with again, and Melanconium, Thielaviopsis, and Colletotrichum
are now recognised to be entirely separate fungi, each of which,
under certain conditions, produces a distinct affection of siiear-
cane, while root disease is attributed to Marasmiiis spp.
A. Howard, working in Barbados, sorted out the confusion,
bringing the situation into line with Went's researches in Java,
and in 1903 brought forward evidence to show that the epidemic
was primarily due to the somewhat obscure infestation of the
canes with Colletotrichwn falcatum, a conclusion which all later
experience goes to confirm.
The situation was met by the adoption of resistant varieties,
of which White Transparent, Caledonian Queen, and some others
were then available, and these have been followed by a succession
of seedling varieties mainly raised in Barbados and BrH"^sh
Guiana. The shot-borer still occurs in damaged cane, Me.jn-
coniiim Sacchari is everywhere abundant on over-ripe or injured
stalks, and Thielaviopsis causes losses of cuttings in dry planting
seasons. Colletotrichum on the other hand has become rare
except when an attempt is made to re-establish the Bourbon,
when the symptoms of the original trouble usually reassert
Photo by]
[C. W. Eiigerlo
Fig. 118
Red-rot of Sugar-cane
From M. T. Cook's Diseases of Tropical Plants, by permission of Macmillan & Co.
DISEASES OF SUGAR CANE
299
Fig. 119 CoLLETOTRICnUM FALCATUM
From Bull. Dept. Pathology, H.S.P.A.
Fig. 121 Melanconium Sacchari : Vertical Section of Stroma
Fro7n Bull. Dept. Pathology, H.S.P.A.
300 DISEASES OF CROP-PLANTS
themselves with undiminished intensity. In some remote dis-
tricts in Trinidad, and to a more considerable but rapidly dim-
inishing extent in British Guiana, the cultivation of the Bomrbon
is still continued.
The statements of some writers that the breakdown of the
Bourbon was mainly due to root disease is not supported by the
evidence. Accounts agree that the crops grew well until they
were about 6 months old, and often for 9 or 10 months, and that
the attack then became general, while good soil, heavy manuring,
and efficient cultivation did nothing to reduce its severity.
This type of incidence is entirely different from that of root
disease. In the trials of Bourbon made from time to time by
planters, and in the plots maintained by experiment stations,
the failure is usually demonstrably due to Collet otrichum.
At the same time the Bourbon cane is very susceptible to
root disease under adverse conditions, and no doubt the losses
occasioned in this way were indiscriminately set down to the
account of " rind disease."
The question that remains most interesting is that of the
origin of the epidemic. Three explanations worth considering
have been offered : —
(i) That a fungus previously occurring as a saprophyte
had evolved a strongly parasitic strain.
(2) That the Bourbon cane, from long vegetative re-
production in the same soils, had degenerated and become
susceptible to parasites previously unable to attack it.
(3) That a parasite previously absent had been introduced
from another region.
A fourth idea held by some of the older school of planters,
that the disease resulted from the use of chemical manures,
was negatived by its prevalence in districts where these had not
been adopted.
The first theory was mainly based on the belief that the disease
was due to Melanconium, which fungus was remembered to have
been noticed for many years. There is no evidence to show how
long the inconspicuous Colletotrichum had been present in the
West Indies, but it was described as a parasite with its present
characters in Java in 1893, and its distribution suggests that it
is indigenous to the Eastern Tropics.
The theory most generally adopted was that of the breakdown
of the " constitution " of the Bourbon cane. It is in several
respects unconvincing. There was no progressive degeneration ;
the transition was a sudden one from a condition apparently as
good as had prevailed in the previous hundred years to a sudden
and almost complete failure. What seems at first sight conclusive
is that some other varieties are recorded as having shown little
or no more resistance. It is too much to assume that degeneration
DISEASES OF SUGAR CANE 301
in their case should have coincided with a similar process in the
Bourbon, so that all became susceptible at the same time to a
fungus previously present, but there is a possibility, on the other
side of the argument, that mass exposure to the fungus de-
veloped on the Bourbon may have led to their infestation, as is
recorded of the Ribbon cane first adopted as a substitute in
Martinique.
The third theory, of introduction from without, involves the
fewest difficulties and is supported by many analogous cases.
The records of the time show that there was an active exchange
of varieties between neighbouring and in some cases distant
countries. The Queen cane, for example, was brought from New
Caledonia to Trinidad in 1874, and sent from Trinidad to Jamaica,
whence it was introduced into St. Kitts in 1888 and passed
into cultivation in one district. It was brought to Barbados
from St. Kitts some years previous to 1893. This cane is recorded
to have proved highly resistant but not immune. That in some
such way as this Colletotrichum falcatum was introduced from the
East and distributed on the original or other varieties to the
various Botanic Stations is by far the most likely theory of a
matter that will probably never be conclusively settled.
The objection that the attack was almost simultaneous through-
out the Lesser Antilles affects all the explanations alike, and is to
some extent lessened by the consideration that the attention
drawn to the disease in one island led to its recognition in another.
The more remote localities in the larger area of Trinidad were
much later in becoming infested, and, as mentioned above,
some continue at least relatively free to the present time. The
outbreak in British Guiana was also distinctly delayed.
Red-Rot.
The disease of sugar-cane caused by Colletotrichum falcatum
Went was first described by Went in Java in 1893, under the
Dutch name " het rood snot " — the red smut. The adoption of
red-rot as its name in English requires that care be taken not to
confuse it with " het rood rot " of Wakker and Went, which is a
sclerotium disease.
Red-rot is the most serious disease of sugar-cane in India,
and is widely, probably universally, distributed through the
tropics. Its epidemic appearance as the cause of " rind disease "
in the West Indies about 1890 has been described above. Owing
to the general adoption of resistant varieties its interest at the
present time in these islands is mainly historical, but it can
sometimes be found by a careful search among seedling varieties,
and still occurs in quantity when the Bourbon cane is re-intro-
duced.
302 DISEASES OF CROP-PLANTS
Symptoms.
On the Bourbon cane the effects of the disease usually became
apparent during the last quarter of the year and increased,
often with startling rapidity, until the fields were reaped. The
mature leaves began to wither from the tip and edges inwards.
Some time later a brown patch appeared on one or more joints,
became wrinkled and sunken, and soon showed signs of secondary
infestation with Melanconimn Sacchari. At reaping time a
large proportion of the canes were dry and rotten, and others were
badly deteriorated.
The symptom accepted as most typical of the disease is the
appearance, when a cane is split open, of a broad, irregular,
more or less central stripe of uneven red discoloration, broken
here and there by whitish patches. It is necessary to examine
stalks not far advanced in the disease to avoid the interference
of the more general discoloration produced by Melanconium.
The writer has seen an appearance very similar to that above
described produced in the lower joints apparently by Marasmius,
and certain diagnosis is only to be obtained from the recognition
of the fungus.
The Fungus.
The acervuli of Colletotrichum falcatum — tufts of black
bristles with the colourless conidia heaped about their base —
are produced in velvety patches on the surface of canes well
advanced with the disease, especially about the nodes, and on
dead or dying leaves or leaf-sheaths. The fruiting of the fungus
in the field is by no means abundant, and usually is decidedly
obscure, in striking contrast with the behaviour of Melanconium.
If a freshly diseased cane is split open and prevented from
drying out too rapidly or becoming mouldy from too much
moisture, the fructifications sometimes appear with great free-
dom on the cut surface.
The conidia are hyaline, unicellular, more or less sickle-
shaped, with bluntly pointed ends, and measure about 25 x 4-5
microns.
Injection.
In the West Indies infection has been regarded as mainly
proceeding from borer holes and other wounds in the stem.
The studies of E. J. Butler in India have shown that while this
means, and the infection of exposed roots or root " eyes," prob-
ably account for the initial entry of the fungus, the infection of
standing canes is too small in amount to give rise to heavy
infestations. These, it has been shown, occur from the use of
infested canes as planting material, in which case the whole
stool, and not individual stalks, becomes infested.
m^ 1
Fig. 120
Rind Fungus of Sugar-cane
F,o>n M. T. Cook's Diseases of Tropical Plants, by permission of MacmiUan & Co.
DISEASES OF SUGAR CANE 303
On resistant varieties the fungus appears to be able to make
headway only in injured or otherwise weakened stalks. Insect
attacks, defective soil aeration, and premature ripening as a
result of dry weather decrease resistance.
Control.
Butler has worked out a successful system of control based
on the information as to infection noted above. Starting with
healthy stock, a thorough inspection is made each year of the
cuttings prepared for planting. All which show reddening at
the ends are discarded. A previous selection of healthy-looking
stools as the source of the sets diminishes the trouble and in-
creases the efficiency of the process. By eliminating in this way
the results of casual infections it is found that in ordinary cir-
cumstances the condition of the stock can be maintained.
The Rind Fungus.
Although Melanconium Sacchari (Cke.) Mass. was not the
originating cause of the epidemic of " rind disease," it certainly
provided the feature which led to the use of that name, and it
seems appropriate to retain the adjective in speaking of this fun-
gus. As Strwnella Sacchari it was described by M. C. Cooke from
material received from Queensland in 1890. As already related
its uniform occurrence on diseased canes received at Kew from
the West Indies led to its being regarded as the cause of the
failure of the Bourbon cane. It is still everywhere abundant in
the West Indian cane-fields, occurring on probably every sickly,
damaged, or dead cane. It appears on the soundest and healthiest
canes with great regularity if these are cut and laid aside either
indoors or out. It occurs in a similar way in Louisiana, in
Mauritius, Java, and the Hawaiian Islands. In India it appears
to be very rarel}'' met with.
Conditions of Occurrence.
The germ-tubes of the fungus are unable to penetrate the
unbroken rind, but gain access through borer holes, torn leaf
scars, cracks, or wounds of any other description. In healthy
growing canes the invasion seems to be definitely restricted to
the tissue affected by the wound. In over-ripe canes, however,
it is able to spread, and when they have stood long in the fields
an ever increasing proportion of them become infested with
this fungus without there being any apparent reason for sus-
pecting preparation by any earlier organism. The view now
generally held regarding the degree of parasitism of which
this fungus is capable is that it is only able to occupy tissue
which has undergone seme degenerative change, due either to a
wound, to the actioi? o a previous organism, to senescence, or to
304 DISEASES OF CROP-PLANTS
death. Johnston and Stevenson attribute some importance to
the infestation of cuttings.
Description.
The first external sign of the occurrence of the fungus is a
Fig. 122
Thielaviopsis paradoxa, Sporophores of Micro- and Macro-conidia
From Wakker & Went
browning of the surface, which is accompanied by souring and
reddening of the interior tissues. Numerous small pustules
appear just beneath the rind. The pustules open and from each
is extruded a black thread which under some conditions becomes
long and curly, in others settles into an irregular heap ; it is made
DISEASES OF SUGAR CANE 305
up of the conidia cemented together by mucilage. The conidia
are unicellular, pale brown, cylindrical, straight or curved, 14-15
X 3.5-4 microns.
The infested cane dries up and becomes very light.
Pineapple Disease of Cuttings.
History and Distribution.
This disease, caused by Thielaviopsis paradoxa (De Seynes) v.
Hohn. was described by Went in Java in 1893. He gave it
the name Pineapple Disease owing to the smell, resembling that
of a ripe pineapple, noticed when a cane containing the fungus
in a fresh and vigorous condition is split open. The fungus
would seem to occur in all cane-growing countries, and, from its
invariable appearance when conditions favour it, would seem
to be universally distributed on sugar estates in the West Indies.
It is a somewhat curious coincidence, considering the name
given to the disease on sugar-cane, that the fungus has since
been found to be the cause of several diseases of the pineapple.
It occurs also on coconut and banana.
Causative Fungus.
The fungus was discovered by De Seynes in 1886 in France,
and again by Went in Java in 1893 in connection with the disease
under consideration. W^ent gave it the name Thielaviopsis
ethaceticus, under which it has become generally known, but in
accordance with the rules of priority the earlier specific name
must be adopted. The fungus grows and fruits in the interior
tissues of the cane. The abundant mycelium produced in the
cells is at first hyaline but turns brown or nearly black as it
matures. Two forms of conidia are produced from the hyphae
within the tissues, sometimes one, sometimes the other pre-
dominating. The macro-conidia are black, thick-walled, oval,
produced in a chain by septation at the tip of a hypha ; the
micro-conidia are short, hyaline, cylindrical or oval, formed by
the sub division of the internal contents of a hypha and pushed
out successively from its open end. No other form of fructifica-
tion is known.
Nature of the Attack.
When trouble is experienced from this fungus it is in nearly
all cases connected with the infection of planted cuttings,
which takes place from the cut ends in a manner strongly sug-
gesting infection from the soil. It is, however, to be noted that
unplanted cuttings, set aside in moist air or transported to a
distance involving close packing for several days, are also very
liable to be attacked. The fungus usually involves the centre
X
3o6 DISEASES OF CROP-PLANTS
of the cane, extending from one or both ends, so that when a
cutting is split open it shows a blackened core, around which the
tissues may retain their normal colour or be somewhat reddened.
The smell is often an aid in diagnosis, being distinctly more fruity
than that of merely sour cane.
Attacks on standing cane are comparatively seldom noticed.
Their occurrence is favoured by wet weather. There is the same
blackening of the tissues as in cuttings. The fungus is un-
doubtedly capable of parasitism and may extend to a considerable
distance up and down from the original point of entry.
In the case of cuttings there is no difficulty about entrance ;
in standing canes it occurs most commonly by way of borer
holes, but may take place through the leaf scars.
Loss of cuttings from this disease usually occurs when owing
to the prevalence of dry weather they remain a long time in the
ground before springing. In such an event the fungus has time
to destroy the whole cutting and the loss of a planting may be
complete. There is no evidence of varying resistance in different
cane varieties, though it is a matter of common experience that
these differ greatly in their germinating capacity.
Control.
The standard measure for the protection of cuttings against
infection is to dip or soak them in Bordeaux mixture, and im-
proved germination has on occasion been secured by this treat-
ment. On the other hand many disappointments have been
experienced when the measure has, nominally at least, been
adopted. When the weather is favourable - the treatment is
not found to be necessary, but since this cannot be depended
on it is usually desirable as a precautionary measure. It needs
to be pointed out to planters, however, that dipping has no
ceremonial value, so that unless, after the cutting has been
handled, placed in the ground, and covered in, there remains
on its ends a well-distributed fihn of blue sediment, the benefits
of the process will be looked for in vain. Nor must it be expected
to give protection if the ends of the cuttings are split or ragged.
Dipping the ends in gas tar has been advocated as affording
a more adhesive coating, but it is difficult to apply on a large
scale and most planters will prefer to take chances, as indeed
many do rather than trouble with the comparatively simple
Bordeaux treatment.
The method commonly in use in applying Bordeaux mixture
is to prepare it in a tank or hogshead and soak the cuttings
either loose or in bags. There is no virtue in soaking and the
crystallization of the precipitate after standing makes it less
adhesive. The points that are of importance are (i) the prepara-
tion of a mixture with a finely divided and adhesive precipitate
and its use while fresh, (2) the agitation of the mixture so that
DISEASES OF SUGAR CANE
307
Tig. 123 Thielaviopsis in Sugar-cane Cutting
Fig. 124 Thielaviopsis in Cells of Sugar-cane
3o8 DISEASES OF CROP-PLANTS
each cutting gets its share of the precipitate, (3) the drying of
the cuttings to allow the film to set, and (4) careful subsequent
handling so that the film is preserved.
DiPLODiA Rot.
The fungus Diplodia Theobromce (Pat.), common throughout
the tropics as a weak parasite and as a saprophyte on the branches
of various trees, occurs also on sugar-cane. A. Howard described
it on this host in Barbados and British Guiana, and E. J. Butler
has found it to be not uncommon in India. Although Howard suc-
ceeded in infecting healthy canes with the fungus it appears from
general experience to resemble Melanconimn Sacchari rather than
the active parasites in the nature of its occurrence. It bears
a close resemblance to Melanconium in general appearance also,
and it is not always easy in the field to discriminate between
them. Under the microscope the spores of Diplodia are at once
distinguishable if mature by their two-celled condition. The
small and crowded pycnidia occur just under the rind, mostly
about the nodes. The little swellings which mark their position
open with a tiny longitudinal slit, from which the spores are
extruded. The interior of the affected cane takes on a general
red discoloration, and later becomes darkened by the presence
of the mature mycelium.
The fungus is mostly found on over-ripe canes or such as have
been seriously damaged or killed by borer or other means.
Wilt Disease.
This disease was first described by E. J. Butler in 1913 from
India, where it was discovered during the investigation of red-rot.
It is found to occur over a large part of India, generally mixed
with red-rot, but sometimes doing considerable damage where the
latter is not present. As a rule the disease has not proved
serious in its effects.
An affection having characters which so far as they can be
judged by figures and descriptions are identical with those of the
Indian disease has been detected in Barbados and in Nevis.
Over a fairly wide area in Barbados in 1915 it occurred in con-
siderable quantity in some fields. Very little further spread
was noticed and the final result on the crop was not important.
The most probable explanation of its occurrence to this marked
extent was that the canes attacked had been somewhat damaged
by the wrenching action of high winds experienced some time
before. Infestation of the stools with Marasmius also seemed
to function as a predisposing cause. The circumstances under
which the fungus is occasionally found suggest that normally
it is present in scattered infections of borer holes.
DISEASES OF SUGAR CANE
309
Causative Fungus.
The disease is due to a very inconspicuous fungus, Cephalo-
sporium Sacchari, Bull., of which the slender colourless hyphae
penetrate all the tissues of the stem. In the hollow which
forms in the centre of diseased canes the fungus collects in a
white fluffy mass and there the hyphae bear on short, mostly
simple branches, terminal heads of one-celled conidia, variable
in size and shape. The fungus can live as a saprophyte on a
variety of materials and probably occurs in the soil.
Symptoms.
As seen externally the effects of the disease in India are
Fig. 125 Cephalosporium Sacchari
From Memoirs, Dept. Agri., India
reported to be very much the same as those of red-rot, namely
the drying-up of the leaves, and, finally, of the stems. In the
Barbados outbreak it was observed that canes might be attacked
rather severely in the lower internodes without the tops being
very noticeably affected. As mentioned above, however, the
disease in this case seemed to be rather suddenly checked. In
Nevis more notable damage was reported.
The discoloration seen on splitting the cane differs froni that
typical of red-rot, being a more uniform and duller red, tending to
muddiness, and without the contrasted white patches seen in
red-rot. In the earlier stages the discoloration tends to run
in vertical lines. A brown patch on the outside marks where
the fungus has killed the underlying tissue. This in the Barbados
examples extended from the nodes, and the internal discoloration
310 DISEASES OF CROP-PLANTS
grew less intense from without inwards. These observations
presumably indicate the course of the infection.
Infection.
According to the experiments made in India the fungus
cannot penetrate the unbroken rind of the internodes, but can
enter by way of wounds, leaf scars, or the root eyes of the nodes.
Cuttings were also experimentally infected by dipping them
before planting in water containing spores and mycelium of the
fungus. It is further reported from India that the infection so
far as it has been followed is not virulent ; spread within the
cane is gradual and communication from one plant to another
slow. A large number of borer holes become infected late in the
season, but the parasite usually remains confined to a few inter-
nodes. Leaves have been artificially infected with the fungus,
but it has not been found occurring upon them in the field.
J. S. Dash carried out inoculation experiments in Barbados
and found that in healthy canes the fungus was only weakly
parasitic and made slow progress beyond the point of infection.
The point of greatest interest in connection with the presence
of this disease in the West Indies is that under certain circum-
stances it can produce effects similar in their nature to those of
red-rot in varieties which are regarded as practically immune
from the latter disease, and may thus to some extent account
for reports of their occasional infestation.
Control.
No general measures are at present called for in respect of
the disease on West Indian canes, which would seem to be more
resistant than those of India. Should it become more serious
it will be necessary to avoid suspected stools as sources of material
for planting and to reject cuttings showing red discoloration.
Top-Rot (Bud-Rot)
In the year 1895 an affection to which the name top-rot was
given began to assume epidemic form in Java. It had previously
been known only through sporadic cases. At an earlier date a
disease with similar characteristics had been described from
Australia under the name of gum disease. Cases have occurred
from time to time in the West Indies which have had somewhat
similar general symptoms and have been described under this
name.
The Javan disease consists essentially in the failure of the
terminal joints of the cane ; the leaves attached to the growing
point may remain normal, or degenerate into a pulp, while the
interior of the nodes immediately below it becomes involved in a
sort of soft rot, producing a glassy appearance of the tissue and
DISEASES OF SUGAR CANE 311
marked off from the healthy outer tissue by a red line. The
young leaves are soft and reddened near their base and withered
towards their tips. In some cases the uppermost leaves are
distorted and fail to grow out properly. The result of this failure
of the top is that while further direct growth is stopped the eyes
are stimulated into growth and produce useless side shoots.
The decayed portion affords an easy means of access to stem-
rotting fungi.
Bacteria are found in the degenerating tissues and some
gumming usually takes place. It has been fairly satisfactorily
established that the Australian disease is due to Bacterium
vascularum, and it is separately described on another page.
J. H. Wakker found in Java several kinds of bacteria present,
and that the condition is associated with abundant rain and high
atmospheric humidity. Wakker' s view is that some disorganiza-
tion is first produced by too rapid growth, which then renders
the tops susceptible to bacterial infection.
In Porto Rico the condition described as top-rot usually
occurs in over-ripe cane, but it is observed that some varieties
rot back after maturity much more quickly than others. This
type of rot occurs particularly in dry weather.
In the froghopper blight of Trinidad the cessation of growth
at the tip of immature shoots, brought about by excessive injuries
to the leaves, is followed by reddening of the region about the
growing point and watery degeneration and death of the soft
upper joints enclosed in the bud. This, and similar injury
arising from root disease or borers, has been called top-rot.
Top-rot in general is obviously no more than a term for the
soft-rotting of the enclosed termination of the stem, and like the
similar bud-rot of the coconut palm includes conditions in
which similar symptoms arise from very different causes.
Gumming Disease, Cobb's Disease.
The disease of sugar-cane caused by Bacterium vascularum
(Cobb) E. F. S. has been recorded in the West Indies only from
Porto Rico (1920), and such accounts of top-rot as have appeared
do not suggest that it has so far occurred in the British islands.
The disease was first adequately described by N. A. Cobb
in Australia, where it occurs in New South Wales and Queensland.
An affection with identical symptoms has been reported from
Brazil. It is also recorded from the Fiji Islands, Mauritius,
Java, Borneo and New Guinea, and is suspected to occur in the
Argentine.
Symptoms.
The specific symptom of the disease is the appearance in the
vascular bundles of a yellow slime and a red stain. When
infested stalks are cut across so as to leave a smooth surface.
312 DISEASES OF CROP-PLANTS
a yellowish gummy ooze collects in droplets on the ends of the
severed bundles. This material dries to a bright yellow stain.
The disease is primarily one of the vascular system, but in
advanced stages the parenchyma is attacked, especially just
below the terminal bud, and cavities are formed which are filled
with the yellow bacterial slime ; this may also ooze from the sto-
mata of the inner surface of the leaf-sheath.
The external signs of the disease are various degrees of
dwarfing, striping of the leaves, dr5dng of tops, and decay of the
terminal bud (top-rot).
Inoculations which demonstrated the causative connection
of B. vascularum with the disease have been described by E. F.
Smith. Wilbrink has recently investigated bacterial vascular
disease as it occurs in Java, where it is characterised by external
and internal symptoms similar to those of Cobb's disease, with
the important exception that no gum nor slime is produced by
it in the canes. The bacterium does not develop gum in pure
cultures, but resembles B. vascularum in other physiological
characters.
Incidence.
Condition of soil, other than lack of drainage, is said not to
influence the prevalence of the disease. Canes rich in sugar,
grown on good land, are subject to it. The evidence suggests
that infection in any way from stool to stool is not very active,
but that the extension of the disease mainly comes from the
planting of diseased sets. Both in Brazil and in Australia the
disease proved exceedingly destructive on susceptible varieties.
Control.
The means of control consist in the planting of resistant
varieties, some of which are nearly or quite immune, or, where
the cultivation of susceptible canes is continued, the careful
selection of healthy sets, rotation of crops, and provision for
good drainage.
Smut.
The Smut disease, due to Ustilago Sacchari, Rab., is common
in the East on the thin canes of the Indian type. C. K. Bancroft
reports its presence in British Guiana, stating that little damage
has been attributed to it. It is often quoted as occurring in
Trinidad, but the reference usually given, to 0. W. Barrett's
record that " a leaf rust, probably Ustilago sacchari, was very
common " is obviously worthless. No other record in the West
Indies is known to the writer.
The disease is at once recognisable by the occurrence of a
long, curved, whip-like shoot, often several feet in length, at the
apex of the cane. This abnormal growth, which probably
DISEASES OF SUGAR CANE
313
represents a flowering shoot, is at first covered by a silvery
white sheath, which soon ruptures, exposing the spores as a
dense mass of black dust.
Direct infection by spores appears to take place at the nodes,
and sets containing the mycelium give rise to infected plants.
The spores germinate in water or moist soil and give rise to
sporidia, which develop yeast-like budding cells.
Fig. 126 UsTiLAGO Sacchari
From Wakker & Wmt
SCHIZOPHYLLUM ROT.
The conspicuous fungus Schizophyllum commune, Fries., is
everywhere common on dead cane as on various dead branches,
bark and other vegetable material. It is sometimes seen on
standing cane, especially the dried or half-dried stalks met with
in reaping and known as " rotten cane." There is no reason
to beUeve that it ever occurs apart from previous serious injury
by borers or by other fungi, except perhaps on cane very much
over-ripe and beginning to dry up by reason of age.
314 DISEASES OF CROP-PLANTS
The Iliau Disease.
The name Iliau, from an expression meaning " hide-bound "
is given to a disease endemic in Hawaii and regarded by H. L.
Lyon, who investigated it there, as having caused more loss than
all other cane diseases combined. The affection is also common
in the more northern parts of Louisiana. The effects are liable
to be confused with a common symptom of root disease, and the
fungus with Melanconium Sacchari, so that very possibly the
affection occurs unrecognised in these islands, but the fact that
it assumes importance in Hawaii only in periods of continuous
cool damp weather would seem to exclude its appearance as a
serious trouble in the sugar-cane districts of the West Indies.
Symptoms.
Iliau is entirely a disease of young shoots, which are attacked
from the soil, and mainly affects the leaf-sheaths, which become
very tightly cemented to each other and to the surface of the
underlying stalk by the growth of a mat of white mycelium
in and between them. The adhesion is much more firm than
that produced by Marasmius and the rind is entered to some
extent. Further useful growth of the shoot after this condition
is attained is entirely prevented, though the stem is often found
variously bent or doubled by the elongation of the joints while
the tip is firmly held. Sometimes the shoot grows through
before the barrier is firmly established. Large numbers of
young shoots, both in young and older stools, are killed out in
this way. Older shoots are not injured. Much more damage
is done to plant canes than to ratoons.
Causation.
The disease has been demonstrated by experiment to be due
to a fungus which has an ascigerous stage, Gnomonia Iliau,
Lyon, and a Melanconium stage, M. Iliau, Lyon. The former
is the commoner form in Louisiana, the latter in Hawaii. The
perithecia are flask-shaped, immersed in the substance of the
leaf-sheath, with long, hard necks which project from the surface.
The spores are spindle-shaped, with two cells. The Melan-
conium closely resembles M. Sacchari in general appearance,
but the spores are larger (7-10x15-28), and are usually filled
with large globules which are quite distinctive.
Control.
Immune varieties are not known, but the Demerara seedlings
grown in Hawaii are the most resistant. Thorough preparation
is recommended as a very ef&cient measure for reducing damage.
DISEASES OF SUGAR CANE
315
Fig. 127
Gnomonia Iliau, Vertical Section of Perithecium and Asci
From Bull. 11, Dept. Pathology, H.S.P.A.
Fig. 128
Melanconium Iliau. Stromata and Conidia
From Bull, it, Dept. Pathology, H.S.Pji.
3i6
DISEASES OF CROP-PLANTS
Cytospora Rot.
The fungus Cytospora Sacchari Butl., originally described
from sugar-cane in India, has been found by J. R. Johnston
on certain susceptible Barbados seedlings in Porto Rico, causing
an affection of young shoots very similar in its nature and effects
to the Hawaiian Iliau disease, but producing also nodal infections
on older canes. The fungus occurs in Barbados, but is not known
to be injurious. The leaf-sheaths of young shoots are firmly
bound together by the mycelium : their exposed portions take
on a very dark, dull red colour, and are profusely covered with
the pycnidia, the comparatively long pointed necks of which
Fig. 129
Cytospora Sacchari : Surface and Sectional Views of Pycnidia
Spore and Sporophore
From Memoirs, Dept. Agri., India
project beyond the surface sufficiently to be readily seen, and to
give a rough sensation when the linger is passed over them.
During wet weather a minute yellowish globule of conidia exudes
from the tip of the beak. The spores are cylindrical and slightly
curved, obtuse at both ends, 3.5x1-1.5 microns.
With the elimination of the susceptible varieties in Porto
Rico no further evidence of parasitic activity on the part of this
fungus has been seen.
Leaf Sheath Rot
Under the name of " rood rot " (red-rot) this disease has
been known from Java since 1890 It is generally distributed
DISEASES OF SUGAR CANE 317
there, but is more common in the hills, where the moisture is
greater. It is not uncommon in some districts in Barbados,
is reported as very common in Porto Rico, and has been recorded
from St. Lucia, St. Croix, and Trinidad. The fungus is very
generally distributed through the islands and no doubt occurs at
times on sugar-cane in all of them, but it nowhere attracts much
attention.
Causative Fungus.
The fungus to which the disease is due produces an orange
or brick-red discoloration of the leaf-sheaths, often on young
shoots. It is distinguished by a copious white radiating my-
celimn, on which occur as it matures large numbers of white
to brown spherical sclerotia like small seeds. The fungus has
been shown by cross inoculations to be identical with Sclerotium
Rolfsii Sacc. described under the heading General Diseases.
Nature of the Attack.
There is no doubt as to the parasitism of the fungus. The
mycelium spreads over the surface and penetrates the tissues
of the leaf-sheath by way of the stomata. On young shoots
the leaves and even the stem may be more or less affected.
Infection apparently always proceeds in the initial stage from the
soil, where it is assumed to be carried over by the resistant
sclerotia.
Effects.
The extent of the damage caused by the fungus on sugar-cane
is uncertain. Some planters attribute considerable importance
to its presence on young shoots, which are dwarfed and reduced
in vitality by its attack. On the other hand it is reported from
Porto Rico that in one instance careful observation for six
months of a field in which it was growing vigorously owing to
the wetness of the season revealed no visible difference in the
condition of infested and uninfested stools. The fungus develops
only under moist conditions.
Red Spot of the Leaf-sheath.
This is the most general of the diseases attacking sugar-cane
in the West Indies. In many districts it is difficult to find a
shoot which does not show more or less evidence of its presence.
It occupies a similar position in Java.
The affection appears on the outer leaf-sheaths as a large
bright red patch, commonly somewhat oval in shape, but ill-
defined, with a dark brown spot of dead tissue in its centre.
Under this spot, by pulling off the successive leaf-sheaths, one
may trace the infection inwards, in the form of red spots which
are smaller and smaller until only a red dot appears at the
3i8 DISEASES OF CROP-PLANT^,
I
limit of penetration. The spots are caused *"by the fungus
Cercospora vagince Kniger, the progressive stages in the develop-
ment of which are represented in the series just described.
The mycelium of the fungus grows in the tissue of the leaf-sheaths,
to which it gains access by way of the stomata, and by passing
from one to the next penetrates towards the middle of the shoot.
The brown central portion of the oldest spots marks the tissue
finally killed by the fungus. Upon it is found a brown external
mycelium with short upright branches, which at their tips
produce the elongated narrow conidia. These readily and quickly
infect other leaf-sheaths to which they may happen to be borne.
The damage done by the fimgus is inappreciable. It finds
its nourishment in the tissues lying between the vascular bundles,
and the latter are little if at all affected until a rather late stage
in the development of the spot.
The newer leaf-sheaths become infected so early from the
spots on the older ones that stripping would usually be rather
useless. The use of top-plants undoubtedly helps to carry over
the fungus, since traces of the infection may usually be seen upon
them. Unless the fungus can be shown to have more importance
than is usually attributed to it its presence does not afford a
reason for the discouragement of the use of such plants.
Ring-Spot.
The leaf disease known as ring-spot was investigated by
Breda de Haan about the year 1893 in Java. It occurs in
India, the Hawaiian Islands and other Eastern locaHties.
In the West Indies it has been definitely recorded from Cuba,
San Domingo, Porto Rico, Antigua, Barbados, Trinidad, British
Guiana and Surinam, and probably occurs in all the islands.
Causation.
The disease is caused by a parasitic Ascomycete LeptosphcBria
Sacchari Br. d. H. It has two spore forms. The conidia are
curved bodies consisting of a central cell with one or two smaller
cells at each end. They are capable of infecting leaves upon
which they come. The ascospores are formed later in small
perithecia. which appear as black dots in the substance of the
leaf near its upper surface ; the spores are four-celled, the
terminal cells with thinner walls.
Symptoms and Incidence
The spots are generally most abundant towards the outer
end of the older leaves, giving rise when abundant to a general
rusty appearance. The spot starts as a purplish fleck on the
surface, which as the fungus grows expands outwards and dries
up in the middle, so that it comes to consist of a small brown or
yellowish patch enclosed by a purple or brownish band, with
Fig. 131
Ring Spot of Sugar-cane (Leptosphaeria Sacchari)
DISEASE^ OF SUGAR CANE
319
Fig. 130 Cercospora vaginae
From Wahber & Went
Fi-. 132 Leptosphaeria Sacchari. Akci and Spore
From Memoirs, Depi. Agri., India
320 DISEASES OF CROP-PLANTS
sometimes a faint yellowish outer margin. The spots are
irregular in shape, somewhat elongated in the direction of the
length of the leaf, usually less than half an inch in length, but
often confluent. When they are abundant they are apt to
cause premature death of the whole leaf.
Marked local epidemics sometimes occur in moist weather.
When the infestation occurs on mature canes, as it often does,
no considerable harm can result, but earlier outbreaks when
at all severe must administer a somewhat severe check to growth.
Their occurrence is usually so erratic that they cannot be anti-
cipated by any preventive measure.
Eye-Spot.
The eye-spot disease of the leaves was originally reported
Fig. 133 Helminthosporium Sacchari
From Memoirs, Dept. Agri., India
from Java, the fungus being named Cercospora Sacchari Br. d. H.
It is now usually admitted to be a Helminthosporium and is
probably identical with H. Sacchari Butl. found in India. The
affection is known to occur in Cuba, Jamaica, Porto Rico,
Barbados and Trinidad. In the last-mentioned island C. B.
Williams reports that during the last few years it has been
found only on D 109 in certain small areas, and only exception-
ally appears before the month of December. In 19 19 a severe
local epidemic two or three months earlier than this soon passed
off. The affection takes the form of elongated spots and streaks,
red to dark brown in colour and developing a straw-coloured
centre. In severe infestations they are crowded together and
the leaves are so generally discoloured that the stools have a
blighted appearance. In such cases the spots may be found
even on leaves just unfolded, and a distinct check to growth can
be detected.
The conidiophores are stout, erect, rather rigid hyphae.
DISEASES OF SUGAR CANE
321
produced from small stromatic masses on the surface of the spot.
The conidia are cylindrical or long elliptical, with very thick
walls, olive green to brown in colour, 3-10 septate.
Red Leaf-Spot.
Red leaf -spot, originally described from Java, is recorded
Fig. 134 Eriosphaeria Sacchari
From Wakker & Went
Asci AND Spore
from the West Indies by Went in Aruba Island and L. H. Gough
in Trinidad. It is caused by an ascomycete with round black
perithecia and two-celled spores, Eriosphcsria Sacchari Went.
The spots are roundish except when two or more interfere.
They begin as red dots with a yellowish border ; when mature
they are reddish-brown, surrounded by a narrow yellow border
that is sometimes scarcely visible. The centre of the spot does
Fig.
135 Cercospora Kopkei
From Wakker 6- Went
not dry out, which serves to distinguish this affection from eye-
spot or ring-spot ; from yellow-spot it differs in the narrowness
of the margin.
Yellow Leaf-Spot.
The yellow leaf-spot, which is very generally distributed in
Java without being the cause of notable damage, was recognised
by Went in Barbados, Trinidad and Demerara. It is caused by
Cercospora Kopkei Krug.
322 DISEASES OF CROP-PLANTS
The affection begins on young leaves and develops thickly
sown irregular spots, bright yellow on the upper surface of the
leaf, covered on the under side in moist weather with a dirty-
white web of hyphae. Later the spots show red dots or stripes,
and in individual instances may become dull red.
On the under surface bundles of short conidiophores project
from the stomata and bear elongated hyaline conidia of varying
length and with i-6 cells.
Brown Leaf-Spot.
Brown leaf-spot is recorded by Gough in Trinidad and as one
of the rarer leaf-spots in Porto Rico by Johnston and Stevenson.
Fig. 136 Cercospora Longipes
From Memoirs, Dept. Agri., India
It is reported to be by far the commonest leaf disease on the
thin canes of India, to which it is practically confined. The
spots appear on the blade of the immature leaf and continue
during maturity. At first they are narrow, oval, about ^ inch
in length, and reddish in colour. A browTi centre early becomes
evident, and a yellow border is formed, so that the spot is tri-
coloured. When fully developed the spots consist of a broad
oval deep-brown ring, with a straw-coloured centre, the whole
reaching to half an inch in length.
The causative fungus is Cercospora longipes, Butler. Long
brown unbranched conidiophores, abruptly bent, form a velvety
DISEASES OF SUGAR CANE 323
cushion on the under surface, and bear hyaline conidia with 5-7
cells,
Sereh.
History.
The Sereh disease caused in Java in the 'eighties of last
century a crisis comparable with the effects of the " rind disease "
epidemic in the West Indies. The disease received its name
from the resemblance of seriously affected plants to bunches of
lemon grass, the Javan name of which is Sereh. There is no
reason to believe that the affection occurs in the West Indies,
but as alarm concerning it has arisen from time to time, and
in view of the possibility of its future occurrence, a brief descrip-
tion based on Went's account of the disease in Java is here
supplied.
In the first years following its discovery the Sereh disease
progressively extended over the greater part of the cane fields of
the country, with serious results. A re-organisation of the
industry on a scientific basis took place, and measures were
found and adopted which made evasion of the disease possible.
These are troublesome and expensive, but no direct remedy
has been found to take their place.
Nature of the Attack.
The outward signs of attack are seen in changes in the habit
of the plant, which may occur to very different degrees.
In the worst cases the internodes are largely or entirely
suppressed, so that there are few stems of any length to be seen.
The leaves are small and expand prematurely, and being crowded
together in two ranks they give to the top of the shoot the
appearance of a fan. Abnormally large numbers of shoots are
produced from each stool. A thick network of roots develops
under the leaf -sheaths of those canes which possess a stem.
In cases of less severity stems of normal length are produced,
but they develop fan-like tops and many of the buds upon their
sprout and form short side-shoots. As in the first type, roots
develop along the stem. In the third and least severe type there
is hardly any outward alteration beyond the formation of the
characteristic short joints at the top of the stems.
An internal symptom which is common to all the types
consists in the discoloration of the vascular bundles entering the
stem from the leaf-sheaths, which are reddened for a longer or
shorter portion of their course and under the microscope are
found to contain a gummy substance. According to recent
findings by H.M. Quanjer, necrosis of the phloem strands in the
vascular bundles is a constant and fundamental character.
Starting with sound canes planted in an infected area the
typical course of the disease on plant cane is that the first crop
324 DISEASES OF CROP-PLANTS
may be only slightly attacked, that from the first replanting
much more severely, and the second replanting may be so badly
affected that a whole field looks Hke a cultivation of lemon grass,
with here and there a cane stalk rising out of the low grass-like
clusters. Ratooned fields are almost invariably badly attacked.
No parasitic organism has been shown to be connected with
the causation of the disease, and it is now usually regarded as
coming within the class of virus diseases.
It will be seen from the foregoing description that the actual
symptoms of Sereh may be somewhat indefinite, and as a matter
of fact the appearances described by Went can all be found
exactly reproduced, including the reddening of the vascular
bundles in the nodes, in stools affected by froghopper in Trinidad,
and are more or less simulated by the results of other affections
elsewhere. In no such case, however, has there been developed
the progressive degeneration which is the essential character
of the disease.
Control.
The system by means of which the disease is avoided consists
in the use of cuttings raised in hill nurseries, which are not
subject to the disease. The canes from these are reproduced
in the plains for only two or three vegetative generations before
they are again replaced with plants from the hills.
Mosaic Disease, Mottling Disease, Yellow Stripe Disease.
History and Distribution.
About 1916 the existence of a serious epidemic disease on
sugar-cane was noticed in the north-western quarter of Porto
Rico. The affection was described by J. A. Stevenson under the
name of mottling disease. Later it was found to be identical
with the " gele strepenziekte " (yellow stripe disease) of Java,
in which island it was described by J. H. Wakker in i8g6.
The disease was known in Java in 1890, and circumstances
point to its having then been present for many years. It was
regarded there as due to recurrent bud variation. Elimination
methods, and its operation as a factor in selection have restricted
its prevalence. It occurs in Java on all types of soil and, with
greater or less severity, on all kinds of sugar-cane except the
wild species.
When attention was first attracted to the disease in Porto
Rico, it was restricted to a single district, but it had then
assumed epidemic form, and its spread was so rapid that in
1919 more than three-fourths of the cane-growing area was
invaded.
E. W. Brandes reports the finding of the disease in Cuba
[C. B. Williams
Fig. 137
Mosaic Disease of Sugar-cane : Stem Canker
DISEASES OF SUGAR CANE 325
in 1919 at three separate points, at one of which it was beUeved
to have been present for nearlj^ twenty years. In 1918 the disease
was found at many places in Santo Domingo, and it has been
discovered in St. Croix on plants from Porto Rico.
A survey of the cane-growing districts of the United States,
made in consequence of the discovery of the disease on planting
material received in Porto Rico from Louisiana, has revealed
its wide distribution in Louisiana and Florida, and its occurrence
in the adjoining States. The river district of Louisiana was in
1919 already badly infested, the proportion of diseased plants
reaching 75 per cent, on some plantations.
At the St. Augustine Experiment Station in Trinidad the
existence of a peculiar and abundant marking of the leaves on
the plot of D 3956 was noted by C. B. WiUiams in 1918 and 1919.
The adjacent plots appeared at this time to be practically free.
Early in 1920 the condition was recognised to be mosaic disease,
and a survey showed that it was then more or less present not
only throughout the Station but in the adjoining district for
several miles east and west, while many new centres of infection
had arisen on estates to which cuttings of selected varieties had
been sent.
Early in 1920 the disease was found to be widely prevalent
in Jamaica in its typical form. It is now common in Barbados.
In the Argentine Republic the disease is reported by G. L.
Fawcett to have been present for many years. It is common on
the Java canes there grown, which exhibit the S3anptoms to
varying degrees, but are not prevented from giving satisfactory
yields. Canes of the White Transparent type are uniformly
affected and have suffered deterioration, which is not, however,
attributed to the effects of mosaic disease. The Barbados and
Demerara seedlings are worst affected, except D 1135, which
is the only one suited to local conditions.
Symptoms.
The one marked and constant primary symptom of the disease
is the peculiar mottling of the leaves.
This is difficult to describe, but is seldom difficult to recognize
after being once learned. Its commonest form is a general
longitudinal streakiness of the green colour, caused by the paling
of many small and ill-defined areas. It is hardly definite enough
to be called spotting, and a form of whitish or yellow spotting
is the appearance most frequently mistaken for it. It is often
best seen by transmitted light, and may be quite conspicuous
in the upper leaves as seen against the sky. In different cane
varieties and under various conditions the amount of the normal
green colour retained differs widely, in extreme cases being
present itself as patches on a lighter background. On certain
varieties in Trinidad the affected leaves as they grow old con-
326 DISEASES OF CROP-PLANTS
stantly develop reddish dots thickly scattered, but as a rule
the variation is only from dark to pale yellowish or whitish
green. Most of the doubtful cases can be determined by ex-
amining the young unfolding leaves, which always show symp-
toms if mosaic disease has infected that particular shoot.
In many varieties of cane the shoots when thoroughly infested
show distinctive markings on the surface of the cane itself. On
the younger joints (intern odes) these are usually in the form of
streaks or stripes, which may be pale yellow on green, or red on
yellow. On older joints the surface may be marked with irregular
whitish stripes, the development of which is followed by cracking
and often by local shrinkage and drying out. The joints of
infested canes tend to be short, and where the symptoms just
described are prevalent there is great loss of weight.
In plants from infested cuttings a further symptom appears
in the form of white opaque spots and streaks in the light-coloured
areas on the leaves. These have no pigment whatever and range
from pin-point spots to streaks several inches long. This white
tissue remains firm and does not turn red or brown as do the pale
spots caused by the feeding of froghoppers.
Fields planted with infected cuttings soon show pronounced
dwarfing of growth, and a similar result is seen in ratoons of the
more susceptible varieties which have become infected after
planting.
Influence of External Conditions.
The disease occurs under all conditions of soil and climate.
Cultivation, manuring, and the use of lime do not notably
decrease its prevalence, but rather tend to bring the leaf markings
into prominence.
Losses due to the Disease.
Regarding the losses which have occurred in Porto Rico,
F. S. Earle stated in 1919 : " We are safe in concluding in a
general way that when infection reaches an average of 60 to 80
per cent., losses of sugar will be from 30 to 40 per cent." A
table published in the Hawaiian Islands in 1912 showed losses
due to this disease varying from 0.5 to 34 per cent., according to
variety. In Jamaica S. F. Ashby records tonnage reductions,
by comparison with healthy canes, of 33 per cent, in D 116 and
27 per cent, in Purple Ribbon.
The losses in advanced cases of the infestation of highly
susceptible varieties extend to the complete failure of the crop ;
in others to reduced yields in all degrees according to the pro-
portion of stunted stools and rotted stalks. The affected canes
have a tendency to be pithy and poor in juice, and on account
of changes which follow the splitting of the internodes the
behaviour of the juice is liable to cause trouble in the factory.
M
i
Jf^<r^
..^^^dhIbhIi
Bii5?u^ilni
t
^•^
PAo^o iy]
Fig. 138
Chlorosis of Sugar-cane
IH. A. Tempany
DISEASES OF SUGAR CANE 327
Causation.
No fungus or bacterium has been found to have any causative
connection with the disease. The evidence points to its associa-
tion with the affections caused by infective viruses, the existence
of which is estabhshed in the mosaic diseases of tobacco, tomato,
cucumber, bean, potato and other plants, and is strongly in-
dicated in diseases such as sereh, potato leaf-roll, and curly-
top of beet. (See Chapter III.)
Communicability.
Cuttings from affected plants invariably reproduce the disease,
and it is in this way that it has been distributed about the world.
The infection of healthy strains brought within the range of the
disease shows clearly that it is in some way transferable. Contact
alone does not produce this result, nor is there any evidence
that the infective principle resides in the soil.
Healthy cuttings planted in fields or in pots from which badly
diseased plants have been recently removed are not infected.
The spread of the disease by infection is observed to be very
erratic, rapid at one time or place, slow or absent at another.
The evidence points to distribution by a carrier, probably an
insect. E. W. Brandes, in greenhouse experiments at Washing-
ton, has proved transmission of the disease from sorghmn to
cane and corn by Aphis maidis. Extensive experiments in Porto
Rico with all the common insects found on sugar-cane have, so
far as reported, had only inconclusive results, as the infections
obtained (with cane-fly, leaf-scale, yellow aphis, and mealy-bug)
were so few that chance infection is not certainly excluded from
their origin. No infections were secured in experiments made by
C. B. Williams, in Trinidad, with several insects, including the
frog-hopper (Tomaspis). Cane-fly and aphides are exceedingly
rare on sugar-cane in Trinidad. In that island there is little or
no evidence of infection in dry weather, but rapid extension of
the disease becomes apparent a month or two from the beginning
of the rainy season.
Artificial infection tested in many ways has only proved
successful when juice expressed from affected plants, with pre-
cautions against oxidation, has been directly inoculated into the
tips of growing canes, and with this method positive results have
been difficult to obtain.
Resistance and Susceptibility.
In Porto Rico all the stcindard varieties, and all the local,
Barbados, and Demerara seedlings are attacked. The Bourbon
cane is exceedingly susceptible and the Striped (Rayada), White
Transparent (Crystalina), and YeUow Caledonia, all of which
are grown in quantity in the iMand, are severely injured. Certain
seedlings imported from and much favoured in Java (P.O.J.
328 DISEASES OF CROP-PLANTS
Series) show a high degree of endurance of the disease, suggesting
that this character has influenced their selection. They become
generally but lightly infested, and make an apparently normal
growth.
Perfect immunity exists, it would appear, only among canes of
the thin Indian type. The well-known Uba cane, the only one of
the type adopted in the West Indies, has been severely tested
without developing any symptoms of infection. The references
to " Kavangire " by Porto Rican and American writers on the
disease really apply, according to more recent information, to
Uba, introduced by way of the Argentine to Porto Rico.
According to Brandes, corn, sorghum, rice, millet, crab-
grass, foxtail, and Panicum have been found to be subject to
infection, but are stated to be attacked with difficulty, con-
tracting the disease when grown in the close neighbourhood of
diseased cane. In Trinidad appearances strongly suggestive of
mosaic have been seen on corn and on so-called " corn-grass "
{Manisurus exaltata).
Control.
Uninfected regions should be protected by careful quarantine,
with special precautions against introduction on highly resistant
varieties, which may transmit the disease without showing signs
of infection in themselves.
Light infestations may be dealt with by pulling up the affected
plants. There is no need to burn or otherwise destroy them
beyond taking precautions against the resumption of growth, as
wilted plants are no longer infective.
Severe infestation should be dealt with by the thorough
clearing of the fields after crop, followed by the use of disease-free
material for replanting. If a rotation crop is interposed, the
security will be greater against reinfection from growing fragments
of the old stools.
Chlorosis, Gall Patches, Moonshine.
Chlorosis as known in the West Indies is a condition regularly
induced in cane planted on certain restricted patches in limestone
districts, in which the leaves become exceedingly pale in colour or
even milk-white. In addition to these permanent locations, the
condition may be manifested more or less over wider areas in wet
years. Chlorosis of this type is reported from Cuba, Jamaica and
Porto Rico ; it occurs to a small extent in Barbados, and is
particularly well-marked in Antigua, where the areas subject to it
are known as gall patches. H. A. Tempany has shown that the
condition is due in Antigua to the presence of soluble material,
indicated by analysis to be sodium carbonate (" black alkali ").
This is believed to originate from interaction between the calcium
carbonate of the soil and sodium chloride brought up in solution
DISEASES OF SUGAR CANE 329
from saliniferous deposits at deeper levels. The condition is thus
not dependent on an excess of lime, as is sometimes supposed,
but on the presence of salt in the soil water, a conclusion which
agrees with the observation that cane will grow quite normally
on soils with a higher lime content than those affected.
No permanent remedy has been found. Experiments in Porto
Rico have shown that temporary recovery of colour follows the
application of iron salts.
In order to complete the review of diseases attacking the
sugar cane, a brief account is added of two diseases which have
been recognised in the Pacific Region for a number of years, but,
as in the case of Sereh, are not known to occur in the West Indies ;
these are the so-called Fiji Disease and the Sclerospora disease.
Fiji Disease.
This disease owes its name to the fact that it was discovered
in the Fiji Islands (about 1905), where it threatened the stability
of the industry ; it was found later in Australia and New Guinea,
and more recently in the Philippines.
The unique and characteristic symptom of the disease is the
occurrence of elongated swellings or galls on the under sides of the
leaves ranging in length from i /12th to several inches and re-
stricted to the large veins. According to H. L. Lyon, similar
galls may be found in the vascular bundles of the stems. A
stalk may grow to a considerable length and develop a succession
of galled leaves, otherwise normal in appearance ; finally and
rather suddenly, only short crumpled leaves, often reduced to
stumps, unfold from the spindle, growth ceases, and the death
of the stalk follows. After the disease has appeared on a single
stalk the whole stool becomes involved and finally succumbs.
The disease is invariably transmitted in cuttings from affected
canes, and the soil may carry the infection for a time with the
result that susceptible varieties may make practically no growth
when planted in such soil. No immune variety of cane has been
found but the New Guinea cane Badila is strongly resistant
and is now the most widely planted kind in Fiji. The disease
has been brought under practically complete control in Fiji by
rigid selection of cuttings only from healthy stools ; experience
has shown that this selection must be maintained even after
disease has been reduced almost to the vanishing point. Plas-
modium-like bodies occur regularly in the cells of the gall-
tissue and this observation has led to the belief that Fiji disease
might be due to a slime-mould (myxomycete) related to Plas-
modiophora brassicce Wor., the well-known cause of club-root
in plants of the order Cruciferae.
330 DISEASES OF CROP-PLANTS
SCLEROSPORA DISEASE.
This disease was described for the first time by Miyake, in
Formosa, where it appeared in 1909. It is believed, however, to be
the same as the leaf-splitting disease known earlier in Australia
and Fiji. It was introduced into the Philippines, apparently in
cane cuttings from Formosa. The earliest symptoms on the leaf
show as a few yellowish stripes parallel with the veins ; the stripes
increase in number until the greater part of the leaf turns pale
and becomes marked by scattered long reddish-brown spots.
At the same time a fluffy growth appears on the under surface
and the leaf dies and dries up or before dying becomes torn or
shredded at the tip. In some varieties of cane the internodes may
be much lengthened or thickened and diseased stalks may be
2-3 feet longer than uninfested stalks. The cause of the disease
is a downy mildew — Sclerospora Sacchan Miyake, which sporulates
abundantly on the under sides of the leaves during the night.
The disease is transmitted by infected cuttings and from plant
to plant under humid conditions by means of the air-borne
conidia. Maize and teosinte {Eiichlcena Mexicana) become
infected readily but endure the disease better than cane. At-
tempts have been made to control the disease by roguing the
young plants which have arisen from infested cuttings before the
spores of the fungus are developed on the leaves, and by careful
seed-piece selection from uninfected stools. If Sclerospora
disease reached the West Indies it would find difficulty in main-
taining itself in the islands having a dry climate.
CHAPTER XXVII
DISEASES OF ROOT CROPS.
Cultivated Aroids.
Although species of Colocasia (dasheens and eddoes) and
Xanthosoma (tannias or cocoes) are widely grown as provision
crops there is little record of their liability to disease in the
southern West Indies. An undescribed affection known as
" el mal " occurs in Porto Rico, and S. F. Ashby has given an
account, summarised below, of a disease of cocoes in Jamaica.
Dry-Rot or Saltpetre.
This is a frequent disease of the tubers of Xanthosoma in
certain parishes of Jamaica, affecting mainly the variety " Com-
mander."
Symptoms.
Leaf development is backward, and the outer leaves have a
tendency to wilt and to become infested with weak parasites,
which produce large pale dry spots.
In diseased tubers the vascular strands are discoloured
yellow or brown. Splits in the internal tissue are frequent, and
when these extend to the surface cavities are formed which become
infested with various scavenging insects, mites, and worms.
Such cavities may come to occupy half of the tuber or more,
leaving the cortex as a relatively sound shell.
Causative Fungus.
The yellowed vascular bundles are infested with a mycelium
of colourless septate hyphae abundant in the spiral vessels. In
the darker strands this mycelium bears long chains of conidia or
gemmae of a yellow or brown hue, generally longer than broad
with rounded ends and narrowing in the middle. Neither hyphae
nor gemmae have been seen outside the vessels. They are well
seen in the large vessels of discoloured roots.
The fungus is described as Vasculomyces xanthosomce Ashby.
Its action is similar to that of the Fusariums causing wilt diseases,
the subsequent rotting being due to the admission of secondary
organisms.
331
332 DISEASES OF CROP-PLANTS
A Jamaica disease resembling this in most particulars was
attributed by G. Massee in 1887 to a fungus named by him
Peronospora trichotoma.
Control.
Certain varieties are immune to the disease and may be
safely grown on infested land. Susceptible varieties should only be
planted on clean land, and care must be taken not to introduce
infection with the planting material or with instruments used in
cultivation.
Fig. 139 Vasculomyces Xanthosomae
Bull. 6, Dept. Agri., Jamaica
In Jamaica and more recently in Hawaii a disease of Colocasia
sp. (taro) has been attributed to a species of Pythium.
Arrowroot.
Burning Disease (Rosellinia).
In 1891 specimens of arrowroot rhizomes from St. Vincent
affected by a disease knowning as " burning " were examined by
H. Marshall Ward. They were reported by him to be badly
affected by a subterranean fungus mycelium, and to have pro-
duced conidia in the manner of the genus Spicaria when kept in a
damp chamber.
No further outside attention seems to have been given to this
affection until South, after a visit in 1911, gave an account of it
in an unpublished report. He described the disease as occurring
in the fields in patches varying in area from a few to several
Fig. 140
Burning Disease" of Arrowroot
Fig. 141
Bacterial Rot of Onion
Bull. 164, New York Agri. Expt. Sta.
DISEASES OF ROOT CROPS 333
hundred square feet, and constant in position from year to year.
The symptoms do not make themselves apparent until the
rhizomes are nearly ripe for digging. At this time the affected
plants have fewer leaves than the healthy ones growing in their
vicinity, and these leaves are often rolled up and somewhat
wilted. Extension of the patch is very slow, but the fungus is
very persistent, and the disease was reported to have reappeared
in the first crop on a field replanted after remaining in bush
fallow for twenty-five years.
In 1915 the present writer visited a field in which the rhizomes
had recently been dug and were lying in small heaps on the
ground. The material from a diseased patch was conspicuous
owing to the black discoloration of the rhizomes. Towards the
base of the heaps sufficient moisture had been retained for the
fungus to continue growing, and there were woolly tufts and
strands of greenish grey mycelium, undoubtedly that of a Rosel-
linia, and an abundance of the typical conidial fructifications.
Allowing for the difference in structure of the two plants, the
appearance of the mycelium in the rhizome corresponds exactly
to that of R. bunodes in lime roots, that is to say, the mycelial
strands are identical and their distribution throughout the
relatively soft tissue of the arrowroot rhizome is similar to that
seen in the bark of the lime. South records the presence of
perithecia of R. bunodes on other plants in close proximity to a
diseased patch.
On the exterior of the arrowroot rhizomes as seen when they
are dug there is little or no loose external mycelium. When
such does occur, the hyphae are of the usual " varicose " type.
On the surface of both rhizome and scale leaves there is a dense
scatter of shiny raised dots and ridges (less than i to about 3 mm.
in diameter) consisting of compact aggregations of dark hyphae
in the superficial tissue. From these the slender compact branch-
ing strands, black with a white core, radiate through the paren-
chyma both of the scale leaves and of the rhizome.
In the latter they mostly follow a radial course, and are con-
spicuous to the naked eye as black dots and lines in a section.
At this stage the rhizomes are still full of starch. At an early
time on the scale leaves, and later on the rhizomes, the black
spots tend to run together until the whole surface is black and
shining, and about this time the parenchyma of the rhizome
may be found generally infested with hyaline hyphae and depleted
of its starch.
The special characteristics of the disease on arrowroot are :
(fl) that the disease occurs in patches which are reported to show
little or no observable increase from year to year ; {b) that in such
patches the disease persists for a long and apparently indefinite
period of years.
The explanation of these features is probably to be found in
the methods pursued in arrowroot cultivation. The fields are
334 DISEASES OF CROP-PLANTS
usually kept in this crop for many years in succession, and, al-
though the crop is an annual one, the ground is never clear of
plants. In the first place it is impracticable to remove the
smaller fragments and offshoots of the rhizomes from the ground,
as shown by the large number of plants which spring up after a
change has been made to some other crop, and, secondly, it is the
custom when digging the rhizomes to break off pieces and return
them to the soil, to provide plants for the succeeding crop. The
persistence of the disease, under suitable circumstances of soil
moisture, may with great probability be thus accounted for, and
it is helped by the custom of leaving the diseased rhizomes
to decay on the spot. The fungus attacks the plants of almost
any crop which succeeds arrowroot on the diseased patches,
but there is no evidence of its persistence on such a crop after
the arrowroot has been completely eliminated.
Its origin, as regards the primary patches, probably goes back
to the cases appearing when the land was cleared of trees. One
can readily conceive of secondary patches occurring, due to
careless disposal of diseased material, or to the germination of
spores on any accumulation of decaying tops or weeds sufficient
to give the fungus a start.
Accurate and systematic observations, involving measure-
ment of the extension of the patches, are wanting, but it does
not appear from the evidence available that the fungus makes
much progress from plant to plant in the soil, and there are no
spreading roots to conduct it as in the case of the tree crops.
Control.
All the plants in a diseased patch and on its margin should
be carefully dug out, dried as far as possible, and burnt with
the aid of added fuel. The spot should be left fallow in the
succeeding crop, kept clean-weeded, and forked from time to
time. Lime should if possible be liberally applied. Any diseased
trees on the land should be disposed of as directed in the section
on Rosellinia diseases in general.
Cigar Disease.
In 1900 a disease of arrowroot in St. Vincent producing a
condition known to planters as " cigar root " was reported on by
A. Howard. It has not been further investigated, but the
writer has recently learnt that it still occurs and renders necessary
the restoration of invaded fields by the use of planting material
from uncontaminated sources, and in severe cases causes the fields
to be thrown out of cultivation in this crop.
On the estate visited by Howard the affected plants first
appeared in a section irrigated by the effluent from the starch
factory and later spread widely through the various fields.
DISEASES OF ROOT CROPS 335
These plants continued in an active state of growth when normal
plants had ripened.
The affection produces an attenuated condition of the
rhizomes, which develop long internodes and abundant roots.
The yield of starch is small and difficult to extract.
Microscopic examination showed that the cells contained few
normal starch grains in the older parts, but many corroded
grains m all stages of disintegration. Preliminary studies sug-
gested that the active agent in this was a filamentous bacterium
which was found to be abundant in the slime deposited by the
mill water.
Cassava.
Cassava is very widely grown in the Lesser Antilles, but sel-
dom on a very large scale. It is grown for starch manufacture in
St. Vincent and is used to some extent as a rotation crop on sugar
estates in Barbados ; a few years ago attempts were made to
extend its cultivation in Trinidad.
Apart from injuries due to insects and mites no disease of
serious importance has been met with on this plant.
WiTHER-TlP.
A wither-tip and dieback disease apparently due to a Gloeo-
sporium occurs occasionally in Barbados, on plants with reduced
vigour, and has been seen to follow on defoliation by caterpillars.
The fungus is probably Gloeosporium manihot Earle. known as
the cause of a similar disease in the United States and Porto
Rico.
Leaf-Spots.
A bacterial leaf disease occurs in Barbados in which a distinct
varmsh-like deposit of bacteria appears on the surface of the
angular leaf-spots produced.
Two leaf-spot diseases unidentified are reported to be
prevalent m Trinidad. A leaf-spot due to Cercospora henninRsii,
All. is common in Porto Rico.
1 -11^"?^^ appropriate circumstances cassava is liable to be
killed by Rosellinia root disease.
Onion.
Bacterial Soft-Rot.
Onions in the West Indies as in North America are liable under
certain cu-cumstances to a soft-rot of the bulb scales caused by
bacteria of the Bacillus cawtovorus t3'pe. Very heavy losses
were experienced on some estates in Barbados after a wet season in
1904, and m the islands where onions are now grown on a large
336 DISEASES OF CROP-PLANTS
scale, as in Antigua and Montserrat, the affection is occasionally
troublesome.
Symptoms.
The disease may become prevalent in the fields about reaping
time, or during storage or transit of the harvested crop. Usually
the rot starts at the neck of the onion, and it may be confined
to the heart, to one or more scales enclosed between others
which remain sound, or the outside may be sound with the whole
interior decayed, or vice versa. The organism is unable to pene-
trate the epidermis and only passes from one scale to another
by way of the base. The presence of the rot may often be de-
tected by a reduction in resistance to pressure.
Conditions of Occurrence.
Various kinds of injury can induce the appearance of the rot»
but the scale of its occurrence in the field is generally determined
by the prevalence of wet weather during ripening and by condi-
tions as to moisture generally. Good drainage and open clean culti-
vation tend to reduce it. Careful harvesting, curing, and packing,
and good ventilation of the place of storage lessen the losses sub-
sequent to reaping. Experience in Montserrat leads to the con-
clusion that not only are onions grown under wet conditions
more liable to decay in the field, but that they suffer much more
during storage and shipment than the lots which are grown in
drier situations.
Sweet Potato.
To a reader familiar with the long and formidable list of
sweet potato diseases occurring in the United States the absence
of recorded diseases of this important vegetable in the West
Indies will appear remarkable.
So far as the storage rots are concerned the explanation is
to be found in the fact that these are so much more severe under
tropical conditions that storage for any considerable period is not
practised. A somewhat similar explanation is probably true in
part of field diseases ; under conditions where they become severe
the sweet potato is not grown. The chief area of production
is the dry and open cane land of Barbados. A third reason may,
perhaps, be found in the existence of sweet potato cultivations
in America on the margin of the climatic range of the plant.
White Rust.
Effects on the leaves ranging from small spots to considerable
pocket-like distortions are caused by infestation with the fungus
Albugo ipomoecB-panduranos (Schw.) Sw. White, somewhat wax-
like swellings appear under the epidermis, and this eventually
DISEASES OF ROOT CROPS 337
bursts to expose the pustules of the fungus, which produce masses
of white spores.
Young stems are to some extent attacked, and S. F. Ashby
has found in one district of Jamaica the potatoes themselves bear-
ing rounded mammillated galls which may exceed two inches in
diameter, and are occupied by a stout non-septate mycelimn
with spherical haustoria bearing oospores characteristic of Albugo.
The galls soon decay and spoil the appearance and keeping qualities
of the potatoes.
The wild Ipomcea biloba, a common plant on sandy beaches, is
often infested with a fungus assumed to be the same as the one
on the sweet potato.
A true rust, Coleosporiiim Ipomoece Burr, is reported common
in Porto Rico, but has not so far been noticed in the Lesser An-
tilles.
Yam.
Wilt.
One or more wilt diseases of which the nature is as yet obscure
occur in Antigua and Barbados and probably in other islands.
Under certain conditions, in Barbados usually dry weather, the
yam vines become black-spotted on the leaves and in sections
of the stem and dry off prematurely in much the same way as
they normally do at the end of the season. Several fungi are
recorded in other countries, associated with black spotting of
yam foliage, but careful and repeated examinations have not
revealed any such on the Barbados material. The indications
are somewhat more in keeping with a root disease.
Tuber Rot.
A rot of the growing tubers has several times been reported
from Antigua in which broviTiing and blackening of the tissues
starts in spots and patches from the outside and leads to the
production of large cavities containing pulpy remains of the
broken-down tissues, and infested with fungi, bacteria, and
scavenging insects. The decay apparently ceases to make pro-
gress after the yams are dug and stored, the rotted tissues being
then found enclosed by layers of wound cork. Material showing
the active condition of the disease has not yet been obtained,
and infection experiments with cultures of the fungi found have
had negative results.
Leaf Blotch.
A Cercospora leaf-spot is common on the yam in Trinidad,
taking the form of large black blotches on the leaves.
CHAPTER XXVIII
DISEASES OF LEGUMINOUS PLANTS
Ground nuts are grown rather extensively as a field crop in
St. Vincent, and to a small extent in some other islands. Pigeon
peas are also grown as a field crop in St. Vincent, and are grown
everywhere in a scattered way, in small holdings, gardens or on
the margins of fields. Cow-peas and Rounceval peas {Vigna spp.)
are fairly common as a rotation crop on arable land. Velvet,
Bengal and Lyon beans {Stizolohium spp.), Sword and Jack beans
{Canavalia spp.) Woolly Pyrol {Phaseolus mungo) and the Tephro-
sias are grown to some extent as cover crops and green dressings.
Bonavist {DoUchos lahlah) and Lima bean {Phaseolus hmatus) are
grown mostly in provision plots and small holdings.
The diseases of peas and beans have had little attention.
In addition to those described below there exist nmnerous
leaf-spots and one or more each of rusts, mildews, and wilts
which have not been studied.
Of the general diseases the most serious are the infections
following bug punctures (especially those of Nezara viridula,
which is partial to these plants) and the rot, affecting all parts
near the soil, caused by Sclerotium Rolfsii, both of which are
elsewhere described.
Ground Nut.
Leaf Rust.
The rust fungus Uredo AracM.Us Lagh. {Uromyces Arachidis
P. Henn.) occurs on ground-nut throughout the Antilles from
Cuba and Porto Rico to Trinida'^ and in South America. The
pustules are small, yellow-brow i fo dark brown, scattered or
thickly crowded, occurring on both sides of the leaf but in greatest
abundance on the under side, and present to some extent on the
stems. The urediniospores are ovoid-round, minutely echinulate,
24-30 microns in diameter, with a yellow epispore.
The rust is more prevalent in wet than in dry seasons, and
worse on heavy or wet than on light land. It has attacked
imported varieties more severely than the local ones.
The incidence of the affection is somewhat erratic. Under
favourable conditions it only occurs in quantity towards the
end of the season, and mostly on falling leaves, but in other
338
DISEASES OF LEGUMINOUS PLANTS 339
circumstances an infestation can bring about the partial defolia-
tion and premature ripening of the haulms, with the production
of a large proportion of shrivelled kernels. W. Robson has
carried out in Montserrat a series of annual tests of the effect of
spraying with Bordeaux mixture. The treatment results in
the control of the fungus, the leaves on the spra^'^ed plots remain-
ing at the end of the season green and healthy, with only a few
scattered pustules, when those on unsprayed plots have on occas-
sion been heavily infested and failing. The effect on the crop is
variable. In some j^ears an obvious and considerable increase
has resulted, in others very little. This is probably to be ex-
plained by the absence of any serious injury from the fungus
when the infestation is developed late.
The indications are that the first spraying should be given
at 6-8 weeks from planting, and that one or two more applica-
tions should be made at intervals of a fortnight.
Leaf-Spot.
A leaf-spot disease produced by the attacks of the fungus
Cercospora personata Ellis {Septoglceum Arachidis Rac.) appears
to be met with in all parts of the world where this crop is grown,
being capable of transmission by spores adhering to the seed.
In St. Vincent, where ground-nuts are grown extensively as a
field crop, it is not in general regarded seriously, but local attacks
are sometimes heavy enough to cause appreciable damage.
This is more or less the position in other islands where the crop
occurs. The disease is more prevalent in wet weather and
more severe in its effects on imported varieties. In India it is
regarded as one of the major diseases of cultivated crops, and is
responsible at times for very great destruction. Its effect is
described as not unlike that of potato blight, the plants being
almost entirely defoliated.
The older leaves are first attacked, and brown to black
nearly circular spots, usually 2-4 mm. in diameter, are produced.
Yellowing and shedding of the leaf may follow.
The conidiophores are brown, tufted, short, and irregular in
outline. The conidia are light brown, mostly more or less
club-shaped, several (usually 3-4) celled, 30-50 X 5-6 microns.
Trials of spraying with Bordeaux mixture have not been
successful either in the West Indies or India. Very satisfactory
control has been achieved in India by the introduction of exotic
varieties with better powers of endurance, which seems to have
reduced the disease to a position similar to that which it occupies
on the varieties established in these islands.
Root Disease.
The " root disease " described by F. W. South is due to the
340 DISEASES OF CROP-PLANTS
fungus Sclerotium Rolfsii, discussed in another section as a general
parasite.
Its attacks on ground-nut have caused very considerable
losses during periods of wet weather.
Kidney Beans.
Anthracnose.
Kidney, Haricot, or bush beans {Phaseohis vulgaris) are grown
to some extent as a garden crop in these islands, usually from
American seed. The bean anthracnose is of fairly frequent
occurrence, being probably in most if not all cases introduced
on the seed, and sometimes causes considerable loss of crop.
It is much less severe on local forms of Phaseolus.
Symptoms.
The disease makes its appearance on the young seedUngs,
and ultimately infects the leaves, stems and pods of the developed
plants. The fungus penetrates the pods to the seeds within,
causing spots to be developed on the latter which form a starting
point for a new cycle of disease. The spots appear on the seeds
as sunken black or brown specks in the case of light attack,
or as brownish or blackened spots, usually in the form of a ring
with a dark or rifted centre, where the attack is severe. On the
seed-leaves, circular or kidney-shaped blotches appear, which
may enlarge until the whole leaf is involved. Elongated cankers
are produced on the stems, and many young plants are lost
from the direct or indirect consequences of stem and root in-
fection. Young leaves may be crinkled owing to the effect
of the fungus on the margins ; older leaves are affected mostly
on the veins, and the portion of the leaf -blade beyond the attacked
vein turns yellow and dies for want of water. On the pods the
lesions start as small reddish pin-point spots, which enlarge
into circular or kidney-shaped sunken areas with a raised rim.
Yellow or pink spore-masses are produced in the centres of these
spots. Severe infestation occurs as a consequence of continued
wet weather, and a general yellow appearance, due to the dying
of the leaves, is then recognizable in the fields.
The Fungus.
The causative fungus is CoUetotrichum lindemuthianum (Sacc.
et Magn.) Br. et Cav. The hyaline spores are at first one-celled,
but later are divided by a single septum. They are surrounded
by a mucilaginous envelope. In mass they are at first pink,
later becoming more or less creamy. They are usually kidney-
shaped, 2.7-5.3x10.6-20 microns.
Control.
Spraying with Bordeaux mixture usually gives disappointing
DISEASES OF LEGUMINOUS PLANTS 341
results. There are notable varietal differences in resistance,
but the adoption of this means of control is complicated by the
existence of strains of the fungus which attack varieties resistant
to other strains. Wells' Red Kidney is reported to be very
satisfactory in this respect, and it is expected that resistant
white types will be made available.
A very considerable degree of freedom from the disease can
however be secured by taking precautions against planting
infected seed. Pods should be selected which have no spots
whatever upon them. It is not sufficient to select unspotted
seed, since a slight infection may not be noticeable. An infection
so slight as this upon a pod would have little significance, since
the fungus would not have penetrated to the seeds within. It is
advisable, however, in order to prevent infection from such a
source, or from merely adherent spores, to dip the pods for ten
minutes in corrosive sublimate solution (i part to 1000 of water).
The seed so obtained should be used to plant a seed plot on clean
ground. Cases arising after this treatment should at the worst
be few, and capable of elimination by hand-pulling.
In using imported seed the best that can be done is to reject
all those which show signs of injury (which in many samples
examined have been very numerous) and to treat the remainder
with a disinfectant.
Bacterial Blight.
This disease which is widespread in the United States has been
frequently noticed on beans in these islands. It is caused by
Bacterium phaseoli E.F. Sm.
The signs of its attack on the seed are, in light cases, yellow
spots or blotches, not definitely circular, frequently on the
side of the seed which was attached to the pod ; in severe cases,
yellowing and shrivelling of the whole seed. On the seed-leaves,
small amber-coloured spots appear, with which may be associated
a light yellow slime. On the foliage leaves the disease produces
irregular water-soaked areas, which are usually bordered with
distinct yellow or red hnes. These areas gradually enlarge
until the entire margin, or half the leaflet, may be involved. The
watery areas later become brown and dry. On the pods, watery
pustules are first formed, which develop into amber-coloured
irregular blotches with uneven green centres ; clumps of the
bacteria ooze out, and dry into yellow crusts on both leaves
and pods. The general appearance of a field, when the prevalence
of humid weather conditions has led to a severe infestation,
is described as suggesting that the plants had been drenched
with hot grease. At a later stage the older leaves look dry and
burnt, and excessive new growth occurs in the attempt to provide
new foliage. Fruiting is seriously interfered with, and the pods
cease to fill out.
342
DISEASES OF CROP-PLANTS
This disease is more capable than anthracnose of withstanding
hot dry weather.
Control.
No fully effective means of control is known. The germs are
able to resist drying for a long period, and become widely spread,
so that probably reinfection does not entirely depend on seed.
The measures recommended for anthracnose should, however,
also reduce considerably the incidence of blight.
Stem Rot.
Bean plants raised in Barbados gardens from American seed
have several times been seen to develop brown decayed patches
on the young stems within the first two or three inches above
the soil. The injury extends deeply into the tissues and often
either girdles the stem or causes it to break. From the nature
of the mycelium found in the lesions the disease is believed to be
due to the soil fungus Rhizoctonia, which has been described
as the cause of stem rot of beans in the United States, and there
is a distinct possibility that the fungus is introduced with the
seed. Pods near the ground are said to be attacked by the
mycelium, which can enter maturing seeds without destroying
them.
Seeds should be carefully picked over before planting, and
those which show even slight injury rejected. The disease can be
prevented to some extent by wide spacing and the avoidance of a
shaded condition of the soil.
Pigeon Pea.
Root and Stem Diseases.
The pigeon pea [Cajanus indicus) is subject in the West Indies
to diseases, probably two or three in number at least, which
cause the death of the whole plant by infestation of roots, collar,
or lower stem. These are additional to the known susceptibility
of this plant to Rosellinia root disease. The occurrence of these
affections seems to be sporadic, though sometimes the losses are
fairly heavy. Their nature is at present almost entirely unknown.
In 1917 old and young trees died off in large numbers but
in a scattered way in several districts of Carriacou, in the Grena-
dines, and a similar occurrence amongst young plants was seen
by the writer at a later date in the island of Bequia. In the
Carriacou material the disease is distinctly of the nature of a
stem and collar canker, and it is evident from the irregular
development of ribs of new wood about the lesions that it may be
present for a considerable time before killing the plant. The
only fungus found to be uniformly present is an Ascomycete
with dark hyphae giving a slaty appearance to the wood, pro-
'.ran.
f St! 0 ]
Photo by] [C. If. Edgerton
Fig. 142 Anthracnose of Bean
From M. T. Cook's Diseases of Tropical Plants, by permission of Macmillan & Co.
Fig. 143 Stem-rot of Papaw
V
DISEASES OF LEGUMINOUS PLANTS 343
ducing a black stroma throughout the affected bark, with long-
necked perithecia in dense clusters more or less united at the base,
ejecting both from these and from adjacent pycnidia white
tendrils of unicellular spores, the former coffin-shaped, the latter
oval or oblong. Infection experiments, made in dry weather,
gave negative results. J. A. Stevenson records the existence of
an undescribed but serious stem canker in Porto Rico, and a
disease closely resembling that described above occurs in Trinidad
and Tobago.
A wilt disease, apparently due to infestation of the roots, occurs
sporadically in Barbados.
Rust.
The rust fungus Uromyces Dolicholi Arthur, occurs on the
leaves of pigeon pea in Trinidad and Porto Rico, and is probably
to be found in the intervening islands. It produces small dark
brown pustules, with powdery spores, on the lower side of the
leaves, and causes discoloration of the foliage.
CHAPTER XXIX
DISEASES OF MINOR FRUIT PLANTS
Pineapple.
The pineapple belongs to an order (Bromeliaceae) the greater
part of which consists of epiphytes and lithophytes (tree and rock
inhabiting plants) and the pineapple itself, while belonging to one
of the soil-inhabiting genera, is closely related in its constitution
and its vegetative structure to those of epiphytic habit. This is
reflected in the fact, emphasized by all authorities on the cultiva-
tion of the plant, that it is most decidedly intolerant of any inter-
ference with free root-respiration, such as is brought about by
soils which are close-lying or tenacious of water. Thus in Antigua,
where pineapples are more regularly grown than in the other
islands of the Lesser Antilles, the industry has practically speaking
been confined to the section of the island which possesses soils of
volcanic origin with a minimum of clay.
Given good aeration the consistency of the soil is of minor
importance ; it appears that the plant will grow well in any soil
rich in humus, but in light soil it is especially necessary
that the supply of plant food be well maintained. The plant
will grow for a time in well-rotted leaf mould, and is grown in
Florida in sand with the addition of chemical manures. It would
seem that some of the difficulties attributed to disease have arisen
through a mistaken belief in the ability of the plant to thrive
in any light soil, which has resulted in the use of land in a
dried out or exhausted condition. It is important to note in
this connection that the plant has a very limited root range.
The plant is well protected against drought, but cannot be
expected to do more than survive in periods when the com-
paratively shallow layer penetrated by its roots is dry. This
points to the need, in dry districts, of close planting to shade the
ground, mulching of the surface with leaves, or the maintenance
of an efficient dust mulch.
Tangle-Root.
The condition known as tangle-root is a frequent accom-
paniment of the root diseases described below, as it also is of
some dry soil conditions. In the affected plants the main
roots, instead of growing directly out into the soil, wind round and
round the rootstock, often quite tightly. The range of the roots
344
Fig. 144
Base-rot of Pineapple Cuttings
Fig. 14.-.
Cross Section of Pineapple Fruit showing
Two Infections of Black-eye
Dull. lo, Dept. Pathology. H.S.P.A.
DISEASES OF MINOR FRUIT PLANTS 345
is thus greatly reduced, and in extreme cases the plant has
hardly any hold on the soil.
The condition arises from the persistence of old leaf-bases
in a leathery condition at the time when new roots are being
produced from the internodes above. The new roots are unable
to penetrate the obstacle thus presented, and are deflected so
that their growth partly or wholly encircles the stem. Failure
of the first formed roots from asphyxiation or other causes, or
decay of the base of the rootstock, induce the condition by
forcing premature development from higher nodes. In planting
out suckers in dry soils it is advisable to remove beforehand the
lower leaf-sheaths so as to allow a free course to the roots ; in
wet situations decay of the old leaves is more rapid and the
operation may be dispensed with.
The existence of the tangle-root condition is not necessarily
a symptom of disease, but it impairs the efficiency of the root-
system, reduces the rate and vigour of growth, and may in extreme
cases cause the failure of the plant under the strain of fruit
production.
Root Disease, Wilt, or Blight.
A condition of ill-health and failure known under one of
these names or an equivalent appears to be familiar in every
country in which any considerable cultivation of pineapples is
carried on. It has not been established as being due to any
particular parasite or any single cause, and it is not probable that
this is the case. Rather it is in its general aspect a form of failure
like the root disease of sugar-cane, and in its simplest form may
be brought about as the direct result of unfavourable conditions,
but in addition may develop characters tending more and more
towards those of parasitic disease according to the nature of the
organisms, usually fungi, which invade the weakened roots or
rootstocks. In this way variations in the final form of the
condition arise which differ in different situations and cause the
confusion of attempts to correlate them. One important question
on which evidence is definitely conflicting, probably for the reason
just given, is that of transmissibility. The disease as it exists in
Hawaii and Queensland has been reported to be seldom or never
communicated ; in Porto Rico, Jamaica and the Philippines it
has been regarded as infectious.
Symptoms.
In the form of the disease seen by the writer in Montserrat,
which is believed to be the one most common in these islands,
the plants showing external signs of the disease first occur in
patches and the affection appears to spread slowly through the
beds attacked, taking several months in the process.
The general appearance is that of a slowly progressive wilting
346 DISEASES OF CROP-PLANTS
and drooping of the leaves, beginning with reddish discoloration
and resulting in withering from the tips, loss of colour, and eventu-
ally the complete drying-up of the plant, frequently when the
fruits are partly grown.
Examination of the plants in various stages of failure showed
quite clearly that it was a question of root decay. The worst
affected plants had hardly any functioning roots, the least affected
were fairly well provided, and the intermediate forms showed
corresponding gradation. In all the specimens the actual
tissues of the stems, apart from the discoloured root traces, were
quite sound, thus differentiating this disease from stem base-rot
due to Thielaviopsis.
Associated Fungi.
In the affection as met with by the writer and as described
by previous investigators in the West Indies fungus hyphae
have been found in the roots and root-hairs from an early stage
of failure. Their presence is referred to by most writers on the
subject but in itself is not of much value as evidence of their
pathogenic nature, especially as no particular fungus has been
found to be well-marked and constant, Fusarium and other
soil-inhabiting tj^pes are met with, but in no case has a necessary
connection with the disease been demonstrated. On several
occasions the mycelium and fructifications of Marasmius Sacchari
have been associated with diseased plants in the West Indies,
but this is not by any means general.
C. W, Carpenter has demonstrated very recently that wilt
in the Hawaiian Islands is due to destruction of the finer roots by
a fungus which he considers to be identical with Pythium butleri
Subram. the cause of root rots of tobacco and ginger and foot-rot
of Carica papaya in India,
Control.
Until the particular types of affection included under the name
of root disease have been investigated and defined there is not
much that can usefully be said on the subject of control.
So far as the disease is primarily due to some unfavourable
condition or conditions of growth, including physically unsuitable,
poorly prepared, insufficiently drained and cultivated soil (all
leading to a restriction of aeration), poverty of soil, lack of water,
and poorly prepared planting material, the means to be taken are
obvious. There can be no doubt that much of the trouble
experienced has originated in this way
The appearance of communication from plant to plant and
the way in which the trouble slowly extends over the beds, as
described above, afford fairly convincing evidence that para-
sitism is an important factor in at least one form of the disease.
Its existence in an affection of this nature introduces the usual
Fig. U6
Thielaviopsis Soft Rot of Pineapple
Bull. lo, Dept. Pathology, H.S.P.A.
DISEASES OF MINOR FRUIT PLANTS 347
difficulties in dealing with soil parasites. Attempts at control
involve the very early eradication of affected plants and their
immediate neighbours, some effort towards sterilization of the
infested soil, the avoidance for a long time (at least a year and
possibly several) of the use of infected land for replanting, and
careful precautions against infecting new land with any sort of
material from the old.
Resistant Varieties
The most hopeful measure lies in the search for and adoption
of suitable resistant varieties. In Montserrat the Red Spanish
variety was reported first to be immune, and later to show a high
degree of resistance to the disease as compared with the Queen
types, which include the Ripley.
Base Rot.
A disease which has been previously described from the
Hawaiian Islands and is apparently indicated in a Porto Rico
reference, was observed in 1914 in a series of plots at the Experi-
ment Station in St. Kitts.
Symptoms.
The plots in question were estabHshed with cuttings of the
Ripley pine, which were disinfected with Bordeaux mixture
before planting. About the time the first fruits were beginning
to ripen a sickly yellow appearance of the plants became very
noticeable and was made the subject of investigation. It was
found that the more seriously affected plants were easily lifted
by hand and the number of living roots was very small. The
underground parts of the stem were rotted to a degree broadly
corresponding to the condition of the top, the worst cases re-
taining only a few roots attached immediately below the bases
of the lowest leaves. No evidence of attack on the remaining
roots and rootlets was found, the loss of roots appearing to
proceed from the rotting of the stem to which they were attached.
The blackened tissues of the stem proved to be filled with
dark hyphae, penetrating the tissues in every direction, lying in
coils in the cells, and producing an abundance of black spores in
simple chains. A large plant with healthy green leaves and a
well-developed root system was found when cleaned to have a
brown spot about the middle of one side of the underground
stem, and when kept for four days in a closed chamber a rot of
the nature observed in the field had extended from this point
throughout the stem accompanied by the advance of the same
fungus.
The fungus was identified in the plant and in cultures as
Thielaviopsis paradoxa, v. Hohn., the cause of pineapple disease
348 DISEASES OF CROP-PLANTS
of sugar-cane cuttings, and was transferred to sugar-cane, upon
which the usual effects were produced.
L. D. Larsen, in Hawaii, inoculated pineapple cuttings with
the spores of this fungus through punctures and by means of a
sprayed suspension, obtaining complete infection by both
methods and the development of the disease in typical form, the
controls remaining healthy.
Losses.
In Hawaii, where the existence of an organised pineapple
industry allows of observations being made on the subject, the
loss of plants in the field due to this disease has been found in
different instances to be 23, 27 and 65 per cent. The loss of
cuttings during transport has on occasion reached similar figures.
Control.
The causative fungus is widely distributed in these islands
and in the soil of sugar-cane fields appears to be universally
present. It occurs in connection with a pineapple leaf-spot and
is the common cause of decay in the ripe fruit after cutting. The
fungus can be generally assumed to be present where pineapples
are grown, and since large numbers of plants can be raised under
these conditions and escape the disease, it is obvious that infec-
tion depends on some local factor. What the predisposing causes
may be has not been ascertained fmrther than that dry weather
after planting seems to favour infection of the sets. This agrees
with experience in the parallel case of sugar-cane cuttings, which,
according to the writer's experience, are destroyed by Thielaviop-
sis only when lack of rain delays their development. The
experiments in Hawaii showed that the removal of as few leaves
as possible from the sets, and the drying out of the cut ends before
planting are effective methods of preventing infection. The
use of Bordeaux mixture gave negative results.
White Leaf-Spot.
A leaf-spot which is fairly common in the West Indies has
been investigated by L. D. Larsen in Hawaii. It occurs on well-
grown plants and its effect on their development is not usually
appreciable. It has been shown to be due to the infection of
insect and spine punctures, or other injuries, by the fungus
Thielaviopsis. Infection and development depend on the pre-
valence of moist and cloudy weather or on the shading of the
plants.
Description 4
Larsen's description is as follows :
" The spots vary considerably, as regards size, shape and
DISEASES OF MINOR FRUIT PLANTS 349
colour. Many are large and white, and are noticeable from a
long distance, while others are small and inconspicuous. In
typical mature spots there is a straw-coloured central area
surrounded by a dark margin. Very often there is a dark centre
within the straw-coloured area, or dark blotches, due to the
formation of the black macrospores within the tissue, may be
scattered over it in an irregular manner. Sometimes long white
arms extend beyond the dark border, and, again, the entire
spot may be white or straw-coloured throughout. The internal
tissue is soft and decayed at first, but this soon dries out, leaving
the injured area dry and sunken. The white or straw-coloured
area is not due simply to drying of the affected tissue, as is often
the case with similar diseases, but comes on while the tissue is
still quite firm, and long before it has begun to dry out. In
early stages the spots may be olive brown in colour and fairly
regular in outline, or they may be white or irregular from the
start. The size of the spots increases very rapidly, so that in two
or three days what was a small brown spot will have become
from two to six inches in length. Even twelve inches is not an
unusual length."
Leaf-Base Rot.
In Jamaica, S. F. Ashby reports the occurrence of a rot of the
leaf-bases, occurring especially in young plants, which is caused
by infestation with Phytophthora parasitica. A secondary
malodorous bacterial soft-rot is usually also present. The
Ripley variety is most susceptible.
Fruitlet Spot, Black Eye, Black Spot.
Nature and Distribution.
For the past twenty years a disease of ripening pineapples
has frequently come to notice in the West Indies, in which the
fruit appears externally quite normal, but when cut open reveals
dark brown or blackish spots situated in the flesh beneath a few
or many of the " eyes." The inappropriate names " black heart"
and " core rot," sometimes used for this affection, have probably
been transferred to it through confusion with the ripe rots due to
Diplodia and Thielaviopsis.
In Antigua the disease is sometimes very common, at others
rare or absent. It occurs in St. Kitts, Montserrat, Dominica and
St. Vincent, and has been found in pines purchased in Martinique.
Similar affections have been described from Jamaica, Queensland
and Hawaii ; in the last two countries, associated with species
of fungi different from that met with in the West Indies.
The disease was investigated by A. Howard in 1901, so far as
this was possible from transmitted specimens, and, in the absence
350 DISEASES OF CROP-PLANTS
of opportunities for prolonged study on the spot, Howard's
observations have been confirmed but not materially supple-
mented by the successive mycologists of the Imperial Department
of Agriculture.
Description of the Disease.
The spots are very erratic in their occurrence and may be
found in few or many of the fruits in a given consignment ;
large and well-grown specimens appear to have no advantage in
this respect. The discoloration may be narrow and linear,
extending inwards for half an inch or more from the base of an
" eye," perhaps with one or more small bulbous expansions which
become cavities, or it may be in the form of a conspicuous spot,
of varying length and width but usually rather sharply defined.
It does not appear that any notable extension of the lesion occurs
during storage of the fruit, nor, apart from the spots themselves
and the disfigurement they cause, that the edibility of the fruit
is affected.
A species of PeniciUium is uniformly found to occupy the
spots, fruiting in small cracks and in the cavities of the ovary.
The beginning of the spots, and the entry of the fungus into the
tissues, is associated, commonly at least, with perforations in
the tough wall of the old floral chamber (the eye), near the
insertion of the style. Species of PeniciUium are generally present
on the withered floral organs, and it is regarded as probable that
the fungus depends for access to the pulp on punctures made by
insects or mites inhabiting the enclosed floral chamber. Mealy-
bugs and mites are usually to be found in this situation. To
establish this theory of the disease it will probably be necessary
to study the general biology of the flower and fruit up to the time
the affection begins to develop, and until this has been done no
remedial measures can be suggested.
Economic Aspect.
The existence of this trouble has a serious reaction on attempts
to establish a pineapple industry. It is impossible to know when
selling, shipping, or buying pineapples whether they will or will
not show a disfigurement which greatly reduces their value for
table purposes.
Thielaviopsis Soft-Rot.
Pineapples which are bruised in gathering or in transit or
which sustain an injury while stiU on the plant are liable to be
attacked by the fungus Thielaviopsis paradoxa, which spreads
rapidly through the fruit and sets up a soft and watery rot. When
the infested tissues are exposed they are quickly blackened by the
abundant development of spores which then takes place. This
is the commonest type of decay in the harvested fruit, whether
DISEASES OF MINOR FRUIT PLANTS 351
green or ripe, and is responsible for the largest part of the losses
which take place in storing and shipping pines. Its incidence in
the field, where it occurs on ripe fruits, is inconsiderable as a rule.
While infection usually takes place through wounds, experiments
have demonstrated that in a moist atmosphere the germinating
spores can penetrate the surface of uninjured fruit.
DiPLODiA Core-Rot.
In the traffic between these islands, pineapples which have
been some days in transit are frequently found to be infected
through the cut end of the stalk by a species of Diplodia. The
fungus passes up the vascular core of the fruit, from which it
spreads to the adjoining pulp. It produces a water-soaked
appearance of the tissues recently invaded, and these are later
darkened by the development of brown coloration in the
mature mycelium infesting them. The fructifications of the
fungus appear as dots under the rind of the fruit near the stalk,
later breaking through as pustules and discharging immature
white or mature black spores which in the latter condition are
two-celled.
Diplodia wound infections of ripe or ripening fruits are very
common in the West Indies (see p. 25), and the one or more
species causing them are quite generally distributed.
Prevention of Fruit Rots.
The measures to be adopted for the prevention of these
troubles are largely matters of handhng and packing arising
principally in connection with export ; though it must be remem-
bered that the intrusive fungi develop so rapidly in a tropical
climate that fruit intended for local consumption is liable to be
quickly spoiled.
In dealing with the fruit it should be taken for granted — it is
indeed usually a fact — that spores are present and only await
suitable conditions for their development. These conditions
are supplied by the presence of moisture and a certain degree of
heat. Unless the fruit is artificially cooled the latter may be
taken as in all cases sufficient, and it is on the control of the
former that success will depend. The requisite moisture may be
supplied by the juices of the fruit on cut or bruised surfaces, or
by the water given off by the fruit accumulating either on the
packing material or in surrounding air enclosed or allowed to
remain still.
It follows that the pineapples must throughout be handled
with the greatest care to avoid bruising or scratching. They must
not be broken off from the stem, but cut with a stalk 3 or 4
inches long, which itself must not be stripped. It would probably
352 DISEASES OF CROP-PLANTS
be of advantage to cut off an inch of the stalk when the fruit
is brought in from the field and then sear the freshly cut end or
dip it in melted adhesive wax. The fruit should be laid out
for twenty-four hours to cool and dry before packing. It is
obvious that the method of packing should be such as to prevent
contact between, or movement of, the fruit, while permitting free
circulation of air, and that storage and stowage should be in
well-ventilated places.
J. E. Higgins, in Hawaii, and Vera H. Charles, in Washington,
have experimented with the use of formaldehyde gas for fumiga-
ting pineapples previous to storage, the object being to kill the
spores adhering to the outside of the fruit. Both report very
considerable improvement in the keeping qualities of fruit so
treated.
Papaw.
The papaw is generally distributed and common throughout
the islands, grown usually in a scattered way as a fruit tree. In
Montserrat a peasant industry exists in the collection of coagu-
lated milk from the unripe fruits of plants which occur in con-
siderable numbers on uncultivated land. Attempts which have'
been seen to establish papaws in field cultivation suggest that
insect and fungus troubles would become severe under these
conditions, and plots of superior imported varieties have in
several cases failed owing to their greater susceptibility to
disease. Only casual attention has as yet been given to the
affections which occur.
Black Leaf-Spot.
The leaves of the papaw are liable to heavy infestation with
the fungus named below, which produces thickly sown and
conspicuous spots, rounded except where they impinge on a
vein, ranging in size from mere specks to about 3 mm. diameter ;
on the upper surface they are dark brown, with a whitish centre
in the larger examples, below they are a uniform velvety
black.
The fungus as usually met with is confined to a conidial form,
Asperisporium Caricce (Speg.) Maub., the uncertain taxonomic
position of which is shown by the variety of genera (Cercospora,
Fusicladium, Scolecothricum, Epiclinium, Pucciniopsis) in which
it has been placed.
The short vertical cylindrical-clavate conidiophores are
crowded side by side in a dense and even layer on the under
surface of the spot. Each has several minute papillae on its
rounded summit, and bears single terminal conidia in succession.
The conidia are thick-walled, rough, irregular in outUne, often
more or less pear-shaped, and i, 2 or occasionally 3-celled. A.
I
Fig. 147
Blossom End Rot of Tomato
Bull. 146, Florida Agri. Expt. Sia.
Fig. Ui
Septoria Leaf-spot of Tomato
From M. T. Cook's Diseases of Tropical Plants, by permission of Macmillan & Co.
DISEASES OF MINOR FRUIT PLANTS 353
Maublanc has recorded a perithecial form associated with the
spots, to which he has given the name Sphcerella CariccB.
The disease is reported from Brazil, Paraguay, Florida.
Bermuda and Porto Rico. It has recently been found in Trinidad,
and the only previous record from the Lesser Antilles known to
the writer is based on specimens forwarded by W. Robson from
Montserrat, with the information that the disease was only met
with in a locality which for some reason has proved distinctly
unsuitable for the cultivation of this plant.
Stem and Fruit Rot.
Localised stem rots of the papaw similar in their appearance
and perhaps identical have been seen in Montserrat, St. Vincent,
Barbados and Trinidad. A similar trouble has been mentioned
in Porto Rico. The causative organism has not with certainty
been isolated in any of these cases. Imported varieties are highly
susceptible, and considerable losses have occurred at times on
the local kinds. Plants growing under very favourable conditions
are liable to attack.
A water-soaked area first appears on the stem, apparently in
any situation, the tissues turn brown, and a depressed and
shrunken patch, which may be several inches long, is produced.
This soon becomes infested with bacteria and with fly and
beetle larvae, and usually leads to the failure of the leaves and the
breaking of the stem. Lesions which in their earlier stages are
similar in appearance may also occur on the fruit.
|A species of Gloeosporium or Colletotrichum has been found
in early and close association with the disease on stem and fruit
in material from Barbados and Montserrat. A Colletotrichum
is exceedingly common on failing leaves.
Cultures of a Fusarium were obtained in St. Vincent, but gave
doubtful results in infection experiments. A similar infection in
India has been produced by inoculation with Pytkmm hutleri.
No actual experience of control has been obtained, but, as
natural recovery sometimes occurs, it seems likely that the
cutting out of the patches in the stem before they are far ad-
vanced, and the treatment of the wound with an antiseptic
such as weak Jeyes fluid, might be effective.
Tomato.
Blossom-End Rot.
Blossom-end rot is a disease of tomatoes which seems to occur
in most if not all of the countries in which the fruit is grown.
In the West Indies the writer has seen it in Barbados, St.
Vincent, and Antigua, on plants grown in black loam, light
volcanic soil, and heavy clay respectively.
2A
354 DISEASES OF CROP-PLANTS
Description.
The affection is not a typical fruit rot. Though as much as
half the fruit may be collapsed and blackened the rest remains
sound unless affected by the secondary organisms which may
enter by way of the original injury. It appears first as a patch of
dark green water-soaked tissue at or near the blossom end of the
fruit, usually when this is one-half to two-thirds grown. The
patch darkens, becomes more distinct in appearance and definite
in outline and may remain small or spread until the distal half
of the fruit is covered. The affected part in a few days becomes
black and leathery, and usually more or less flattened or sunken.
Causation.
The disease has been the subject of much investigation, and
has been attributed by different authors to numerous fungus or
bacterial parasites. Several of these are liable to occur in the
later stages of the affection, and may extend its scope, con-
verting it into a real rot, but it is now coming to be generally
accepted that in its origin the affection is non-parasitic. Its
association with certain physical conditions, especially with
variations in water supply, has frequently been pointed out, and
C. Brooks published in 1914 the results of a wide range of studies
made to determine the influence of physical and chemical factors
on its prevalence.
The disease is most severe on vigorous plants in a very active
state of growth, and a sudden check to the water supply of
plants in this condition is one of the most usual ways in which
the disease is induced, while Brooks has further shown that
continued excessive watering produces the same effect. Most
nitrogenous manures, and especially stable manure, increase its
incidence, and liberal applications may cause the losses to become
very heavy. Aeration of the soil decreases the tendency to the
disease, and lime has some effect in modifying the influence of
heavy watering. The evidence available gives no clear indication
of the actual nature of the disease, or even of the mode of opera-
tion of the factors which induce it, but recognition of these
factors provides, so far as they are under control, the means of
reducing its incidence.
Control.
Where the plants are artificially watered care must be taken to
make the supply regular but not excessive. With either natural
or artificial watering the drainage and aeration of the soil should
be as good as it can be made. The prevention of excessive
transpiration by shelter and light shade, the effect of which may
to some extent be obtained by close planting, have good results.
The soil may also be protected by trash of some description.
In an instance coming within the writer's experience heavy
DISEASES OF MINOR FRUIT PLANTS 355
losses were stopped by restricting watering to one application in
the early morning, trashing the soil, and shading the plants
lightly with muslin.
There is distinct varietal difference in susceptibiHty to
blossom-end rot. Brooks reports trials in which the percentage
of loss was zero with Bonny Best, low with Stone and Golden
Queen, medium with Earliana and Ponderosa, and heavy with
Matchless.
Leaf-Mould.
The leaf-mould disease of tomato foliage has been seen by the
writer on plants in St. Vincent, Barbados and Trinidad, grown
from American seed. It is reported by J. A. Stevenson to be
destructive to the first-crop (December-February) plantings in
Porto Rico, but absent from the second (April-June) crop. In
Trinidad its prevalence appears to be one of the main factors in
the failure of this crop during the wet season.
Description.
The disease is caused by the fungus Cladosporium fulvum Cke.
The conidiophores emerge on the underside of the leaves, forming
dense velvety patches of indefinite extent, white or grey at first,
later smoky brown. On the upper surface the leaves show yellow
patches which turn brown as the tissues die. The fungus also
appears in streaks aloiig the stem, and may form scattered patches
on the fruit. The conidia are elliptical, somewhat elongated, very
variable in size, mostly one- but sometimes two- or three-
septate,
The disease spreads rapidly imder moist and shaded con-
ditions, and where these continue it can largely defoliate the
plants. It is inhibited by dry air and sun.
Control.
No varietal resistance to the disease appears to have been
recorded. Spraying with Bordeaux mixture has been recom-
mended as effective in the early stages of infection, but Stevenson
reports that repeated applications started before the fungus
appeared had no good effect in the experiments observed by him.
Choice of season for planting and avoidance of humid situa-
tions are the measures indicated where this affection has proved
troublesome.
Septoria Leaf-Spot.
Leaf-spot is recognised as the most serious disease of tomatoes
occurring in the United States, where it has earned distinction
as the " tomato blight." It was first reported from the Argentine,
is widely distributed in Europe, and occurs in Austraha and
356 DISEASES OF CROP-PLANTS
South Africa. It is destructive in Porto Rico, and, though it
does not seem to have been recorded from the Lesser Antilles,
may with some confidence be expected to occur.
Description.
A recent and full investigation of leaf-spot has been made by
G. H. Coons and E. Levin. The disease is mainly one of the
leaves, beginning with small water-soaked spots on the underside,
which later penetrate the leaf and dry into definite more or less
circular spots, firm in texture, dark or greyish-white, with a
black or brown margin. Typically the spots are small, not
exceeding one-eighth of an inch in diameter, but they may by
coalescence approach to half an inch. In each spot the pycnidia
of the causative fungus occur to the number of 3 to 10, appearing
to the naked eye as black dots, usually situated on the upper
surface of the leaf.
The lower leaves are first attacked, and as the spots become
numerous they turn yellow, wither, and fall. In severe infesta-
tions the disease extends to successively younger leaves until
only terminal tufts are left. The attacks of the fungus on stems
and fruits are of minor importance. The quantity and quality
of fruit produced are affected in proportion to the injury to the
leaves, and total failure of crop may ensue. The disease readily
attacks seedlings and young plants, and is often carried by them
into the field.
Causative Fungus.
The disease has been demonstrated by infection experiments
to be caused by the fungus Septoria lycopersici Speg. The
mycelium is of two types, a hyaline thin-walled vegetative form,
and a dark closely septate thick-walled type associated with
reproduction. The former is intercellular with knob-like haus-
toria. The spores from the pycnidia are filiform and hyaline,
60-120 X 2-4 microns, with 2-8 septa. The thickness of a spore
is not uniform throughout and the shape of the ends varies from
pointed to globose. The spores are exuded under the influence
of moisture in mucilaginous masses which disintegrate in water.
Conditions of Occurrence.
The spores are washed and splashed about by rain and are
readily transported during the handling of the crop under wet
conditions. In dry weather the disease does not spread, and the
spores do not appear to be able to tolerate drying for more than a
few days. Infection is stomatal, and takes place for the most
part and with greater effect on the lower surfaces of the
leaves. The source of the infectious material initiating an out-
break appears to be mainly the remains of previous crops. It is
not known that infection can be introduced with the seed.
DISEASES OF MINOR FRUIT PLANTS 357
The mycelium does not grow weU at high temperatures, and
it is quite likely that this character prevents the disease from
assuming importance in these islands.
Control.
The use of clean seed in clean soil is the best means of pre-
vention. Training the plants on stakes reduces infection and
enables spraying, when it becomes necessary, to have more
effect. Under favourable conditions the periodic use of Bordeaux
mixture has given good results, but these are in general uncertain,
and J. A. Stevenson reports that no benefits resulted from re-
peated applications in trials made in Porto Rico.
Bacterial Wilt.
During a visit of the writer to St. Vincent in 1917, some
attention was given to a wilt disease which was found to be
occurring extensively at the Experiment Station, Kingstown, in
beds of hybrid tomato plants (Ponderosa x Native) and in a
row of the variety Earliana from American seed.
The symptoms of the affected plants, the appearance of the
associated bacterium under the microscope and on agar cultures,
and its virulence when inoculated into healthy plants, leave no
reason to doubt the identity of the disease with the wilt due to
Bacterium (Bacillus) solanacearum, E. F. Sm.
This disease is very prevalent in the southern United States,
and makes the growing of tomatoes impossible over large tracts
of country. A closely similar, probably identical, affection has
been reported in many countries of the Old World. Several
other members of the natural order Solanaceje, especially egg-
plant, potato, and Datura spp. are liable to serious diseases
produced by the same organism. The whole series has been
grouped together under the name of Brown Rot of Solanaceae,
and very fully treated in Volume III of E. F. Smith's " Bacteria
in relation to Plant Diseases."
So far as the records of the Imperial Department show, the
disease has only been previously reported in the Lesser Antilles
on one occasion, on tomatoes from Union, St. Lucia, in January,
1904.
Symptoms.
The outstanding character of the affection on tomatoes, as
seen in St. Vincent, is the wilting, usually sudden, of the whole
plant, which has just the appearance which would be produced
by severing the base of the stem from the roots. No preliminary
browning of leaves or stems was observed. When the bacterium
was artificially introduced into a single stem, wilting above the
point of inoculation took place in from two or three days to a
week, while downward infection proceeded only slowly.
358 DISEASES OF CROP-PLANTS
Since wilting of the same type is produced by Fusarium dis-
ease, and would ensue from any other cause which cut off the
supply of water through the stem, it is necessary for the purpose
of a field diagnosis to have some further evidence of the presence
of this particular disease. Confirmation fairly satisfactory for
this purpose may be obtained by cutting across the stem at
intervals with a clean sharp knife. When the region of infestation
is reached the woody ring in the stem is found to be discoloured
brown, and in a short time dirty white or brownish beads of
bacterial ooze collect on the ends of the cut vessels. In the
region of serious infestation this discharge is copious enough to be
quite apparent to the naked eye. The pith may or may not be
discoloured ; the principal seat of the disease is in the vascular
tissues, from which circumstance, when the infestation of any
section of the stem is complete, the wilting of all parts of the
plant beyond that section follows.
Susceptibility.
There seems to be no evidence of varietal resistance in tomatoes
given in connection with the published studies of this disease.
Solanum mammositm is reported from Porto Rico to be resistant,
and the suggestion has been made to use it as a stock for grafting
tomato and egg-plant. At the St. Vincent Station natural
infection occurred to a considerable extent in American varieties
of the Ponderosa type (which includes Earliana). The type with
small round fruits, common in these islands and reputed to be
native, proved fully susceptible to inoculations. A bed of F 2
hybrids between Ponderosa and Native was with few exceptions
destroyed when in full bearing. It is very much open to doubt
whether the survivors were really resistant to the disease, in view
of the known lessening of susceptibility to infection in matured
plants. From whatever circumstances arising, some of the plants
did resist artificial infection at this stage, and cuttings were taken
from the survivors for further trial. A batch of hybrid seedlings,
inoculated through needle pricks when a few inches high, suc-
cumbed in five days, showing at the point of introduction of the
bacterium an infestation which was greatest in the protoxylem
and the immediately adjacent cortical parenchyma. Attempts
to infect tomato fruits with Bact. solanacearum resulted in slight
local injuries which developed no further. This is in agreement
with the results of previous investigators.
Infection.
The plants in which natural infections occurred were trans-
planted seedlings, in some cases very widely spaced, and the
distribution of the diseased plants was of a sporadic type. In
one rather close-planted row of Earliana several scattered plants
died when coming into fruit without infecting their neighbours^
DISEASES OF MINOR FRUIT PLANTS 359
The failure of attacked plants was completed at one stroke, and
the indication given by this feature was confirmed in all the
cases examined by the finding of the most severely affected
sections at or near the base of the stem, and in one or more of the
principal roots.
It seems on the whole most likely that infection had pro-
ceeded from the soil, probably, in some cases examined, through
wounds made by removing basal shoots. The possibility remains,
however, that wounds kept moist by nearness to the soil may have
been more susceptible to infection from the pruning knife than
those higher up, and it is somewhat easier to conceive of the
knife becoming contaminated than the soil in some of the cases
noted, where no Solanaceous plants appear to have been grown
in recent times.
Control.
According to E. F. Smith, it seems probable that the organism
is strictly a wound parasite. He has shown that it may be
transmitted by insects, and is inclined to attach great importance
to the action of nematode worms in opening the way for root
infections, regarding their presence as prohibitive of tomato-
growing in infected soil. The St. Vincent plants did not show any
obvious signs of the presence of nematodes. The further pre-
caution on which most dependence must be placed, according to
the author quoted, Ues in avoiding even trifling injuries to the
roots in transplanting. For this reason, if the seed cannot be
germinated in situ, the transfer must be made with great care
while the seedlings are still small enough to make the avoidance
of injury possible. It is also obvious that infections may be
easily made in pruning. A branch may be infested for a part of its
circumference while not yet showing signs of wilt, and the cutting
of it will smear the knife with large numbers of the causative
bacteria. It would be safest to avoid pruning altogether when
infected plants are known to be present.
It should be noted in conclusion, that it is reported from
North Carolina and from Sumatra that ground nuts grown on
infested soil have proved susceptible to the attacks of Bacterium
solanacearum.
Bubbly Fruit-Rot.
At the Kingstown Experiment Station, St. Vincent, in 1917,
there came under the writer's observation a bacterial rot of
tomatoes of a very swift and decisive nature.
Bacteria which invade injuries, and cause ripe rot of tomato,
as of other fleshy fruits, are not at all uncommon, but the one in
question was distinguished by its ability to cause very destructive
effects in fruit of any age, and of any size from the small "Native "
to the large fruits of Earhana. Its outstanding character was the
36o DISEASES OF CROP-PLANTS
formation of very large or very numerous gas bubbles under the
transparent skin of the fruit, replacing the pulpy tissues as they
were rapidly broken down. Finally, the whole content of the fruit
except the seeds was reduced to slime and gas, which escaped
sooner or later by the rupture of the pendent skin, leaving the
latter empty on the stalk. Needle-point inoculation of half- ripe
fruits in the evening gave rise to a hollow an inch in diameter in
two or three hours, and by next morning the process was com-
plete. Smearing the skin with the slime produced did not
cause infection unless a crack or other wound was present. The
natural infections examined appeared to have their origin in bug
punctures, made by Nezara and Leptoglossus. The losses due to
the disease were significant but not severe.
Growths of the bacterium, apparently pure, were obtained by
transfers from one fruit to another, and consisted of long, actively
motile rods. Cultures were obtained on nutrient agar but time
could not be spared from other work to continue the study
further. Inoculations from diseased fruits into succulent stems
had no effect.
No other record has been seen of a disease answering to this
description.
CHAPTER XXX
DISEASES OF PLANTS NOT CLASSIFIED.
Agave.
Anthracnose.
Various species of Agave, including sisal, are subject to a
leaf disease of wide distribution caused by Colletotrichum Agaves
Cav. There are records of its occurrence in the West Indies in
Antigua, Trinidad, and British Guiana.
Dark spots are caused on the leaves which may remain local
or result in a general sickly appearance and, according to reports,
the eventual death of the plant.
The pustules, which produce masses of pink or orange-coloured
conidia, are usually arranged in concentric rings.
Removal of infested leaves and spraying with Bordeaux
mixture may be expected to control the disease.
Rose.
Black Spot.
The commonest disease of roses in the West Indies as else-
where is the black spot of leaves caused by infestation with the
fungus Diplocarpon Rosce Wolf {Actinonema Rosce Fr.).
The disease reveals itself usually in mature or old leaves,
which show first brownish blotches with fringed edges and set
with black dots. Later the blotches turn black and examination
with a lens shows upon them a raised pattern of radiating lines.
The roundish dots are at first covered by the cuticle, which later
bursts irregularly and reveals beds of two-celled conidia. The
radiating lines mark the course of sub-cuticular hyphae, while
another set of hyphae penetrate the tissues of the leaf.
The result of the infestation is that the affected leaves turn
yellow and fall. When this is confined to old leaves not much
harm is done, but under weather conditions favouring the disease
repeated defoliations may occur, which weaken the plants and
result in some dieback of the twigs.
The disease may be controlled by timely applications of
Bordeaux mixture or lime-sulphur, but, as deposits left by these
sprays are objectionable in appearance, a preferable treatment,
which is simple and at least as effective, is dusting with a mixture
of fine sulphur 9 parts and powdered arsenate of lead, i part.
361
362 DISEASES OF CROP-PLANTS
The first application should be made on the appearance of the
first signs of the disease or at the beginning of seasons in which
experience has shown it to be prevalent.
Mildew.
Certain varieties of roses, particularly of the rambler class,
are subject to severe attacks of powdery mildew on developing
shoots and young fohage. Only the Oidium stage has been
observed in these islands, consisting of a white surface mycelium,
on and about which a powdery deposit of conidia occurs. The
fungus is assumed to be the conidial stage of the common rose
mildew Sphcerotheca pannosa Lev.
Dusting with sulphur, or with sulphur 9 parts and arsenate of
lead powder i part, forms the most effective means of control.
Other Diseases.
A disease referred to by J. B. Rorer as canker occurs in
Trinidad, and an affection probably identical is somewhat
common in Barbados. Small purplish spots appear on the stems
and gradually increase in size, sometimes even girdling the branch.
The bark becomes shrivelled and sunken and in the later stages
is covered with the bright red stalks and red globular heads of a
species of Stilbella (Stilbum). Affected branches should be
pruned out, or the whole crown is liable to become infested.
Rorer records a leaf-spot due to Cercospora rosicola Pass.,
which causes rounded spots which are yellowish with a distinct
brown or purplish margin, and also reports very severe effects
from infestation of the stems with the algal disease, described
in Chapter XVL
Fig. 149 Black Spot of Rose
Fig. 150 Rose Mildew
PRINCIPAL REFERENCES
The Author regrets that his transfer to a situation with more limited
library facilities has prevented the preparation of a complete list of the
sources to which acknowledgment is due. The following list is intended
for the direction of readers to some of the principal papers dealing with
the subject concerned.
CHAPTER I. NOTABLE GROUPS CONCERNED IN FUNGUS
DISEASES.
Duggar, B. M. Rhizoctonia. Fungous Diseases of Plants, pp. 444-452.
Duggar, B. M. Rhizoctonia crocorum (Pers.) D.C. and R. solani Kiihn
(Corticium vagum, B and C). Ann. Missouri Bot. Gard. II,
pp. 403-458. 1915.
Petch, T. On Lasiodiplodia. Ann. Roy. Bot. Gard. Ceylon, IV. p. 445.
1910.
Peltier, G. L. Illinois Agr. Exp. Sta. Bull. 189 (1916). Rhizoctonia.
Stevens, G. L. The genus Meliola in Porto Rico. Illinois Biol. Monographs
II. iv. (1916).
CHAPTER II. DISEASES CAUSED BY BACTERIA.
Smith, E. F. Bacteria in relation to Plant Diseases. Vols. I — III.
Bacterial Diseases of Plants. 1920. Carnegie Institute, Washington.
CHAPTER III. DISEASES CAUSED BY INFECTIVE VIRUSES.
Allard, H. A. Papers on Mosaic Disease. Jrn. of Agric. Res. Ill, 295 ;
V, 251 ; VI, 649 ; X, 615 ; XIII, 619.
CHAPTER V. DISEASES DUE TO NEMATODES.
Butler, E. J. The Rice Worm and its Control. Mem. Dept. Agric. India
X, i. (1919).
Taylor, A. M. Black Currant Eelworm. Jrn. Agric. Sci. VIII, 246. 1917.
CHAPTER VI. THE RELATION OF INSECTS TO PLANT DISEASES.
Pierce, W. D. and Rand, F. V. A Co-ordination of our Knowledge of
InsectTransmission in Plant and Animal Diseases. Phytopathology X.
p. 190.
CHAPTER VIII. ENTOMOGENOUS FUNGI.
Morrill, A. W. and Back, E. A. Natural Control of White Flies in Florida.
U.S. Dept. Agr. Bur. Ent. Bull. 102. 1912.
Johnston, J. R. The Entomogenous Fungi of Porto Rico. Bd.ofCommrs.
of Agr. P. R. Bull. 10.
CHAPTER X. FUNGICIDES AND THEIR APPLICATION.
Butler, O. Bordeaux Mixture. Phytopathology IV, 125. 1914.
The Cuprammonium Washes. Phytopathology VII, 235. 191 7.
Thomas, C. C. Seed Disinfection by Formalin Vapour. Jrn. Agric. Res
XVII, 33.
363
364 DISEASES OF CROP-PLANTS
CHAPTER XII. THE CONTROL OF DAMPING-OFF.
Hartley, C. and Pierce, R. G. Control of Damping-off of Coniferous
Seedlings. U.S. Dept. Agric. Bull. 453. 1917.
CHAPTER XV. ROOT DISEASES.
Rosellinia Root Diseases.
Fetch, T. Root Disease of Tea. Circ. and Agr. Jrn. R. B. G. Ceylon.V.
No. II, 1910.
Nowell, W. Rosellinia Root Diseases in the Lesser Antilles. West
Ind. Bull. XVI, 31. 1916.
Sclerotium Rots.
Taubenhaus, J. J. Recent Studies on Sclerotium Rolfsii Sacc. Jrn.
Agric. Res. XVIII, 127. 1919.
CHAPTER XVI. STEM AND LEAF DISEASES.
Pink Disease.
Brooks, F. W. and Sharpies, A. Pink Disease. Dept. Agr. Fed.
Malay States Bull. 21. 1914.
Pink Disease of Plantation Rubber. Annals of Applied Biology II, 58.
Thread Blights.
Burt, E. A. Corticiums causing Pellicularia Disease of the Coffee
Plant, Hypochnose of Pomaceous Fruits, and Rhizoctonia
Disease. Ann. Missouri Bot. Gard. V. 119. 1918.
Stevens, F. L. and Hall, J. G. Hypochnose of Pomaceous Fruits.
Ann. Mycologici VII, 49. 1909.
Fawcett, G. L. Fungus Diseases of Coffee in Porto Rico. P. R. Agric.
Exp. Stn. Bull. 17. 1915.
Horsehair Blight.
Fetch, T. Horsehair Blight. Ann. Bot. Gard. Ceylon VI, 43.
Algal Disease.
Rorer, J. B. Algal Disease of Cacao. Proc. Agr. Soc. Trinidad XVII,
345- 1917-
Butler, E. J. Red Rust; p. 413. Fungi and Disease in Plants.
Calcutta, 1918.
CHAPTER XVII. DISEASES OF CACAO.
General.
South, F. W. Fungus Diseases of Cacao. West Indian Bull. XII,
277. 1912.
Canker and Pod-rot.
Rorer, J. B. Bulletin Dept. Agric. Trinidad IX, 79.
Fetch, T. Circulars and Agric. Jrnl. R. B. G. Ceylon V, No. 13. 1910.
Van Hall, C. J. J. Meded. Laboratorium von Plantenziekten, Java.
No. 30.
Witch-Broom.
Stahel, G. Dept. v. d. Laudbouw, Suriname. Bulls. 33, 39.
Monilia Pod-rot, Sphceronema Diseases.
Rorer, J. B. Enfermedades y Plagas del Cacao en el Ecuador. 1918.
Anthracnose.
South loc. cit. Bancroft, C. K. West Ind. Bull. XI, 34. 1911.
Van Hall, C. J. J. Meded. Proefstat Midden — Java Nos. 6, 1912 ;
14. 1914-
PRINCIPAL REFERENCES 365
CHAPTER XVIII. DISEASES OF COCONUT.
Red Ring Disease.
Nowell, W. West Indian Bull. XVII, 189. 1919 ; XVIII. 73. 1920.
Cobb. N. A., ibid. XVII. 203. 1919.
Bacterial Bud-rot.
Johnston, J. R. Bureau of Plant Industry, U.S.A. Bull. 228.
Ashby, S. F. Bull. Dept. Agric. Jamaica II. viii. 299. 1915.
Rorer. J. B. Board of Agriculture. Trinidad. Circ. 4. 191 1.
Phytophthora Bud-rot.
Ashby. S. F. West Indian Bulletin XVIII, 61. 1920.
Stem-bleeding Diseases.
Petch, T. Circ. and Agr. Jrnl. R. B. G. Ceylon IV, xxii. 1909.
CHAPTER XIX. DISEASES OF LIMES AND OTHER CITRUS
TREES.
Red Root Disease.
South. F. W. West Indian Bull, XII, iv, 490. 1912.
Collar Canker.
South, F. W. Rpt. Agric. Dept. Dominica. 1912-13.
Fawcett, H. S. Phytopathology X, 397. 1920.
Galls.
Tenney, L. S., Hedges, F. and Bureau Plant Industry, U.S.A. Bull.
247.
Hedges, Florence. Sphseropsis tumefaciens. Phytopathology 1,6 3.
1911.
Anthracnose of Limes.
Clausen, R. E. Phytopathology II., 217. 1912.
Grey Blight.
Burt, E. A. Ann. Missouri Botan. Gard. Ill, 335. 1916.
Citrus Anthracnose.
Clausen loc. cit.
Scab.
Stevenson, J. A. Dept. Agric. Porto Rico. Bull. 17. 1917.
Canker.
Hasse, Clara H. Pseudomonas citri : the cause of Citrus Canker.
Jrn. Agric. Research IV. 97. 1915.
Stevens, H. S. Florida A. E. S. Bulls. 124, 128.
Wolf. F. A. Jrn. Agric. Research VI. 69.
Jehle, R. A. Quart. Bull. PI. Board, Florida I. 1916, 2 ; 1917, 24.
Oil Spot.
Fawcett, H. S. California A. E. S. Bull. 226.
Charcoal Rot.
Wilson. O. T. Notes upon a market disease of limes. Phytopathology
VIII. 45. 1918.
CHAPTER XX. DISEASES OF COFFEE.
General.
Fawcett. G. L. Fungus Diseases of Coffee in Porto Rico. Porto
Rico Agr. Exp. Stn. Bull. 17. 1915.
366 DISEASES OF CROP-PLANTS
Viruela.
Fawcett, loc. cit.
Maublanc, A. and Rangel, E. Bull, de Soc. Mycol. de France XXX,
41. 1914.
Sclerotium Disease.
Stahel, G. Dept. Land. Suriname. Bull. 42. 1921.
CHAPTER XXI. DISEASES OF HEVEA.
Stahel, G. De Zuid-amerikaansche Hevea-Bladziekte. Bull. 34,
Dept. Land. Suriname. 1917.
CHAPTER XXII. DISEASES OF MINOR FRUIT TREES.
Avocado.
Anthracnose.
Rorer, J. B. The Fungus Diseases of the Avocado. Bull. Dept. of
Agric. Trinidad XVIII, p. 132. 1919.
Mango.
Anihyacnose.
Rorer, J. B., ibid., XIV, 164. 1915.
CHAPTER XXIII. DISEASES OF BANANA.
General.
Ashby, S. F. B«ll. Jamaica Dept. Agric. II, vi, 95. 1913.
Panama Disease.
Ashby loc. cit.
Brandes, E. W. Banana Wilt. Phytopathology IX, 339. 1919.
Moko Disease.
Rorer, J. B. Phytopathology I, 45. 1911.
Black Spot of Leaves et seq.
Ashby loc. cit.
CHAPTER XXIV. DISEASES OF CORN AND SORGHUM.
Dry Rot.
Van der Bijl, P. A. Dept. of Agric. South Africa, Sci. Bull. 7. 1916.
Head Smut.
Potter, A. A. Jrn. Agric. Res. II, 339. 1914.
McAlpine, D. Jrn. Dep. Agric. Victoria VIII, 5, 290. 1910.
CHAPTER XXV. DISEASES OF COTTON.
General.
Edgerton, C. W. The Rots of the Cotton Boll. Louisiana Bull. 137.
1912.
Internal Boll Disease.
Nowell, W. The Fungi of Internal Boll Disease. West Indian Bulletin
XVI, 152.
Internal Disease of Cotton Bolls in the West Indies, ibid., XVI, 203 ;
XVII, I.
PRINCIPAL REFERENCES 367
Angular Spot.
Rolfs, P. H. Angular Leaf-spot of Cotton, S. Carolina Bull. 184,
1915-
Smith, E. F. Bacterial Diseases of Plants, p. 314, 1920.
Bacterial Boll Disease, Diplodia Boll Rot, Anthracnose.
Edgerton, C. W., loc. cit.
Curly Leaf, Loggerhead.
Nowell, W. Two Physiological Affections of Sea Island Cotton.
West Indian Bulletin XIV, 304. 1914.
Yellow Leaf Blight.
Atkinson, G. F. Some Diseases of Cotton. Alabama Bull. 41. 1892.
Alternaria Leaf-Spot.
Faulwetter, R. C. The Alternaria Leaf-spot of Cotton. Phytopath-
ology VIII, 98. 1918.
Leaf Mildeiv.
Wakefield, E. M. On two species of Ovulariopsis from the West
Indies. Kew Bulletin. 1920. 235.
Crinkled Dwarf.
Harland, S. C. Genetics of Crinkled Dwarf Rogues. West Indian
Bulletin XVI. Nos. i, 1916 and 4, 1918.
Mayi Cotton.
Harland, S. C. Note on a peculiar Type of Rogue in Sea Island Cotton.
Agricultural News. Barbados, XIX, 29. 1920.
Mottled Hybrids.
Nowell, W. West Indian Bull. XIV, p. 310. 1914.
CHAPTER XXVI. DISEASES OF SUGAR-CANE.
General.
Wakker, J. H. and Went, F. A. F. C. De Ziekten van het Suikerriet
op Java, Leyden. i8g8.
Howard, A. On some Diseases of the Sugar-Cane in the West Indies.
Annals of Botany, Vol. XVII, pp. 373-411. 1903.
Johnston, J. R., et al. Diseases of Sugar-Cane in Tropical and Sub-
Tropical America, especially the West Indies. West Indian
Bulletin, Vol. XVI, pp. 275-308. 1918.
Johnston, J. R., and Stevenson, J. A. Sugar-cane Fungi and Diseases
of Porto Rico. Journal of the Department of Agriculture of
Porto Rico, Vol. I, No. 4. 1917.
Butler, E. J. Fungi and Disease in Plants, Chap. XI. Calcutta,
1918.
Cobb, N. A. Fungus Maladies of the Sugar-Cane. Hawaiian Sugar
Planters' Association, Bull. 5. 1906.
Root Disease.
Wakker & Went, Howard, Johnston, Johnston & Stevenson loc. cit.
Bud Rot.
Wakker & Went, Howard, Butler, loc. cit.
Rind Fungus.
Howard, loc. cit.
Johnston, J. R. History and Cause of Rind Disease. Journal Board
of Commissioners of Agriculture, Porto Rico, Vol. I, pp. 17-45.
1917-
368 DISEASES OF CROP-PLANTS
Pineapple Disease.
Wakker & Went, Cobb, loc. cit.
Howard, A. Field Treatment of Cane Cuttings, West Indian Bulletin,
Vol. Ill, pp. 73-88. 1902.
Fetch, T. Thielaviopsis. Ann. Roy. Bot. Gard. Ceylon IV, 5 1 1 . 1910.
Diplodia Rot.
Howard, A. Diplodia cacaoicola, a Parasitic Fungus on Sugar-cane
in the West Indies. Annals of Botany, Vol. XV, pp. 683-701.
1901.
Butler loc. cit.
Wilt Disease.
Butler, E. J. & Khan, A. H. Some new Sugar-Cane Diseases. Memoirs
Dept. of Agriculture India, Vol. VI, No. 6. 1913.
Top Rot.
Wakker & Went, loc. cit.
Cobb's Disease.
Smith, E. F. Bacteria in relation to Plant Diseases, Vol. III. Wash-
ington, 1914.
Smut.
Butler, loc. cit.
Iliau.
Lyon, H. L. Iliau, an Endemic Cane Disease. Hawaiian Sugar
Planters' Association. Bull. XI. 1912.
Cytospora Rot.
Johnston, J. R. and Stevenson, J. A., loc. cit.
Leaf-sheath Rot.
Wakker & Went, Johnston & Stevenson, loc. cit.
Sereh.
Wakker & Went, loc. cit.
Lyon, H. L. Three Major Cane Diseases : Mosaic, Sereh and Fiji
Disease. Bull. Hawaiian S. P. A. Bot. Series III, pt. i. 1921.
Mosaic Disease.
Stevenson, J. A. An Epiphytotic of Cane Disease in Porto Rico.
Phytopathology, Vol. VII. 418. 191 7.
Brandes, E. W. Mosaic Disease of Sugar-Cane and other Grasses.
U.S. Dept. Agric. Bull. 829. 1919.
Lyon, H. L., loc. cit.
Chlorosis.
Tempany, H. A. The Gall Patches in Antigua Soils. W. I. Bulletin
Vol. XVI, pp. 137-144. 1917.
Fiji Disease.
Lyon, H. L., loc. cit.
CHAPTER XXVII. DISEASES OF ROOT CROPS.
Aroids.
Dry Rot.
Ashby, S. F. Jamaica Dept. Agric. Bull. II, vi, p. 150.
Onion.
Bacterial Rot.
Stewart, F. C. New York (Geneva) Agr. Exp. Sta. Bull. 164. 1899.
PRINCIPAL REFERENCES 369
CHAPTER XXVIII. DISEASES OF LEGUMINOUS PLANTS.
Ground Nut.
Fungus Diseases of Ground Nut in W. I.
South. F. W. West Ind. Bull. XI, p. 157. 1911.
Leaf Spot.
Butler, E. J. Fungi and Disease in Plants. Calcutta, 1918.
Kidney Beans.
Anthracnose, Bacterial Blight.
Muncie. Michigan, Sp. Bull. 68. 1914. Tech. Bull ^tS lor?
Barrus, M. F. Phytopathology I, p. 107. 191 1.
CHAPTER XXIX. DISEASES OF MINOR FRUIT PLANTS.
Pineapple.
General.
Henrickson & lorns. Porto Rico Agric. Exp. Stn. Bull. 8 1909
Larsen, L. D. Hawaiian Sugar Planters' Association Bull. 10 1910
Stockdale, F. A. West Ind. Bull. VIII, p. 158. 1908.
Papaw.
Black Leaf-spot.
Maublanc, A. Bull. Soc. Mycol. de France XXIX, p. 353. 1913.
Tomato.
Blossom End Rot.
Brooks, C. Phytopathology IV, v, p. 345. 1914.
Septoria Leaf-spot.
Coons, G. H. and Levin, E. Michigan Tech. Bull. 25. 1916.
GLOSSARY OF WEST INDIAN TERMS
ANAUCO — the mountain immortelle {E. micropteryx, Poepp).
B. 208, ETC. — seedling sugar canes raised by the Barbados
Department of Agriculture.
BLACK BLIGHT — investment of leaves, fruits, etc., with a sooty
covering of fungus mycelium (see page 44).
BLACK EYE PEA — a form of cow pea.
BLUGGOE — a coarse cooking banana in Grenada (the moko of
Trinidad).
BOCARE — the lowland immortelle (£". glauca, Willd).
Bourbon cane — a variety of sugar-cane, in general cultivation
in the W.I. until near the end of the Nineteenth Century ;
believed to have been introduced from Otaheite.
Canary banana — see Chinese banana.
CANE-FLY — Stenocranus {Delphax) saccharivorus.
CASSAVA — Manihot utilissima.
Chinese banana — Musa Cavendishii, Lamb ; also called Canary
or Governor banana,
coco — Jamaican name for Xanthosoma sp.
corn — confined in the West Indies to maize.
COTTON STAiNERS — plant bugs of the genus Dysdercus.
COW-PEA — Vigna catjang.
Criollo cacao — a superior and more delicate variety of cacao
producing white beans, apparently at one time in general
cultivation in Trinidad, but now almost extinct in its
pure form.
CUSHION — the raised areas on cacao stems developed in con-
nection with repeated localised production of flowers
and fruit.
CUTLASS — a large and heavy knife used for chopping, pruning
and light digging.
D. 109, ETC. — seedling sugar-canes raised by the British Guiana
Department of Agriculture.
DASHEEN — a cultivated aroid of the genus Colocasia, a common
food plant in the wetter islands.
372 DISEASES OF CROP-PLANTS
EDDOE — a cultivated aroid of the genus Colocasia, a common
food plant.
EYES — dormant buds, especially in sugar-cane.
FROGHOPPER (sugar-cane) — Tomaspis saccharina, a serious pest
in Trinidad.
GALEA — Calophyllum calaha, a guttiferous tree used for wind-
breaks.
GOVERNOR BANANA — see Chinese banana.
GREEN BUG — Nezara viridula.
green scale — Coccus viridis (Lecanium viride).
Gros Michel — the common large-fruited commercial banana.
Guinea corn — Andropogon Sorghum, a variety with compact
heads.
immortelle — ^leguminous trees of the genus Erythrina, used
generally for shading cacao fields.
imphee — Andropogon Sorghum, a variety with loose panicles.
LEAF blister MITE (cotton) — Eriophyes gossypii, Banks.
lemon grass — Cymhopogon citratus (DC) Stapf, a scented coarse
grass.
mandarin — Citrus nohilis, an orange with small dark foliage
and red loose-skinned fruit.
muscovado — sugar made on the old open-pan system.
okra — Hibiscus esculenius, the succulent young capsules of
which are used as a vegetable.
PAPAW — Carica papaya.
PEN manure — manure accumulated for several months on the
floor of stock pens, usually with liberal additions of cane
trash or other litter.
PIGEON PEA — Cajanus indicus.
plantain — Musa paradisiaca, Linn., a distinct type of cooking
banana tending to be restricted to forest soils.
PLANT-CANE — the first crop of sugar-cane from planted cuttings,
pois Doux — Inga spp. Leguminous trees used as windbreaks.
RATOONS — the second and subsequent crops of sugar-cane
obtained without replanting.
ROUGH LEMON — Gitrus Hmonum, Risso, a lemon with large rough-
skinned fruits.
Sea Island cotton — long staple cotton of the type developed
in certain islands off the coast of S. Carolina.
SHADDOCK — Citrus decumana, var., producing a very large and
rather dry fruit.
SILK FIG — ^Trinidad name for a variety of banana mostly grown
as temporary shade on cacao estates ; fruit with a thin
smooth skin.
GLOSSARY OF WEST INDIAN TERMS 373
SOUR ORANGE — CUtus vulgaHs Risso, a tree with a scattered
distribution about estates or villages, useless in itself,
but valuable as a stock.
STRAINER — the fibrous canvas-like material developed at maturity
from the sheathing base of the coconut leaf.
Sudan grass — Andropogon Sorghum sudanensis, Piper.
SUPPLIES — plants or cuttings used to fill up gaps in a planted
field.
SYRUP — cane juice concentrated in open pans.
SWEET POTATO — Ipomcea batatas.
tannia — cultivated aroids of the genus Xanthosoma.
TRASH — the old dry leaves, especially of sugar-cane and banana.
Uba — a thin reed-like sugar-cane of the Indian type, used to
some extent in the West Indies for planting on poor land.
yam — Dioscorea spp.
INDEX
Page
Aciinotiema Rosce 361
Adams, J. F. - 260
iEcia ; jecidia ; aecidiospores - - - 18
Mcidium oxalidis ..... 256
^gerita 43
Agaricaceae 40
Agave, diseases of 361
Albugo 36
A . Ipomoece-pandurance - - . - 336
Alectra 58
Alfalfa, collar rot of 28
Algae and Fungi, distinguished - - 3, 6
Algal Disease 152
Allard, H. A. 49
Alligator cacao (Theobroma pentagona) - 167
Alternaria 43
leaf-spot 284
A . tenuis - . 284
American Leaf Disease .... 226
Ammonium sulphide - - - - 97
Amphispores 18
Angular spot - - 13, 268-71, 273 et set].
Anstead, R. D. 239
Anthracnose ... 13, 14, 22, 44
Agave 361
Avocado 233
Cacao 174
Cotton - - . 271, 272, 277-8
Grape-Vine 236
Kidney beans .... 340
Lime 207-9
Mango 237
Spotting 210, 217, 224
And see Citrus.
Antigua.
Agave Anthracnose .... 361
Agricultural conditions - - . 117
Brown rust of corn .... 256
Cotton disease 279
Entomogenous fungi - - - - 74
Pomes Root rot - - - - - 147
Onion disease 336
Pineapple cultivation and diseases 344,349
Root disease of corn - - - - 260
Sugar diseases ... 293, 296, 328
Yam diseases 337
Aphelenchus cocophilus - - - 65, 181
Aphides 66, 67, 79
Aphis maidis ...... 327
AposphcETia Ulei ..... 233
AraUa hedges .... X35, 160
Areolate mildew of cotton - - - 286
Arrowroot Diseases - - . . 332-3
Red Root disease .... 203
Rosellinia .... 130, 132, 332
Arthur, J. C. 237
Aschersonia 79
Ascomycetes - • .20, 32, 36, 43
Ascospores 6, 7, 233
Ashby, S. F. 183 et seq., 203, 241 et seq.,
263, 326, 337, 349
Aspergillus 43. 74
A . ftavus 74, 79
Asperisporium Cariae .... 352
Atkinson, G. F. 268, 283
Authorities referred to - - - 363-9
Avocado
Diseases of 235
susceptible to Rosellinia - 135, 139, 235
Bacillus coli ...
B. Musce . . - -
Bacteria classified
defined - - . -
Diseases caused by
Fungi, relation of to -
Bacterial blight of beans -
Paga
- 185
- 247
- 45
35, 43
45-47
3. 45. 47
- 341
Boll disease of cotton - 270, 271-4, 277
bud-rot 184-6
soft-rot of onion .... 335
Wilt of tomato 357
Bacterium malvacearum 266, 268, 271, 272, 277
B. phaseoli 341
B. solanacearum ... 46, 337 et seq.
B. tumefaciens 16
B. vascularum - • - - 311 c< seq.
Diseases 241-55
trash for controlling leaf -stalk rot - 189
Bancroft, C. K. .... 293, 312
Barbados
Agricultural conditions - - - - 121
Banana disease .... 242, 246
Cassava diseases 333
Cotton diseases - - 273, 277, 286, 287
Cotton stainers absent - - - 264
Entomogenous fungi . - - . 74
Foot-Rot 213
Onion disease 336
Papaw diseases 353
Powdery Mildew of Grape-vine - 237
Rose diseases .... 362
Shaddock Splitting . . - . 223
Sugar cultivation - - - - 289, 294
Sugar diseases 296, 297 et seq., 308 et seq., 328
Yam Wilt ----- 337
Barber, C. A. - - - - 58, 126
Bark disease of coffee - - - - 229
Bark rots I5, i73
Barrett, O. W. 312
Basidiomycetes - - 18, 38, 44, 149, 231
Basidiospores 6, 19
Beans, diseases of .... 340-2
Berkefeld filter 50
Bequia, cotton boll disease - - - 266
pigeon pea disease ... 343
Bichet 156
Bird Vine 56-S, 114
Bitten-Leaf disease of coconut
Black-arm disease of cotton -
Black Blight ....
and see Sooty Mould
" Black cacao " - . -
Black-eye disease of pineapple
Blackhead disease of banana -
Black leaf-spot of papaw
Black melanose - . - -
" Black rust " of cotton -
Black spot
banana leaves -
pineapple - -
rose . - -
Bleaching powder
Blight, Pineapple
" Blights " of coSee -
Blodgett, F. M. - - -
Blossom bhghts -
Avocado
Lime
Blossom-end rot of tomatoes -
Blue Moulds, see PeniciUiums.
Bluggoe banana - - -
Bonnygate disease of banana
191
270, 271
.32,37,114
167
349
254
352
217
284
251
- 349
361
- lOI
- 345
- 225
94
- 13
- 235
- 207-9
71. 353
- 248
- 253
375
376
INDEX
Page
Bordeaux mixture 22, 85, 87 et seg., 149,
150, 151, 153, 167, 172, 186, 188, 189,
igr, 209, 215, 217, 227, 234, 252, 254,
270, 275, 296, 306, 339, 340, 347, 355, 361
Formula 88
Physical properties . ... 89
plus formalin loi
Preparation 90
Testing 91
Bordeaux paste - - - 91,105,188,189
Bordeaux powder 99
Botryodiplodia 25, 177
B. elastica 159
B. Theobromoe - . - - . jjg
Botrytis ... - 43, 80, 233
B. cinerea 212
Bourbon cane
Epidemic 297, 301
replaced in West Indies - - lo, 298
susceptible to disease - - - 295, 327
Bracket fungi 201
Brain, L. Lewton - . . . 271, 277
Branch Galls of Limes - - - . 206
Brandes, E. W. 241 et seg., 324, 326
Breadfruit, susceptible to Rosellinia 135, 139
British Guiana
Agave aiithracnose - - - 361
Bourbon cane epidemic - - 298, 301
Cotton rust 283
Foot-Rot
Hevea disease -
Leaf blight of coconut -
Ked ring of coconut
Red Root disease of limes
213
232
193
177
203
92, 308, 312
71.
274,
Sugar diseases
Witch-Broom disease
Wither-tip
British Honduras
Bud rot of coconut ...
Panama Disease ...
Red ring of coconut
Brooks, C.
Brown eye-spot of coffee
Brown leaf-spot of sugar - . -
Brown rot, Solanacese - - .
Bubbly fruit-rot of tomato
Bud rot
Coconut - - 16, 113, 183-8
distinguished from red ring -
Sugar .......
Bug punctures - 14, 29, 264, 338,
Burgundy mixture 22, 91-3, 209, 270,
Burgundy paste - - - 92,
" Burning " disease of arrowroot - 332-4
Burrill, T. J. 45
Burt, E. A. - . 149, 151, 210, 292
Butler, E. J. - 25, 183, 187, 308
Butler, O. - ■ - - - 89, 93
Calabacillo cacao ...
Cacao
Bonnygate disease ...
Canker
conditions favourable to
Corticium Stevensii
Diseases of - - - -
drying up of young fruit
Pink disease of - - .
pod-rot - . - . 9, 14, 120, 159
red rust of 152
RoselUnia diseases of 126, 127, 132, 139-41
shade trees - - - 11, 156, 160, 172
Caledonian Queen variety of cane - 298
Cambium n
Canary type of banana -
- 167
- 254
i, 162-8
- 155
- 149
155-76
14
148
Canker
Cacao
Citrus
- 247
15
9, 120, 162-8
10, 112, 215, 218-21
Page
Canker
Nutmeg - - 239
Rose 362
Capnodiae. See Sooty Mould.
Carbon Bisulphide 63
Carpenter, C. W. - - - - 293, 346
Carriacou, pigeon pea desease - - 342
Carruthers, J. B. 164
Cassava, cultivation and diseases - - 335
Cassytha 56
Castillu, Rosellinia on - - - - 130
Cephaleuros mycoidea - 152, 210, 235, 239
C. virescens 152
Cephalosporium 24, 42, 79, 205, 229, 243
C. Sacchari 260, 309
Cercospora - - . - 43, 228, 251, 337
C. coffeicola 228
C. henningsii 335
C. kopkei
C. longipes -
C. musarum
C. personata
C. rosicola -
C. Sacchari -
C. vaginae
Chaetodiplodia
C. grisea
Chamberland filter
321
322
252
339
362
320
318
25
159
50
Chamcluco banana 24 i
Charcoal rot of Citrus ... - 223
Charles, V. H. 352
Chestnut bark disease - - - - 10
Chibble leaf 278
" Chinese " banana ... 242, 245, 249
Chionapsis 210
Chlamydospores - - - 19, 21, 189, 243
Chloride of Lime. See Bleaching Powder.
Chlorinated potash. See Eau de Javelle.
ChoanepJwra infundibuUfcra - - - 36
Chlorophyll absent in Bacteria - - 45
absent in Algse 3
reduced by Mosaic disease - - 49
Chlorosis of sugar-cane - - - 70, 328
Chupon wilt of cacao ... 164, 165
" Cigar " disease of arrowroot - - 334
Citricolum variety of Cladosporium - 214
Citrus
Anthracnose - - - - 210, 211, 219
Canker - - - 10, 112, 215, 218-21
Diseases 194, 224
Fruit injuries and rots - - 221-4
Scab - . - . 215-7, 222
Cladosporium 43
C. ciiri 216
C. fulvum 355
C. herbarum - . . . . 214.
Chmate, effects of - - - - 7, 9, 12
Coal tar for protecting trees - - - 106
Cobb, N. A. 59 et seg., 180, 181, 249, 311
Cobb's disease of sugar-cane - - - 311
Coccidae. See Scale insects.
Coccus Viridis 79
Coconut
diseases of 177-93
dropping of nuts 185
immune from Rosellinia Pepo - - 127
trash for controlling leaf-stalk rot iHg
Coffee, diseases of - - - - 225-31
Coffee, leaf rust 17
Coleosporium Ipomoeoe .... 337
Collar canker of limes .... 206
Collar rot 15
Colletotrichum - - - 22, 35, i75. 353
Colletotrichum Agaves - - - - 361
C. falcatum 298, 301
C. glceosporioides 14, 23, 207 et seg., 235, 238
C. gossypii 277
C. incarnatum i75
C. lindemutkianum 34°
INDEX
Colletotrichum luxificum
Colocasia, diseases of
" Congo " banana
Conidia, Conidiospores
Coniothecium
Page
170
331
245
6
222
ConvolvulacesB -54
Cook, O. F. - - - - - 71
Cooke, M. C. 226, 275
Coons, G. H. 356
Cooper's Cattle Dip 56
Copper compounds as fungicides - - 88
Copper Sulphate 56, 93, loi, iii, 188, igr, 270
Coremium disease of coffee ... 230
Corn, diseases of . - . . 256-62
Com smut 16, 20, 257
Corrosive sublimates - - - - 99, 105
Cortex, infections in the - - 14, 16
Corticium genus 40
Corticium javanicum 148
C. lilacino-fuscum - . . . . 148
C. (PelHcularia) koleroga - - - - 151
C. salmonicolor --.... 148
C. Stevensii ------ 149, 240
C. vagum 26
Cotton, boll diseases 9, 12, 67, 263 et seq.
diseases 263-87
leaf blister mite 10
perennial - - - 273, 283, 287
Rivers type -...-. 270
superfine type 270
shedding of young fruit - - - 14
soft rot - 14
stainers. See Dysdercus ... 114
strains, resistance of - - - - 275
vascular wilt disease - - - . 243
Cremastogaster spp. 106
" Crinkled-dwarf " in cotton - 84, 286
CrioUo cacao and canker - - - 179
Crop rotation. See Rotation
Crossing 10, 287
Crown gall disease 16
Cuprammonium washes (suphate and hydrate)
93
Curly leaf of cotton - - - 278-80, 281
Curly top of beet 51-3
Cuscuta Americana 54
Cytospora 42, 316
C Sauhari 316
D 95 sugar-cane
D 109 „ „ ...
D 116 „ „ - . -
D 118, 145, 159, 216, 625, canes
^ 1135. 3956, canes -
Damping off . - . . 107-9, 278
Dash, J. S. 310
Date palm, diseases of - - - 236-8
Debility diseases - 2, 5, 11, 12, 225
Decay 3
Deficiency dieback of hme - - . 201
Dematiace® 42, 43
Dendropemon caribceus - - - - 58
Diaprepes 198, 206
Diatrcea - 80
Dieback
Cacao
Coconut
of twigs
Diplocarp:
• 157-61
- - 193
14. 15. 25, 120
- - 361
275-6
- - 351
Rosce
Diplodia" boll rot
core rot of pineapple
Dieback, Avocado . . . -
Dieback, Cacao. See Dieback.
Dieback, Lime - - 196-200,
Pod rot 160,
Rot, citrus fruit
Rot, Sugar-cane . - - .
D. cacaoicola
235
308
159
Diplodia epicocos
D. gossypina
D. maydis -
D. Nalalensis
D. rasa
377
Page
- - - - 193
- - - - 275
- 260
- 198, 222
- 159
D. Theobromce - • 157-60, 198, 222, 308
D. Zece 259, 260
Diplodias 5, 13, 14, 15, 25, 44, 69, no, 153
171, 178, 193. 196, 255
Control of 26
Discomycetes .--... 43
Diseases caused by bacteria ... 45-47
caused by fungi 3-44
caused by infective viruses - 48-53
caused by nematodes - - - 59-65
classified i, 2
leguminous plants ... 338-43
minor fruit plants ... 344-60
minor fruit trees ... 235-40
legislation against ... 112-4
nature of ----- i^ 2
non-parasitic - - . . . 69-72
permanent crops of . . - 155, 240
prevention and control of . - - 8i-6
root crops 331-49
Disinfectants, use of - - 99-ior, 142
Dominica
Agricultural conditions. • . . 119
Cacao pod-rot i6r
Citrus diseases . - - - ig^ et seq
Coffee industry 225
Entomogenous fungi - . - 73^ 74
Horsehair blight 152
Lime diseases - - - igS et seq.
Lime trees, life of ... jgj
Mistletoe in 58
Orange diseases 72
Pink Disease 148
RoseUinia in ... 127, 130, 134
Wither-tip, 1922 207
Dothideales .... 38, 44
Dothidella Vlei 233
Downy Mildews (Phytophthora ) - 14, 21
Drainage
for Root Diseases - - - 137-8, 143-5
for Red Ring disease ... - 182
for Red Root disease .... 305
for Sugar diseases .... 2^6
Drost, A. W. - - - . 161, 170, 241
Dry-rot disease of Xanthosoma - - 331
of Maize 259
Duggar, B. M. 27, loi
Dunlop, W. R. 177
Dusting with fungicide - - 99, 222, 237
Dysdercus spp. - - - - 80, 263 et seq.
Earle, F. S. 199, 326
Eau Celeste 93
Eau de Javelle loi
Edessa medittabunda 264
Edgerton, C. W. - - - 271 tt seq.
Eelworm black-rot 249
Eelworms. See Nematodes.
" El mal " disease 331
Empusa 79, 80
Endogenous formation of spores - - 6
Enfermedad del Machete - - . . 173
Engler's classification of fungi - - 43
Eremothecium cymbalariae - - . 30, 265
Eriosphceria Sacckari - - - - 321
Erysiphacece. See Powdery Mildews.
Euascomycetes 37
Eubasidii ...... 3^^ ^q
Eumycetes 35. 43
Eutettix tetiella 53
Exoascus ... ... 43
£. Theobromee 170
378
Exogamous formation of spores
Exophthalmus . . - -
Eye-spot disease of sugar
INDEX
Page
6
198, 206
- 320
Faulwetter, R. C.
Fawcett, G. L. -
Fawcett, H. S. -
- 268, 284
151, 226 et seq. 325
212 et seq.
329
13
94
236
147
236
212
167
203
Fiji Disease of sugar
Fission Fungi. See Schizomycetes
Flowers, diseases of
" Flowers of sulpbiur
Fames australis -
Fomes Root Rot
Fomes Stem Rot
Foot Rot of citrus
Forastero cacao -
Forest land, effect of clearing 11, 137,
Formaldehyde vapour - - 100, iii, 352
Formalin ... - 100, 108, in, 262
Froghopper blight of sugar-cane 68, 294,
296, 311. 324
Fruit
diseases of - - - - 13. iiOi 238
handling of - - no, 221, 223, 351
Trade requirements ... - 221
Fruitlet spot of pineapple . . - 349
Fulton, H. P. 209
Fumago 33
Fungi
and Algae distinguished • - 3
Classified 34-44
Diseases caused by .... 3-44
Entomogenous .... 73-8°
Host's resistance and susceptibility 8
Imperfecti 40-43
Natm-e of 3
Reproduction and infection - - 6
Their place in Nature ... 4
Fungicides, use of, 87-101. See also Spraying.
Fungus diseases
Notable groups .... 17-44
types of 12-17
Fusarium diseases, control of - - - 246
F. cubense 242, 243
F. moniliforme 260
F. vasinfecium 243
Fusariums 8, 17, 23, 43, 44, 229, 260, 331, 353
Fusicladium macrosporum - - 233
Gaillard 33
Galba susceptible to Rosellinia - - 134
Gall patches 328
GaUs 15. 62-3, 206
Gas tar 306
Gasteromycetes 44
" Gele strepenziekte " . . - . 324
GemmsB -...--- 6
Gibellula 43. 80
Gibberella saubinetii 260
GlcBosporium limetticolum - - 207 et seq.
G. Mangiferae 238
G. manihot 335
G. spp. 13. 22, 353
Glomerella 22, 23, 35
G. gossypii 277
Glossary 361-3
Gnomonia 22
G. Iliau 314
Gough, L. H. 321
Grand Cayman, Bacterial bud-rot in - 184
Grapefruit,
dieback ....... 199
diseases of 210-24
Grape-Vine, diseases of - - - 236-7
Graphiola Phanids - - - - 43> 236
Graphium 43
Page
Green bug. See Nezara viridula <
Green scale 79
Green Transparent sugar-cane - - 495 |
Grenada '
Agricultural conditions - - - 123
Banana disease 248
Cacao pod-rot 161 |
Cacao thrips 80 1
Date palm disease . . - - 236
Entomogenous fungi - - - - 74 i
Foot Rot Z13 !
Grey blight 210
Horsehair blight 152
Legislation against disease - - 114 [
Nutmeg cultivation ... - 239 i
Red ring of coconut ... 177, 180 I
Rosellinia root disease - - 130, 132 j
Shade trees, absence of - - - i57
Stem rot of coconut .... 189 I
Grey Blight of Lime ... - 210
Gros Michel banana - - - 241, 245, 247 1
Ground Nuts
diseases 338-40. 359
Root disease - - . . 146, 339
Gum disease of sugar-cane - - - 310 ]
Gumming
Coconut - • .... 189
Lime - 197, 210, 312, 213 1
Sugar 311
Gummosis ... - 15, 72, 212-4 i
Hard leaf-bitten disease of coconut - 190
Harland, S. C. - 70, 72, 270 et seq., 287
Harrison, J. B. 162
Hart, J. H. 164. I77
Hartley, C. 108
Haustoria in fungi ... 8, 18, 19
Head smut of sorghum . - . 258, 262
Heart leaf disease of banana - - 256
Hedges, F- 206
Helminthosporiose 258
Helminthosporium - - - - 43. 258
H. inconspicuum 258
H. Sacchari 320
H. turcicum ------- 258
Helopeltis bug 68
Helvellineaa 44
Hemiascomycetes .... 36, 43
Hemibasidii 38. 44
Hemileia. See Cofiee leaf rust.
Hennings, P. i59
Heterodera radicola - ~ - 59. 62, 229
Heteroecism 17
Heteroptera 30
Hevea, diseases of - - - - 232-4
Hevea, leaf spot IJ
Hibiscus susceptible to RoselUnia - - 135
Higgins, J. E. 352
Hill's Seedlings 297
Himantia stellifera - - • - 43. 292
" Honey dew " 8
Hormodendron 79
Horsehair blight - - - - 4. 29, 151
Howard, A. i6r, 181, 290, 298, 308, 334. 349
Hymencea Courbanl 127
Hymenomycetes 44
Hypertrophied twigs of cacao - 170. i7i
Hyphse in Fungi - - - - 4. 6, 8
Hyphomycetes. See Moniliales.
Hypochnaceae - 40
Hypochnus ochroleucus - - - " „ ^49
Hypochreales 38, 44
lliau disease of sugar cane - - - 3^4
Imperial Department of Agriculture 126, 350
Insects
enemies of 73-oo
INDEX
379
Insects
injuries from - - - -
Internal boll disease of cotton
Iron sulphide . - - .
Isaria
/. Barberi
Ithyphallui coralloides
Jamaica
Banana diseases 205, 241 </s«j
Bitten-leaf disease -
Bud-rot of coconut
Coffee eye-spot
Dieback of coconut
Hard or Little Leaf-bitten disease
Leaf-blight of coconut
Leaf-stalk rot
Legislation against disease
Mosaic disease
Panama disease
Pineapple diseases
Red Rust
Root crop diseases
Sorghum rust
Stem-rot of coconut
Wither-tip
Jassid bugs ...
Jehle, R. A. - - -
Jeyes fluid - - - .
John BuU tree -
Johnston, J. R. - 177, 18
Page
66-8
263-8
97
■ 43
- 80
292
250-5
- 191
- 183-7
- 228
- 193
190
- 193
- 185
325 et seq.
241 et seq.
• 349
- 152
- 331-2
- 261
- 190
- 208
53. 67
- 219
191. 353
- 266
185, 292, 316
Kelsall, A. - - - -
Kernel smut of sorghums
Kidney beans, diseases of
Kuehneola Gossypii -
go
261
340-2
• 283
Larsen, L. D. 348
Lasiodiplodia 25
L. nigra 159
L. Theobromce 159
Leaf-base rot of pineapple - - - 349
Leaf-blight of coconut - - . . J93
of Maize 258
Leaf-blister mite of cotton - - - 122
Leaf blotch of yams .... 337
Leaf-curl of Sea Island cotton - - 70
Leaf Diseases - . - . 12, 148-54
of date palm - . - -
Leaf-dwindling disease of coconut
Leaf mildew of cotton
Leaf-mould of tomato
Leaf-roll of potato -
Leaf-rust
coffee
ground nuts - - . .
Leaf-scorch of maize-
Leaf sheaf rot of sugar-cane -
Leaf spots
Cassava
Ground nuts - - . .
Lime
Nutmeg
Tomato 355
Leaf-stalk rot of coconut ... 188
Lee, H. A. 220
Leptostylus prcemorsus - - - 206
Lemon, diseases of - - - - 210-24
Leptoglossus 360
Lepiosphceria Sacchari - - - 318
Leptospora Musce 243
Levin, E. 356
Lime, use of - - 142, 147, 296, 354
Lime-sulphur solution 94-7, 149, 151, 209
821, 361
236
190
285
355
51
17
338
258
316
355
338
210
239
- 194-224
- - 195
13, 119, 125,
Page
Lime Trees
diseases of - -
natural habitat
Limes, Anthracnose of
And see Anthracnose.
Rosellinia disease of - - g, 130, 140
Little-leaf disease of coconut - - - igo
Liver of sulphur 97
Loggerhead disease of Sea Island cotton
70, 280-3
LorantbaceaB 56
Love Vine 54-6, 114
Lyon, H. L. 314, 329
24
159
284
284
175
Macroconidia
Macropkoma vestita
Macrosporium Leaf-spot - . .
M. nigricantium
Maize. See Corn.
Mal-di-goma, See Foot-Rot.
" Male " cacao
" Man cotton " 84,
Mandarin, diseases of - - 194, 210-24
Mango
Blossom blight 235
diseases of 237-8
Manjak for protecting trees - - - 106
Manns, T. F. 260
Manure, organic or chemical 200, 201, 289,
296, 300
Manuring ir, 12, 71, 82, 156, 160, 195, 200,
245. 284, 354
Manzana banana 242
Marasmius - - - 17, 28, 44, 152, 170
Marasmius root disease of banana - 246
M. perniciosus - - - - 29, 171
M. Sacchari - 172, 260, 290 et seq., 297, 346
M. sarmentosus 152
M. semiustus ...... 246
M. sienophyllus 246
Massee, G. - 149, 164, 191, 226, 298, 332
Maublanc, A. - - - - - 227, 233
Mealy-bug ----.. 7^^ 79
Melanconiales 42, 44
Melanconium Iliau 314
M. Sacchari - - - 298, 308, 314
Melanopsammopsis Ulei ... 232
Melanose 217
Mehola 33
Mercuric Chloride 99
Metamasius sericeus - . . - . 192
Metarrhizium anisoplice . - - 77, go
Microconidia ----- 24, 243
MUdew
fungicide for - - - - - 97
54-8
172
42
42. 44
Rose
See also Downy, Powdery.
Mistletoes
MonUia pod rot - - - -
Moniliaceae - . . . -
Moniliales - . . . -
Monocotyledons ----- 16, 183
Montserrat
Agricultural conditions - - - 118
Brown rust of corn .... 256
Cotton disease ... 265, 274, 279
Diplodia Dieback of Lime - - 197, 198
Fomes Root Rot .... 147
Grey blight of Lime .... gio
Ground-nut leaf rust - . - - 33g
Lime growing in - - - - ig8
Onion disease 336
Pineapple diseases - - 345, 347, 349
Moonshine. See Chlorosis.
Moor
J. C.
Moro Disease of banana -
Morrill .....
38o
INDEX
Page
49-51
■ 283
■ 67
Mosaic diseases - - - .
cotton
infection -----
symptoms 49
sugar-cane - - - - 49, 112, 324-8
tobacco 50
transmission 51
effect of germicides - - - - 50
Mottled hybrids of cotton - - - 287
Mottling disease of sugar - - - 324
Mucedinaceaj ...... ^z
Mucors, the 36
Mulching - - - 156, 160, 200, 201, 244
Mycoidea parasitica 152
Mycologist
origin of term i
province of 59
Mycelium, the 4, 6, 8
Myriangium Duriizi 79
Myxomycetes 34. 43
Nail-head rust 214
Necator decretus 149
NestrJEB - - - 38, 44, 164, 201
Nematode diseases of coffee - - - 229
Nematodes - - 13, 59-65, 177, 183, 359
Nematospora 36
N. Coryli 265
A'. Lycopersici ---..- 31
Neocosmospora ------ 38
Nevis cotton disease - - 265, 279, 287
Sugar diseases - . - . 308
See also St. Kitts.
Purple cotton 283
Nezara 360
N. Viridula - - 80, 263 et seq., 338
Nutmegs, diseases of - - - 149, 239
- 5, 9. 17
17, 40, 292
- 292
- 292
20, 42, 362
- 222
- 243
- 227
335-6
35, 43
- 2r, 189
- 38. 79
206, 210-24
- 271, 277
Obligate parasites
Odontia, spp. ...
O. S ace hart - . - -
O. Sacckaricola - - -
Oidium -----
Oil Spot on citrus fruits
Okra, vascular wilt disease
Omphalia flavida
Onion, diseases of - -
Oomycetes - - . -
Oospores . - . .
Ophionectria coccicola
Orange, diseases of -
Orton, W. A. -
Ovulariopsis - . .
O. Gossypii
Palmyra palms. Bud-rot of - - - 183
Panama disease of bananas - 84, 241-6, ;247
Papaw, diseases of - - - - 352-3
Parasitic diseases
classified ------ i
infection, sources of - - - . 81
prevention and control - - 81-86
Parasites explained 4, 5
classified 5
phanerogamic 54-8
Passalora Hevece . . . . 233
Patouillard
127, 159
Peach yellows; Peach rosette - - 51-3
Pellagra 2S9
Peltier, G. L. 27
Penicillium - . - 43, no, 213, 350
Perisporiaceae 32, 37
Perisporiales 43, 44
Perithecia of Red Root disease - - 205
Peronospora trichotoma - - - - 333
Peronoscoracea 36
Pag«
193
210
237
292
Pestalozzia palmarum ....
Petch, T. 147, 151 et seq., 189, 193
Phakospora vitis
Phallus aurantiacus
Phaseolus vulgaris
Phloem-necrosis - 234
Phoma 44
Phomopsis 42
Phomopsis Citri 217
Phycomycetes 33. 43
Phyllachora graminis 260
Phyllachora spp. 38
Phyllosticta 239
Physiological disease - - - - 2, 48, 69
Phytophthora and Gummosis - - - 212
P. faberi 177, 183
P. omnivora 164
P. palmivora . . . . 183, 186-8, 189
P. parasitica - . - 188, 213, 274, 349
P. terrestria 189, 213
Phytophthora of cotton . - . - 274
Pod-rot - - - 160, 173, 175, 274
And see Downy MUdews.
Pierce, R. G. 108
Pickering, S. U. 88
Pigeon pea, diseases of - - - - 342
Pineapple, diseases of - - - 344-53
" Pineapple " fungus of sugar-cane 192, 305-8
Pink disease 15, 148
Plantain immune from Panama disease 245
Plasmodiophora brassicce . - . - 329
Plectasci 43
Pod-rot. See Cacao.
Poix-doux susceptible to Rosellinia - 135
Polyporacese 40
Potash and cotton disease - - 283, 284
Potassium permanganate - - 97, iii
Potassium sulphide. See Liver of Sulphur.
Potato blight 10
Powdery Mildew of Grape vine - - 236
Powdery Mildews (Erysiphacea) 20, 37, 94
Prantl's classification of Fungi - - 43
Prescott, S. C. 241
Prestoe, H. 225
Protoascomycetes .... 36, 43
Protobasidii .... 38, 40, 44
Protoplasm, resistance of - - - 8, 9, 67
Pruning of cacao trees - - - 168, 173
Pseudococcus calceolariae. See Mealy Bug.
Psorosis of citrus 214
Puccinia purpurea - - - 256, 261
sorghi 256, 261
Purple ribbon sugar-cane - - - 326
Pycnia ; pycnidia ; pycniospores 18, 26
Pyrenomycetes 38, 43
Pythiacystis citrophthora - - - 212, 313
Pythium 3^
Pythiacystis 36
P. butleri 293, 346, 353
P. de Baryanum 28
P. palmivorum 187
Quanjer, H. M.
Quicklime
323
296
Rainfall, influence of 115, 157. I93. I99. 294
Ramularia areola 286
Rangel, E. 227
Ratooning - - - 289, 294 ri $e?., 324
Red leaf-Wight of cotton - - - 283
Red leaf -spot of sugar - - - - 321
Red or claret banana ... - 245
Red ring disease of coconut - - 177-82
Red root disease of limes - - 203-5
Red rot - . - 10, 301, 310, 316
Red rust ... - - - 152
Avocado 235
Nutmeg 839
INDEX
Page
Red spot of sugar leaf -sheaf - - - 317
Reinking, A. 183
Resistance of plants to disease and insects
8-10, 68, 83
Resistant varieties, disadvantages of - 84
Rhizoctonia - - - - 26, 43, 44, 342
Rhizomorphs - - - - 4, 217, 252
Rhizopus nigricans 36
Rhynchota, plant feeding - - - 66
" Rind disease " of cane - - - 301, 323
Ring-spot of sugar-cane ... 318-20
Robson, W. .... 274, 339, 353
Rogers, J. M. 199
Rolfs, F. M. 270
" Root rot," " Rood snot" • 301, 316
Root crops, diseases of - - - 331-7
Root diseases 15, 23, 117, 119, 123, 126-47
Corn 260
Ground Nut 339
Pineapple 345
Sugar-cane .... 289-97, 345
Rorer, J. B. 148, 161 et seq., 177, 184, 235,
237, 248, 362
Rose, diseases of .... 361-2
Rosellinia 5, 9, 17, 43, 83, 120, 126-46, 160
204, 235, 240, 335, 342
R. bunodes - - 127, 128, 130-2, 137, 333
R. paraguayensis- - - - 128, 132-3
R. pepo 123, 127-30, 131, 136, 238
Rosenbaum, J. 177
Rotation of Crops 9, 11, 20, 81, 116, 258,
294, 296
Rots of fruit 25
Russeting of Citrus fruit - . . . 222
Rust-mites 222
Rusts (Uredineoe) ... 5, 19, 44.
classified 17-19
cotton 283
grape-vine 237
on leaves 13
pigeon pea 343
sorghum 261
Rutaceas 220
SaccharomycetacesB. See Yeasts.
St. Kitts-Nevis
Agricultural conditions in
Fomes lucidus in -
Curly leaf of cotton
Date palm disease
Loggerhead disease
" Man Cotton "
Pineapple diseases -
Root disease of corn
Sugar diseases
St. Lucia
Agricultural conditions
Avocado
Banana disease
Brown Rust of corn
Cacao pod-rot
Citrus diseases
Entomogenous fungi
Horsehair-blight -
Legislation against disease -
Pink Disease - . - .
Red rust ....
St. Vincent
Agricultural conditions -
Angular spot of cotton
Arrowroot diseases
Brown rust of corn
Cacao thrips ....
Citrus trees, absence of
Cotton diseases, 265 et seq., 274
Grey bhght
Ground nut diseases
116
147
279
236
70, 280
286
347, 349
260
301
235
256
- 161
194 el seq.
■ 73
- 152
- 114
. 148
- 152
121
- 268
332-5
- 256
79
- 194
279.
283, 286
- 210
338, 339
St. Vincent
Mistletoe - - . .
Papaw diseases
Rain, influence of -
Red ring of coconut
Root disease of corn -
Rosellinia
Sea Island cotton diseases
Shade trees
Tomato diseases
Saltpetre disease
Sanders, G. E. .
Saprophytes
cacao ...
fruit and flowers -
leaves - . .
Saprophytism explained
Scab disease of citrus
Scale insects
and dieback
381
Page
- - 58
- - 353
. 12, 266
- - 177
- 260
- 130, 133
70, 83
- - - - 156
353. 355, 357 et seq.
- - - ' 331
. - 90
161, 16S
- - - 13
- - - 4, 5
- - - 213
73. 79. "8, 209
- - - 199
212, 214, 219
35, 43. 45
40
- 313
313
- 330
43
31
230
146
Scaly bark citrus disease
Schizomycetes
Schizophyllum ... -
rot of sugar-cane .
S. commune - - . . -
Sclerospora Sacchari -
Sclerotinia ...
Sclerotia in fungi
Sclerotium disease of Liberian co
rots
S. coffeicolum 230
S. rolfsii ... 16^ 146, 317, 338
Scolecotrichum 233
Sea Island Cotton, close season 10, 82, 114
losses through bacterial boll disease - 271
non-parasitic diseases - - - . 70
And see Cotton.
Sealy Seedling cane - . - . 296
Seaside Mahoe 266
Seeds, disinfection of - - 99, 257, 278
infection through . - - . 82
Septobasidium - - . . 40, 79. 2I0
S. langloisii 210
SepioglcBum Arachidis .... 339
Septoria
leaf-spot of tomatoes -
S. ly coper sici
Sereh disease of sugar-cane
Shaddoclv, diseases of
Shade Trees, question of -
Shellac varnish for fruit rot
Shield-scale fungus -
" Shot-borer " beetle
Silk cotton
- 356
ii-3. 323
210-24
- 156
- no
74. 77
- 298
- 266
Silk fig " banana 250
Sisal, anthracnose of .... 361
Slime-fungi 34, 329
Smith, Erwin, F. 45, 184, 243, 268, 271,
312, 357
Smith, Long&eld 280
Smoke-clouds to prevent dew . - 234
Smuts (U slilaginece) 5, 44. And see Corn.
Smut disease of sugar cane . - - 312
Soft rot of cotton boHs .... 274-3
Soil
aeration 354
grade immaterial with Red Ring - 181
infection ...... 259
influence of 70, 115, 201, 203, 229, 245,
280, 344
poverty - ...... 294
St. Vincent 195
treatment - 142
virgin . - . . • 11, 181, 225
Sooty Mould 8
Sorghums, diseases of - . . 258, 262
Sorosporium reilianum ■ - . 256, 62
South, F. W. 126, 130, 147, 174, 203, 206
246, 260, 271, 274, 332, 339
382
INDE^t
Sphacelotheca reiliana - - - - 262
5. Sor?,hi 261, 262
Sphcerella Caricce 353
Sphaeriace® - - - ... 233
Sphaeriales 44
Sphaeronema black spot - - - - 173
Sphaeropsidales - - - - 44
Sphaeropsis .... . 16, 42
S. tumejaciens 206
Spharostilbe - - - 17, 38, 43, 203
S. coccophila .... 38, 79
S. fiavida - - .... 239
S. musarum .... 204, 252
S. repens 203, 204
Sphcerotheca pannosa - - . - - 362
SphaBriales - 38
Spicaria 332
Sporangium . - . . 6, 153, 189
Spores
action of - . . . . g
classified .... - 6
of diplodia - - - - - 26
of rusts 17
of smuts 19
Sporidia 19
Sporodochium, the 43
Sporotrichum 43, 80
Spraying - 20, 22, 34, 56, 109-13
Anthracnose control .... 209
Citrus diseases 220
compared with parasite fungi - - 76
cotton - - - . 270, 274, 275
injury from no, 270
Machines no
method of 109
object of 85
Tomato diseases 357
West Indies, little used - - - 87
Atid see Bordeaux, Sulphur, etc.
Stahel, G. - - - 170, i7r, 231, 232, 234
Staining of seed cotton, cause of - - 265
Stell, F. 177. 189
Stem-bleeding disease of coconut - - 189
Stem canker of pigeon pea - - - 342
Stem diseases .... 14-16, 148-54
Stem rot
Banana 246
Beans 342
Coconut ....... 189
Stevens, F. L. 33
Stevens, H. E. 213
Stevenson, J. A. 215, 260, 324, 343, 355, 357
Stigmatomycosis. See Bug Punctures.
Stigmonoses 66, 264
Stilbaceas 43
Stilbum ..... 201, 205, 362
S. flavidum, Siilbella flavida • - • 226
Stinkhorn fungus of sugar - - - 292
Stockdale, F. A. .... 177, 183
Stimips and logs favour root diseases 131,
133-4, 137
Sugar-cane
as remedy for damping off - - - 109
conditions favouring - - . . 288
diseases ot 288-330
resistant to Rosellinia - - - - 127
weevil-borer 192
Sulphate of Ammonia ... 201, 297
Sulphur, use of 20, 94-7, 142, 237, 362
Sulphuric Acid - - 10 r, 108, 109
Surinam. See Witch Broom.
Susceptibility of plants ... 10, 68
Sweet potato diseases .... 336
Tangle-root of pineapple - . - . 344
Taubenhaus, J. J. 97
Telia ; Teleuto-sori ; Teliospores - 18
Tempany, H. A. ... 70, 328
43.
187.
80
189, 346
- 350
298, 305
190, 192, 254, 305, 347, 350
49. 240
Page
115
206
153
■ 159
38.39
191
Temperature, effect of
Tenney, L. S. -
Thalli through algal attacks
Thelephora pedicellata
Thelephoraceaa ...
Thermesia gemmaialis
Thielaviopsis
soft-rot of pineapple
T. ethaceticus
T. paradoxa
Thread blight
Threadworm. See Nematodes,
Thrips on cacao -
Thyridaria tarda
TilletiaceaB ...
Tip-wither -
Tobacco extract -
Tobago
agricultural conditio!
Malvaceous weeds
Red ring of coconut
Stem-rot of coconut
" Tomato blight "
Tomato, diseases of -
Tomosis
Top-rot of sugar-cane
Tortola, boll disease of cotton 263, 263
Trees, age and size attained by - 15, 198
structure of 102
surgery of 104
Trenches for isolation. See Drainage,
Trichosphceria Sacchari - . .
Trinidad
Agave anthracnose
Agricultural conditions
Avocado anthracnose
Banana disease ....
Bud rot of coconut in
Cacao pod-rot ....
Cassava diseases ....
125
- 267
- 177
- 189
- 355
353-60
183, 340
298
Corticium koleroga
Cotton mildew
Entomogenous fungi
Grey blight ....
Horsehair blight
Leaf-blight of coconut
Leaf-dwindling disease
Legislation against disease
Mango disease ...
Marasmius diseases of sugar
Mistletoe ....
Panama Disease
Pink Disease -
Powery Mildew of Grape Vine
Red ring of coconut
Red rust ... -
Root disease of cacao
Rose canker - - . .
Stem-rot of coconut
Sugar cultivation ...
Sugar diseases 294, 297, 311
Wither-tip " " " ".
Tropics, their influence on fungi
Tuber rot of yams ...
Tuberculariacea ...
Tylenckus angustus - . .
T. cofjecz
T. devasiatrix ....
T. musicola ....
T. ribes ....
T. similis
361
123
235
246
183-5
161
335
- - 151
- - 286
73. 77
• 210
- - 152
- - 193
190
■ 114. 185
- 237
- 292
- - 58
- - 242
- - 148
- - 237
- 147, 180
- - 152
- - 176
- 362
288, 289
4
- 337
- 43
- 63
- 230
- 63
65, 249
- 64
65. 249
Uba cane ...... 296
Uncinula necator .... 236
United Fruit Co. 241
Uredinece. See Rusts.
Uredinia ; Uredo-sori ; Urediniospores - 18
Uredo Arachidis 338
INDEX
i83
Vredo Gossypii -
U. pallida -
V. Sorghi - - -
Uromyces Arachidis -
V. DoUchoU
Vsiilaginecs. See Smuts.
UsHlago Maydis, Mays-i
U. reiliana
D. Sacchari - - -
U. ZecB - - -
Page
283
260
261
338
343
257
262
312
257
Van Breemen, P. J. .... 147
Van der Bijl, P. A. 147
Van Hall, C. J. - - - 155, 157, 168
Vascular system of stem ... 16, 17
Vasculomyces Xanthosomce - - - 331
Ventilation .importance of 139, 140, 227, 237
Vermicelle. See Love Vine.
Verticillium ....
Virgin Islands
Mistletoe
Cotton boll disease
Virgin SoU. See Soil.
Viruela disease of Coffee
Virus, infective. See Disease.
43
226
Wakker, J. H. - - 290, 297, 311, 324
Ward, H. MarshaU 332
Watery disease of cacao. See Monilia Pod Rot.
Weevil-borer 192
West Indies
Coffee decline - - - - . 225
West Indian methods of cultivation - 85
sugar industry, decline of - - 10, 11
unfavourable to Rust fungi - - - 17
Vegetable accumulation ... ,^
White fly 77, 78
White leaf-spot of pineapple - 348
Page
White rust of potato .... 336
White Transparent sugar-cane 295, 298,
325. 327
Williams, C. B. - - - - 320, 325, 326
Wilson, O. T. 223
Wilt disease 8
pigeon pea 343
pineapple 345
sugar-cane .... 260, 308-10
yam 337
Windbreaks, for Lime trees - - - 199
infection from ... 126, 135, 139
use of 78, 118
Witch broom disease of cacao 10, 16, 39, 168-72
Wither-tip
Cassava 335
Lime 207-9
Wood rots. Lime - - - 15, 102-6, 201
Woods, A. F. 68
Woody excrescences of cacao - - - 176
Wound parasites 5
Xanthosoma, diseases of
Xyleborus perforatts -
331
298
Yam diseases . . . .
Yeasts
Yellow Caledonian cane -
Yellow leaf-bhght of cotton -
Yellow leaf-spot of sugar
Yellow stripe disease of sugar
Zonal leaf-spot of coffee -
- - 337
36, 43. 189, 191
- - 327
. 283
- 321
- - 324
Zygomycetes
Zygospores -
• 229
I, 153
36, 43
36
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