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DISEASES OF GLASSHOUSE PLANTS

“ Stripe ’’ disease of the tomato. [Frontispiece

DISEASES OF
GLASSHOUSE PLANTS

y 7

lg > tt By
Ww. F. BEWLEY, D.Sc.

DIRECTOR OF THE EXPERIMENTAL AND RESEARCH STATION, CHESHUNT

WITH A FOREWORD BY
SIR JOHN RUSSELL, D.sc., F.RS.

DIRECTOR OF THE ROTHAMSTED EXPERIMENTAL STATION

LONDON :

ERNEST BENN, LIMITED

(BENN BROTHERS)
8, Bouverie Street, E.C. 4
1923

PREFACE

THE main object of this book is to bring before growers
of glasshouse plants the fundamental principles of
disease control, in the hope that it may be of some
assistance to them in the course of their business. Its
preparation was stimulated by a desire to supply more
detailed information in reply to many inquiries than
can be set forth in routine correspondence.

An attempt has been made to describe the chief
diseases of glasshouse plants in this country. Structural
details of the causal organisms have been purposely
omitted as these do not concern the practical man, but
special attention is paid to methods of control.

I have to acknowledge my indebtedness to Sir John
Russell, D.Sc., F.R.S., and Dr. W. B. Brierley for the
kindly assistance they have so freely given during the
preparation of this work.

I am indebted to Mr. E. R. Speyer for the photographs
on pages 42, 69, 88, 114 and 161, and to Mr. O. Owen
for kindly reading the manuscript.

For permission to publish the frontispiece and the
material and photographs on pages 56, 57, 73—84,
127—133 I am indebted to the Association of Economic
Biologists, in the official journal of which they first
appeared.

Finally, I have to record my gratitude to Miss Helen
Marchant, who kindly undertook the preparation of the
manuscript.

W. F. B.

CHESHUNT, September, 1923.

a Le on TOM, 2" een ra ewe ' ey neh. fy?
oo) et ad ea . . .

eS SU apis Haye Gehrke

1g Wal

FOREWORD

Since Dr. Bewley left the Rothamsted Experimental

Station to proceed to the daughter station at Cheshunt

he has had unrivalled facilities for studying the diseases
of the plants grown under glass in that area. Few
people realize the highly specialized nature of the market
garden glasshouse industry in the Lea Valley, or the
extraordinary skill of the more successful of the growers
engaged therein. The traveller to Cambridge on the
London and North Eastern Railway line sees a multitude
of glasshouses between Enfield and St. Margaret’s, but
probably does not know that here are grown the bulk
of the English tomatoes and cucumbers which find their
way to the London markets and the industrial regions
of the North, and that here too the growth of plants is
carried out to so high a level of efficiency that growers’
have actually sold palm-trees to Africa and peaches to
New York.

Naturally, enterprises of this sort are possible only
to those with keen powers of observation and a sound
instinct for the growth of plants. Among these men
there is a great body of empirical knowledge which
Dr. Bewley has been able to explore. Further, he has
been able, at the Experiment Station at Cheshunt,
to test thoroughly whatever seemed worth following up,
and from his own investigations he has succeeded in
elucidating much that was shrouded in mystery.

The book thus combines the best empirical knowledge
of the grower with the results obtained by the scientific

7

8 DISEASES OF GLASSHOUSE PLANTS

observer; the facts have been carefully sorted, the
deductions thoroughly examined, and, in consequence,
Dr. Bewley has been able to present to the growers
sound information which cannot fail to help in the
management of their nurseries, while his record of
observations will prove of great assistance to plant
pathologists.
K. J. RUSSELL.
September, 1923.

CONTENTS

CHAPTER PAGE
I. HYGIENIC CONDITIONS OF GLASSHOUSES IN
RELATION TO HEALTH AND DISEASE IN PLANTS - 13

Situation of a Nursery: (a) Slope, (6) drainage, (c) re-
lation to neighbouring high land.—Glasshouse Con-
struction: (a) Light in relation to plant growth,
(6) glasshouse construction in relation to the light
factor, (c) glasshouse construction in relation to air
capacity and the ventilation factor.—The Heating of
Glasshouses.—The Humidity of Glasshouse Atmo-
spheres.—Watering.—Mulching.—Pruning and Train-
ing.—Disposal of Dead and Diseased Tissues.—Sources
of Infection of Plant Diseases: (a) Water supply,
(6) straw manure, (c) imported plants, (d) baskets,
sacking, etc., (e) nursery workers.

II. DISEASED CONDITIONS OF PLANTS DUE TO
ENVIRONMENTAL FACTORS — LIGHT, HEAT,
HUMIDITY, SOIL, ETC. - - - - - - - - 383

Light.—Heat.—Water: (a) Humidity of the atmo-
sphere, (6) soil moisture.—The Soil.—The Physical
Condition of the Soil: (a) Water-holding capacity,
(5) food-holding capacity, (c) aeration, (d) anchorage,
(e) degrees of compactness.—The Chemical Conditions
of the Soil: (a) Soil reaction, (b) balance of plant foods,
(c) soil population.—Malnutrition.—Chlorosis.—Stig-
monose.—Gas Injury.—Diseases of Unknown Origin.

III. DISEASES DUE TO FUNGI- - - - : - - at oe

(1) Root Diseases: ‘‘ Damping off ”’ of tomato seedlings ;
“damping off’ of cucumber seedlings; phytophthora
foot rot of the tomato; botrytis foot rot of the tomato ;
macrosporium foot rot of the tomato; verticillium
collar rot of the tomato; crown canker of the rose;
fusarium root rot of the tomato; sclerotium root rot
of the tomato; root rot of the carnation; rhizoctonia
root rot of the sweet-pea ; sclerotium disease of bulbs ;
botrytis disease of bulbs; fusarium and penicillium
bulb rots.

IV. DISEASES DUE TO FUNGI—Continued : - - - -

(2) Wilt Diseases: Verticillium wilt of the tomato ;
verticilium wilt of the cucumber and melon; verti-
cillium wilt of the sweet-pea; fusarium wilt of the
tomato ; fusarium wilt of the cucumber and melon.

VY. DISEASES DUE TO FUNGI—Continued - - : - - 88

(3) Stem, Leaf, and Fruit Diseases. Stem Diseases :
sclerotinia stem rot; stem canker of the tomato;
botrytis stem rot of the tomato; stem canker of the
rose; rose graft disease. Leaf diseases: Cercospora
leaf spot of the cucumber; colletotrichum leaf spot of
the cucumber ;_ cladosporium leaf spot of the cucumber ;
alternaria leaf spot of the cucumber ; powdery mildew
of the cucumber; downy mildew of the cucumber;
tomato leaf mould or ‘‘ mildew”; leaf spot disease
of the tomato; carnation rust; septoria leaf spot of
the carnation; leaf mould of the carnation; macros-
porium leaf spot of the carnation; powdery mildew
of the carnation; ‘‘die back’ of the carnation;

9

10 DISEASES OF GLASSHOUSE PLANTS

CHAPTER

rust of the chrysanthemum ; leaf blight of the chrysan-
themum ; powdery mildew of the chrysanthemum ;
leaf spot of the chrysanthemum; downy mildew of
the rose; leaf blotch of the rose; downy mildew of
the sweet-pea. Fruit diseases: ‘‘ buckeye’’ rot of
tomato fruits; rhizoctonia rot of tomato fruits;
botrytis rot of ‘tomato fruits ; rhizopus rot of tomato
fruits; penicilliwm and fusarium rots of tomato fruits;
other rots of tomato fruits.—Gummosis of Cucumber
Pruits.—General Surface Diseases: Potato blight on
the tomato; macrosporium disease of the tomato ;
“‘ nailhead ”’ spot of the tomato.

VI. DISEASES DUE TO BACTERIA - : - . -

Wilt Disease of the Cucumber.—Foot rot of the
Cucumber and Melon.—Angular Leaf Spot of the
Cucumber.— Stripe ”’ Disease of the Tomato.—Wilt
Disease of the Tomato.—Grand Rapids’ Disease of the
Tomato.—A Tomato Canker.—Tomato Fruit Diseases :
Soft Rot; Brown Rot.—Soft Rot of the Arum.

VII. MOSAIC DISEASES - - - - = . = .

General Symptoms of the Disease: Mottling, Abnor-
mality, Distortion.—Symptoms of Tomato Mosaic
Disease.—Symptoms of Cucumber Mosaic Disease.—
Pathological Anatomy of Diseased Plants.—The
Infectious Nature of the Disease.—Properties of the
Virus.—Transmission of the Disease.—Cross Inocula-
tions.—Carrier Plants.—The Effect of Environmental
Conditions.—The Control] of the Disease.—The Deter-
mination and Elimination of Infection Centres.—The
Determination and Elimination of Agents by which the
Diseases are Spread.—The Determination of Cultural
Conditions Necessary to Increase the Resistance of
Susceptible Plants.—The Breeding of Immune Varieties.

VIII. GENERAL REFLECTIONS AND OQuSIDESA TIONS
ON DISEASE TREATMENT - *

Soil and Water Sterilization. —Steam Sterilization : :
(2) ‘Box and grid’’ method, (6) ‘‘small grid”’
method, (ec) ‘‘ tray ’? method, (d) tank method, (e) drain-
pipe method.—Surface Sterilization with Hot Water.—
Deep Sterilization with Hot Water.—The Boiler Tray
Method.—Sterilization by Baking.—Sterilization with
Cresylic Acid.—Sterilization with Formaldehyde.—
Sterilization by Drying.—The Effect of Different
Methods of Soil Sterilization upon Plant Growth.—
The Effect of Sterilization upon Plant Disease.—Water
Sterilization.

IX. GENERAL REFLECTIONS AND CS ae
ON DISEASE TREATMENT—Continued -

Spraying and Dusting.—Fungicides. Jie fee “Mix.
ture.—Burgundy Mixture.—Ammoniacal Copper Car-
bonate.—Sulphur Fungicides.—Spreaders.—Liver of
Sulphur and Flour Paste.—Lime Sulphur and Flour
Paste.—The Process of Spraying.—The Effect of the
Spray on the Plant.—Soil Fungicides.—Dusting.—
Breeding.—Selection. —Hybridization. —Conclusions

(1) Plant hygiene—(a) The elimination of centres of
infection, (6) cultural methods ; (2) spraying, dusting,
and sterilization ; (3) resistant varieties.

APPENDIX - 2 > E ‘ 2 ‘
SELECTED BIBLIOGRAPHY - - - “ - 2
INDEX - : - a 4 . Z

PAGE

121

140

154

175

LIST OF ILLUSTRATIONS

Stripe disease of the tomato - - - - - - Frontispiece
PAGE

Fie. 1. Diagram illustrating an experiment on the relation of waterlogged
soil conditions to chlorosis of the tomato - - - - 40

Fias. 2 and 2a. Cdema of the geranium, showing swellings on the leaves
and stem - - : - - - - - 42
Fic. 8. (a) Hyphe; (b) thin-walled “‘summer” spores; (c) thick-walled
resting spore ; (d) a pycnidium ; (e) a perithecium ; (f) ascus con-

taining eight asco-spores - - - - - = 55
Fic. 4. Tomato seedling attacked by ‘‘ damping off ”’ disease at (a) - 56
Fie. 5. Hyphe of Rhizoctonia solani - - - - - 658

Fia. 6. Phytophthora cryptogea: (1) Reproductive bodies—(a) conidium ;
(b) zoospores. (2) Typical arrangement of conidia showing how
one conidium after another is produced on the main axis—(a) empty

conidium ; (b) young conidium - - - - - 60
Fie. 7. Phytophthora foot rot of the tomato - - - - 63
Fie. 8. Botrytis foot rot of the tomato - - - - - 64
¥ie, 9. Botrytis sp. growing on tomato stem - - - - 64
Fic. 10. agai foot rot of the a: (a) spores ; (b) diseased

stem - - - . - - 65
Fic. 11. Colletotrichum tabificwm on sears roots, = kyeiest. ee

black sclerotia - - 69
Fic. 12. Sclerotium disease of the tulip - - - - - 69
Fic. 13. Botrytis disease of the tulip: (a) bulb showing black sclerotia ;

(6) fungal growth on the bulb - - - - - 72
Fie. 14. Verticillium wilt of the tomato. Old diseased tomato stem showing

the fungal outgrowth at the base - - - - e — 5G
Fie. 15. Verticillium wilt of the tomato: (a) wilted plant six weeks after

inoculation with V. albo-atrum ; (6b) contro] plant - : - 5

Fic. 16. This photograph shows the wilted plant in Fig. 15 after being sub-
mitted to shade and an average temperature of 25° C. for thirty
days. The wilted leaves have fallen off, but the plant has recovered

and made good growth in the top - - - - - 179
Fie. 17. Sweet pea, showing right-hand branch wilted as the result of

inoculation with a pure culture of V. albo-atrum - 79
Fig. 18. Spores of Fusarium vasinfectum : (a) microspores ; (b) sickle spores ;

(c) chlamydospores - - - - - - 85

Fie, 19. Sclerotinia sclerotiorum on the cucumber, showing typical sclerotia 88
Fie, 20. Tomato stems showing ‘‘ cankers”’ made by Diplodina lycopersici 90
I!

12

Fia. 21.

Fie. 22.
Fia. 23.
Fic. 24.
Fie. 25.
Fia. 26.

Fia. 27.

DISEASES OF GLASSHOUSE PLANTS

Botrytis stem rot of the anes seatarea a aes pisaons at (a)
and (6) - - =
Spores of Cercospora melonis - - - - é

Cucumber ‘‘ leaf spot”? caused by Colletotrichum oligocheetum -
Colletotrichum oligochetum growing on cotton wool, straw and wood

Powdery mildew of the cucumber - - = 3 =
Tomato ‘“ mildew ’’ caused by Cladosporium fulvuum, showing the
fungal masses on the underside of the leaf - - es
(a) Rust of the carnation ; (6) uredo-spore ; (c) teleuto-spore ;

(d) spore mass ; (e) stigmonose of the carnation - - é

Fias. 28 and 29. Chrysanthemum rust, showing the dark spore masses -

Fria. 30.

Fia. 31.

Fia. 32.
Fic. 33.
Fia. 33a.

Fig. 34.

Fie. 35.
Fia. 36.
Fic. 37.
Fia. 38.

Fie. 39.
Fia. 40.
Fic. 41.
Fia. 42.

Fia. 43.
Fie. 44.

Fra. 45.
Fria. 46.
Pia. 47.

Septoria leaf spot of the Syne: (@) diseased leaf; (b) a
pycnidium ; (c) spores - = F =
Powdery mildew of the rose: (a) diseased leaves ; (b) summer
spores ; (c) a perithecium ; (d) ascus containing eight asco-spores
Rose leaf blotch: (a) diseased leaf; (6) spore cluster ; (c) spores

“ Buckeye ” rot of tomato fruit’ - - - = 2
*¢ Foot rot ’’ of the melon due to Bacillus carotovorus: (a) inoculated
plant; (b) control - - - - - - js

Gummosis of the cucumber caused by Cladosporium cucumerinum :
(a) leaf lesions ; (b) fruit lesions ; (c) hyphz and spores ; (d) spores

Typical appearance of bacteria pathogenic to plants - =
“ Dieback ”’ of the carnation caused by Fusarium sp: - “
“ Streak ’? disease of the pea - - - - =
Arum disease caused by Bacillus carotovorus, showing diseased
corm and roots ° - - - S < =
Mosaic disease of the tomato - - - ~ c
Mosaic disease of the cucumber, showing the distorted leaves -
‘Box and grid ” for steam sterilization - - - -

Steam sterilization by the small grid method: (a) diagram show-
ing apparatus in position ; (b) section illustrating the method of
placing the grid in the trench; (ce) section showing the soil in
position ready for steaming ; (d) portion of grid showing position
of the holes ; (e) water trap - - - - -
Steam boiler - ~ - - - - - -
Tray method of Pheer BOWES Saye and pipes ready © Ree
in position -

Trays down ready for steaming to begin - - 7 ‘4
Tank method of steaming - - - - - %

Drain pipe method of steaming - - - ~ =

PAGE

90
93
94
94
100

100

105
108

109

110
112
114

114

118
121
132
132

138
143
143
157

159
161

161
161
162
163

DISEASES OF
GLASSHOUSE PLANTS

CHAPTER I

HYGIENIC CONDITIONS OF GLASSHOUSES IN
RELATION TO HEALTH AND DISEASE
IN PLANTS

One of the most important things that every grower of
plants has to learn is the intimate relationship which
exists between the health of a plant and the many factors
- which make up its environment. A discussion of plants
and their diseases with growers of wide knowledge and
experience invariably culminates in a general agreement
that the ravages of many plant diseases may be restricted,
if not entirely prevented, by providing suitable con-
ditions of growth. In the case of outside crops the
cultivator is largely in the hands of the weather, but
with those grown under glass the conditions should be
thoroughly well under control. Great advances have
been made in this direction since the building of the first
commercial glasshouses, but those who have opportunity
to study the existing appliances and methods realize
only too well that there is yet much to learn, and that a
great deal of very careful investigation will be needed
before we may be reasonably satisfied with our methods,

Situation

It has long been recognized that the position of a
nursery is an important factor in relation to the health
13

14 DISEASES OF GLASSHOUSE PLANTS

of the plants to be grown, and careful attention to this
point when choosing a site will save the grower much
worry and perhaps serious loss in ensuing years.

(a) Slope-—An important point is the slope of the
land. Wherever possible a slope with a southern aspect
should be chosen; but the degree of slope is also im-
portant, for it has a considerable bearing upon two
factors of the plant’s environment: (1) the temperature
of the air within the glasshouses, and (2) the efficiency
of the drainage. In houses built in the block formation
without dividing partitions there is a tendency for the
heated air to accumulate at the highest part of the block.
This uneven distribution of temperature has a detri-
mental effect upon the total yield per acre and increases
susceptibility to diseases. Several instances have been
observed in big blocks of tomato houses built on severe
slopes, where ‘“‘mildew” (Cladosporium fulvum) has
completely destroyed the plants at the highest parts
of the block. This is because the rapid spread and
vigour of attack is intensified by reason of the accumula-
tion of moist hot air at these parts. Reducing the
number of houses in a block by building dividing parti-
tions at every seventh or tenth house has invariably
resulted in preventing a recurrence of severe attacks of
‘“‘ mildew.” To maintain an even temperature, therefore,
the slope must be gentle. The presence of depressions
in the surface of the ground should be avoided by careful
levelling, as at these places the atmosphere tends to
stagnate. By taking careful records it has been
demonstrated that the percentage humidity of the air
over a depression is appreciably higher than that above
the general level of the ground, and observations have
shown that certain diseases generally begin at these places.

(6b) Drainage.—Upon the nature of the soil and the
slope of the land depends the all-important factor of
drainage, and consequently the former must be taken
into account when the drainage of any particular site
about to be chosen is considered.

HYGIENIC CONDITIONS OF GLASSHOUSES § 15

The tomato, one of the most important crops under
glass, is especially sensitive to the drainage factor, and
serves as an example of the way in which plants react
to the water conditions existing round or near the roots.
Generally speaking, tomato plants thrive best in a well-
drained alluvial soil, such as is found in many parts of
the Lea Valley. It is well known that the tomato
produces its highest yields in soils where it may be given
abundant water which readily drains away through the
subsoil, and some of the best crops have been produced
in the neighbourhood of sandpits, where the water may
be seen literally pouring away. On the other hand,
the application of too much water on lands where the
drainage is poor will give rise to a poor, unhealthy crop.

While the best results are obtained with abundant
water accompanied by efficient drainage, it is of
paramount importance that where these conditions do
exist the water supply must be sufficient to keep the
soil uniformly moist and yet allow for loss by drainage.
Should a deficiency occur even for a day in the water
supply of a tomato nursery the effect on the plants will
be evident at a later date. Where insufficient water
is provided over a number of days “ blossom end rot”
of the tomato fruits is almost sure to appear a few weeks
later. When the result is as marked as this, however,
the bad effect of insufficient watering is obvious; but
in many cases the outward signs are not so evident and
are to be looked for in a reduced yield and vitality.
Lands, however, in which the drainage is too rapid are
in the minority, and by far the greater amount of trouble
arises from insufficient drainage. The existence of
insufficient drainage in the subsoil is not always apparent,
but careful observation on the grower’s part will generally
reveal it. This is the case when on apparently ideal
soils, with a suitable inclination, there exists a hard pan
or impervious stratum of soil at some distance below
the surface. The existence of such may be a limiting
factor in the health of the crop, for if the pan is saucer-

16 DISEASES OF GLASSHOUSE PLANTS

shaped the water will lie in the depression and harmful
effects invariably follow. Cases similar to this have
been investigated, and all remedial measures were of no
avail until the pan was broken up mechanically.

(c) Relation to Neighbouring High Land.—In choosing
the site for a future nursery it is important to consider
the relation of the particular area to the height of the
surrounding land, for upon this depends two other
important factors: (1) the height of the water table in
the soil, and (2) the extent of drainage water passing
through the site.

Upon the level of the water table depends the nearness
of stagnant water to the plant roots. Should the water
table be high the plants become “ chlorotic ” or yellowish
in appearance as soon as the roots reach the waterlogged
soil. The “tops”? become weak and sickly, and the
vitality of the plants is much reduced, with the con-
sequence that they become an early prey to parasitic
fungi or bacteria.

Such conditions have been found to exist at the base
of hills which border low-lying plains near to rivers,
and sites in such regions should be avoided wherever
possible.

Upon the surface drainage water depends to an
important extent the question of contamination with
fungus and bacterial diseases introduced by such drainage
water from the neighbouring high lands.

Two cases, personally investigated by the writer, in
which epidemics originated in this manner will serve to
emphasize the importance of this factor.

In the first, the tomato nursery in question was
situated in a natural depression at the base of a wide
ridge, and running close to the block of houses was a
natural watercourse draining the high land. <A heap of
soil, part of which had been used each spring during the
previous six years for raising seedlings and young plants,
lay at the side of the watercourse. In 1919 tomato
seedlings raised in nurseries on the top of the ridge had

HYGIENIC CONDITIONS OF GLASSHOUSES 17

suffered badly from ‘‘ damping off ”’ caused by a fungus,
Phytophthora cryptogea. During the winter 1919-20 a
heavy rain caused the drainage water from the high land
to flood the depression below. The heap of soil by
the watercourse was half submerged for about eight days,
and remained in a sodden condition during the rest of
the winter. The nursery in the depression had previously
been free from ‘‘ damping off,’ but in 1920 an epidemic
broke out which caused the death of thousands of young
plants and threatened to prevent the cultivation of
tomatoes for that season at least. Careful tests showed
that the heap of propagating soil was badly infected,
and indicated that infection had been introduced in the
drainage brought down during the winter flood.

In the second case, a tomato nursery lost 70,000
plants from the same disease, and the infection was
traced to the surface drainage water brought from
another nursery, where tomatoes, asters, and wallflowers
were badly attacked. It is thus evident that disease
organisms in contaminated drainage are of immense
importance, especially in areas where nurseries are con-
gested together, and if a site is chosen in which there is
danger from this source, the nursery should be protected
by laying drains round it, so as to divert the natural
drainage.

Glasshouse Construction

By reason of the marked improvements which have
resulted from the evolution of modern glasshouses from
the older types, and the consequent alteration in glass-
house management, the grower is now in a better position
to control the environment of his crops and to elimi-
nate many troubles which formerly were of common
occurrence.

Of all the many factors which are intimately connected
with the construction and management of glasshouses
there is probably none which has not a direct bearing

2

18 DISEASES OF GLASSHOUSE PLANTS

upon crop production and the ability of the plant to
resist disease. Not only are the physical, chemical,
and biological factors of the soil of great importance,
but a whole series of others which are related
to the supply of heat and light are equally important.
Among these may be cited the aspect of the house,
angle of the roof, size and quality of the glass, size,
position, and number of purlins, posts, bars, etc., amount
of air space, and system of ventilation.

(a) Light in Relation to Plant Growth.—The importance
of the light factor will be immediately realized when
one considers that practically 95 per cent of the plant
substance is produced from the carbon dioxide of the
air by the chemical activity of light, working in con-
junction with the chlorophyll, or green colouring matter,
of the leaves. All parts of the spectrum rays do not
have the same effect upon plant development, for the
orange and red rays are especially concerned with the
assimilatory processes, while the blue, indigo, and
violet rays stimulate growth.

Contrary to the general idea, plants grow. most at
night and least in the day; but during the day they
manufacture their food from the air under the influence
of light. Investigations in the past have indicated that
light has a markedly inhibiting effect upon plant growth,
but, on the other hand, that it assists the development
of the supporting tissues. When grown in darkness
plants become yellowish white in colour owing to reduced
chlorophyll production. They become etiolated and
develop long, weak leaves and shoots. It is thus obvious
that the light intensity and the nature of the light rays
which penetrate the glass have an important bearing
upon success in glasshouse culture. This being so, it
is important to consider the many factors in glasshouse
construction which influence the quality and quantity of
light available to the plants the houses may contain.

(6) Glasshouse Construction in Relation to the Light
Factor—When compared with the modern types, the

HYGIENIC CONDITIONS OF GLASSHOUSES 19

earliest glasshouses appear as dimly lighted structures.
In their construction a large amount of timber was ~
used, and the glass, of inferior quality, was cut into small
panes, in the fitting together of which big overlaps were
allowed. Gradually, as the importance of good lighting
was better realized, there commenced an evolution in
methods of building which to-day finds expression in
our much-improved structures, and which must still
continue if the best results are to be obtained. In the
process of development less heavy and clumsy super-
structures and larger glass of superior light-transmitting
qualities were employed, with a consequent improvement
in the health of the crops. While improvements in
house construction have continued to make steady pro-
gress, it is unfortunate that so little attention has been
given in the past by scientific investigators to glasshouse
construction in relation to light intensity. Important
contributions, however, have been made recently by
Stone (48), whose work clearly shows the immense
importance of further study along these lines.

His investigations were concerned with a study of
light intensities in glasshouses of different. construction
and under different conditions, and while important
results were obtained, the ultimate proof of the superiority
of any given type of house will depend upon direct
evidence obtained from a study of crops grown in such
houses. Unfortunately these investigations cost money,
which, in these days of strict economy, is difficult to
obtain; but the ultimate end of such investigations
must lead to information of great economic importance,
far outweighing in money value the comparatively small
sum required to gain it.

Stone showed that morning light is more intense than
afternoon light, his experiments giving a difference of
intensity varying from 10 per cent to 30 per cent, in
accordance with the time of the year. These results
indicate that the orientation of glasshouses in relation
to a north and south aspect has an important effect upon

20 DISEASES OF GLASSHOUSE PLANTS

the yield and health of the crop, and suggests that houses
built so that the crop receives the maximum morning
light will give the best results. Another point of im-
portance, in view of the uniform construction of glass-
houses in this country, is the effects produced by roofs
of different angles. Past work has shown that the more
nearly the roof is placed at right angles to the sun’s
rays the more light is transmitted. Records made by
Stone in February showed that a house with a roof
angle of 46° gave 18 per cent more light than one with a
32° roof angle.

Considerable differences exist in the light-transmitting
properties of various types of glass, as was evident to
purchasers of this material shortly after the war years ;
and it is common knowledge that irregular surfaces,
flaws, bubbles, etc., in the glass are detrimental.
Experienced growers are extremely careful in selecting
glass for their houses, and panes with flaws are invariably
rejected. Bubbles and irregularities act as lenses and
may cause “ scorching ”’ in leaves near to the glass, and
the former frequently are painted over by growers of
such delicate plants as lilies. Stone has shown that the
amount of light excluded by different samples of glass
may vary from 13 per cent to 36 per cent of the total
light.

Researches of some interest to practical glasshouse
growers were performed by, Flammarion (21), who
compared the light and heat values of white glass with
those of red, green, and blue glass. He showed that the
temperature within the house varied with the colour
of the glass. Blue, in virtue of its great absorbing power,
gave the lowest temperature, while green, red, and white
gave increasingly higher temperatures. The height of
sensitive plants grown in a red house was fifteen times
as great as that ina blue house. Plants in a red house
came into bloom first, and their foliage was lighter in
colour than those grown in a white house, while the
foliage under blue glass was still darker incolour. After

HYGIENIC CONDITIONS OF GLASSHOUSES 21

three months Flammarion found that the height in the
red house was 0:5 metres, in the white house 0-38 metres,
in the green house 0-1 metres, and in the blue house
0-035 metres.

While plants in the red house were the highest they
were not the heaviest, for the weight of stem and leaves
in the white house was 8-4 gms., in the red house 4:6 gms.,
in the green house 0-3 gms., and in the blue house 0-15 gms.
Lettuce plants grown under white glass developed large,
thick leaves with good heads ; under red glass the leaves
were drawn, long, and drooping; under green glass
they made a slight growth; while under blue glass
practically no growth occurred. Experiments with peas,
beans, and coleus plants yielded similar results.

Further experiments were concerned with root develop-
ment under different glasses, and indicated that the root
systems were greater under white glass than under red
glass, while poor root action was exhibited under green
and blue glass. Similarly, the mechanical tissues showed
better development in plants under white glass than
those under red, green, or blue.

Flammarion showed that the colour of many flowers
can be changed by growing them under different coloured
glass. Thus certain blue flowers become pink in a white
glasshouse and white under red, green, and blue glass.
Also, if pale blue lilac flowers are placed under a dark
bell jar they become a clear red-violet. He also showed
that the normal amount of red pigment in coleus leaves
is reduced under red glass, and practically disappears
under green and blue glass.

These experiments of Flammarion are of practical
interest to growers of plants under glass as pointing
to the importance of using glass of consistent purity,
because the preponderance of any group of light rays
has an important bearing upon the plant cultivated
under their influence.

(c) Glasshouse Construction in Relation to Air Capacity
and the Ventilation Factor.—Just as the light conditions

22 DISEASES OF GLASSHOUSE PLANTS

of the modern glasshouse are an immense improvement
upon those of the earliest houses, so advancement has
been made in air capacity and ventilation.

There is still, however, much to be done in the latter
respect, for the ventilation of modern glasshouses is far
from satisfactory from a plant disease standpoint. It
is true the air capacity has been greatly increased by the
building of higher and larger houses, but the methods
of circulating the air need much improvement. It is
a well-known fact that the incidence of many diseases is
so intimately connected with stagnant air rich in water
vapour that they may be greatly reduced, and in many
cases entirely checked, by increasing the amount of
ventilation.

Necessary investigations should be concerned with a
study of the number, size, and position of ventilators in
houses of different shapes and sizes which are required
to produce the correct number of complete changes of
air in any given time. There is evidence to show that
the mere movement of air in a house, apart from intro-
ducing fresh air and expelling the old air, has a beneficial
effect upon the plant, and adequate experiments should
be arranged. The importance of the relation between
the cubical contents of a house and the area covered
is apparent from several standpoints. High houses,
exemplified by the “aeroplane” type of tomato house
(14 or 15 feet wide, 7 feet to the gutter, and 11 feet to
the ridge), allow unrestricted development of the plant,
the tops of which in low houses become congested
towards the end of the season. Another effect of abun-
dant head space is improved ventilation and circulation
of the air.

On the other hand, the larger volume of air in houses
of the high type requires more heat to raise it to a given
temperature and maintain it at this than does the smaller
volume of a low house. The evolution of the larger,
better lighted glasshouses has produced a marked change
in methods of crop management, with the result that

HYGIENIC CONDITIONS OF GLASSHOUSES 23

modern crops are better grown and less susceptible to
disease than were those in the earliest glasshouses.

The Heating of Glasshouses

- While it is common knowledge in a general way that
_ different crops require different temperatures to enable

them to resist disease and produce their maximum yield
there is yet but scanty information as to the exact
limits of temperature required by each crop. It is,
however, obviously of great importance that this informa-
tion should be obtained as soon as possible. Not only
do different crops require different temperatures, but
each requires a different temperature at particular stages
of growth. A plant’s activity reaches its maximum
at the optimum temperature, other conditions being
suitable. Below the optimum temperature growth is
retarded and vitality reduced; while above this
temperature the plant also suffers, for the rapid growth,
unaccompanied by proper maturing of the tissues, not
infrequently leads to death through disease.

Recent investigations have shown that the temperature
of the soil is equally as important as that of the air,
and if the two temperatures are very different the plant
suffers accordingly. For instance, cucumber plants
grown in cold, damp beds in a warm house frequently
wilt badly, and will die if the soil conditions are not
improved quickly. This is caused by the inability of
the cold roots to supply the leaves with sufficient water
to compensate for the loss of water by transpiration.
The soil and air temperatures have an important bearing
upon disease. If they are too low or too high the resist-
ance of the plant is appreciably lowered and it becomes
an easy prey to disease. On the other hand, the optimum
temperature for the plant may coincide with that of a
fungal root or stem parasite which is attacking it, and it
may be beneficial to lower or raise the temperature slightly
to make the conditions less favourable for the disease.

24 DISEASES OF GLASSHOUSE PLANTS

Investigations upon temperature requirements are
thus of great importance, both from the point of view
of the grower and the pathologist, but unfortunately
they are expensive to carry out, for specially controlled
chambers are necessary.

The present methods of heating glasshouses by means
of pipes in which either steam or hot water circulates
are undoubtedly expensive, as is indicated by the fuel
bill each season, and to those who have studied the
question it is obvious that many improvements could
be made. It is a little surprising, perhaps, that the
various associations of glasshouse growers have not
enlisted the services of a skilled engineer to study the
heating question, for this would certainly lead to a
considerable saving in the cost of heating.

The Humidity of Glasshouse Atmospheres

The importance of this factor deserves to be appreciated
to a much greater extent by glasshouse growers than it
is at present, for upon it depends such important matters
as the condition of foliage and flowers, the setting of
fruit, and the incidence of leaf, stem, flower, and fruit
diseases.

As with temperature, so the optimum conditions of
humidity vary with the kind of plant grown. Ferns
and plants of similar habitat thrive in an atmosphere of
high humidity which would result in disease in plants
like the tomato and vine. If the atmosphere is too dry

the leaves, flowers, and fruits of most plants are under-.

sized and hard, but if too moist there is a tendency
towards a soft, sappy growth possessing a low disease
resistance. ‘The humidity of the atmosphere is a limiting
factor in many fungous and bacterial diseases of the
aerial parts of plants.

In general, a large proportion of moisture in the air
is favourable to abundant spore production, while a
dry atmosphere with suitable air currents is favourable

F

HYGIENIC CONDITIONS OF GLASSHOUSES — 25

to the efficient dispersal of the spores. The presence
of a film of moisture on the surface of the plant enables
the spores to germinate and enter the tissues. Con-
sequently high humidity should be avoided when plants
are exposed to infection.

An intimate relationship exists between the humidity
_ and the temperature, for an increase in the latter produces
a proportional decrease in the former. The humidity
in glasshouses of the taller types is generally less than
that in low houses, for the larger volume of air in the
former requires more water to raise it to a given degree
of humidity than the smaller volume in the latter.
Consequently leaf diseases are the more easily controlled
in houses with high gutters and ridges.

Watering

The process of watering plants is probably one of
the most important of all horticultural practices. Many
experienced growers have reduced it to a fine art, but
their methods are largely intuitive and difficult to explain.
Insufficient watering prevents the plant from reaching
its maximum development and produces physiological
disorders, but error on this side is preferable to watering
in excess. Excessive watering leads to waterlogged soil
conditions, except on extreme sandy soils. An excess of
water in the soil produces a deficiency of air about the
plant roots, and serious complications result. Such
conditions produce a reduction in yield and resistance
to disease, and in extreme cases death by suffocation.
The careless splashing of water about the base of plants
causes harm in many ways. It may chill the plants and
so reduce their vitality ; it changes the physical nature
of the soil, which puddles and dries hard at the surface ;
and it leaves the lower foliage hanging in water, pro-
viding ideal conditions for infection by certain organisms.
It increases the rate of spread of such diseases as “ buck-
eye rot” of the bottom trusses, “foot rot” and

26 DISEASES OF GLASSHOUSE PLANTS

‘“‘ damping off ”’ of tomatoes, and also “‘ canker ” of the
cucumber and melon. Surface watering by means of
hoses is almost universally adopted by the practical
man, and ingenious methods have been devised to
regulate the rate of distribution.

In this country overhead watering is still in its infancy,
but it possesses the advantage of uniform distribution.
A dry atmosphere can be moistened quickly, which is
an advantage during hot summer days, but a disad-
vantage in dull weather, as the excessive moisture on
the leaves encourages disease. Also, because it is purely
mechanical, it does not cater for the special needs of
individual plants.

The use of perforated hoses, laid on the ground in such
a manner that the issuing jets make impact with the
soil midway between the plants, has lately been adopted
and has proved an efficient and labour-saving device.
Sub-irrigation has not so far found favour with the
practical man, but experimentally it has proved
extremely successful. The absence of surface moisture
prevents the rapid spread of fungus parasites, and
consequently diseases such as “damping off” and
** foot rot ”’ are avoided.

Mulching

The process of mulching, so familiar to nurserymen,
fulfils the dual purpose of conserving the soil moisture
and providing a top-dressing of plant foods. While it
is recognized that on some lands mulching is a necessary
adjunct to the growing of profitable crops, the view is
gaining favour that on other lands the cost of this practice
is out of all proportion to the effect produced.

During the years of war stable manure was difficult
to obtain, and where procurable was obtained at a high
cost. In the tomato industry, where mulch is applied
at the rate of 20 to 25 tons per acre, these conditions
induced some nurserymen to refrain from mulching their

HYGIENIC CONDITIONS OF GLASSHOUSES 27

crops. As a result it was found that on moderately
heavy soils the unmulched tomato plants did not produce
a lower yield than those mulched with stable manure,
and preliminary experiments at Cheshunt Experimental
Station have confirmed this. It is apparent that the
need for mulching varies with different nurseries, and
nurserymen will be wise to carry out experiments on
their own soil.

While mulching on some lands may not appreciably
affect the yield per acre, it does bear an important
relation to plant disease. Mulching with contaminated
stable manure may introduce serious diseases into pre-
_ viously healthy houses, but the use of such materials
as spent hops, peat, or straw has a beneficial effect, by
preventing splashing of soil on to the lower parts of the
plant, and helps to keep the houses clean. ‘‘ Buckeye
rot’ of the bottom trusses of tomatoes may be con-
trolled by mulching sufficiently early to prevent the
disease organisms from being splashed on to the trusses
from the soil; but while early mulching will result in
healthy bottom fruit, it must not be applied too early.
Experiments have been performed whereby a mulch of
straw was laid on the ground prior to planting, the straw
having to be moved aside to allow the plants to be
placed in position. It was found that such plants grew
very slowly compared with those planted in the usual
way. ‘Temperature records showed that the soil covered
with straw was many degrees colder than that uncovered,
and accounted for the slow growth of plants in the former.
On removing the straw mulch for ten days the soil
became warm and the plants grew rapidly. It is
necessary, therefore, to allow the soil to become
thoroughly warmed before applying the mulch. The
kind of mulch, its condition, and rate of application
have an important bearing upon plant diseases, for,
apart from the introduction of disease organisms, such
factors influence the plant’s susceptibility to disease.

To obtain the best results mulching should be regulated

28 DISEASES OF GLASSHOUSE PLANTS

by the individual crop requirement and cannot be
performed by mere rule-of-thumb methods.

Pruning and Training

These processes are necessary for the production of
clean, good quality fruit, for, as a general rule, plants
which are pruned produce fewer flowers and fruits, but
these are of a better quality than plants which are not
pruned. Methods of pruning and training are of con-
siderable importance in disease control. Not only does
skilful pruning determine flower and fruit production,
but by opening up the foliage it allows free circulation of
air and prevents stagnation, which is so favourable to
disease. At the same time the resulting wounds are
important. All cutting should be done with a sharp
knife, and the wound must be quite clean. Where
leaves and laterals are removed, they must be cut off
close to the point of origin. Stumps must not be left
to dry up and decay, for these provide suitable entrances
for fungi of all kinds. Even those fungi which normally
do not attack the living plant may do so after growth on
these half-dead tissues. It is especially important to’
realize this in tomato cultivation, where careless pruning
and defoliation, which leaves jagged wounds, is the
primary cause of stem rot by Botrytis sp.

Disposal of Dead and Diseased Tissue

The problem of destroying the remains of healthy
and diseased plants is one which every grower has to
face, and the degree of freedom from disease exhibited
by his crops depends very considerably upon the
thoroughness of this process. In many places the dead
plants are allowed to lie for many months in large heaps
close to the houses. The plants decay and become
centres of infection from which disease is spread broad-
cast. There is only one satisfactory way of disposing

HYGIENIC CONDITIONS OF GLASSHOUSES 29

of such remains, which is to burn them immediately
they are removed from the houses. Every nursery
should possess an incinerator for this purpose, and the
beneficial results of its use will far more than compensate
for the initial cost.

Sources of Infection of Plant Diseases

(a) The Water Supply.—The possibility that con-
taminated water supplies are a frequent source of
infection to glasshouse crops was indicated in 1919,
when there were attacks of “damping off” of tomato
seedlings, caused by Phytophthora cryptogea, and “ buck-
eye rot” of tomato fruits, caused by Phytophthora
parasitica.

By spraying healthy seedlings and bunches of fruit,
under properly controlled conditions, with samples of
the suspected waters it was possible to reproduce the
diseases in question, and it thus became evident that a
centre of infection existed in certain nursery waters.
A suitable method of examining water supplies was
devised by filtering large volumes through special wire
and cotton wool filters, and a systematic examination
of the waters used in nurseries in the Lea Valley district
was made.

Important results (8), were obtained, for it was demon-
strated that while water from some sources was free
from contamination, that from others contained many
fungal and bacterial parasites of glasshouse crops.
Water from the Water Company (Metropolitan Water
Board) and deep artesian wells was generally free from
contamination. Samples from wells receiving surface
drainage contained large numbers of plant pathogens,
deep wells of this kind being only slightly less con-
taminated than shallow wells; but shallow wells placed
some distance away from the glasshouses were generally
purer than those surrounded by houses. Wells whose
waters contained a large amount of decaying organic

30 DISEASES OF GLASSHOUSE PLANTS

matter were richer in fungi than those whose waters
were clearer. Water from brooks and ponds was also
highly contaminated, but a slight reduction in the
number of fungus species results from thoroughly clearing
away the vegetation on the banks. Water which had
passed through an uncovered tank was more highly
contaminated than that from a covered tank. The
importance of a pure water supply is obvious, and every
nursery should possess a deep artesian well, suitably
protected from contamination by surface drainage.

(b) Straw Manure.—Straw manure has frequently
been regarded with suspicion as a possible source of
infection with plant diseases, and investigations at
Cheshunt Experimental Station have indicated that such
may be the case (7). Some disease-causing fungi thrive
on this material and readily permeate large masses
when they gain access to them. Our investigations were
concerned primarily with the introduction of cucumber
anthracnose caused by Colletotrichum oligochetum, but
they also showed that other plant parasites may be
introduced in this manner. Straw manure from different
sources was examined, and while many were found to
be free from plant pests others were badly contaminated.
Manure from country farms was mostly free from disease,
but that obtained from town stables was frequently
contaminated. Heaps of straw manure lying adjacent
to those of decaying plant debris on commercial nurseries
were found to be contaminated.

Growers should exercise particular care in the choice
of their straw manure, and that which is obviously
contaminated with half-rotten potatoes and roots should
be avoided.

(c) Imported Plants—Growers whose land is free
from disease should be warned against importing plants
from other nurseries, but where this is unavoidable the
imported plants should be isolated until it is certain
they are free from disease. Jt must be clearly understood
that it is easier to keep disease away from a nursery than to

HYGIENIC CONDITIONS OF GLASSHOUSES 31

eradicate it once it has gained entrance. Our experience
in the Lea Valley has shown quite clearly the danger of
growing plants other than those raised from seed on the
nursery, and many serious diseases have been introduced
in this manner.

(d) Baskets, Sacking, etc.—Another source of infection
which becomes more and more evident is the contamina-
tion carried in “ strikes ”’ or baskets, and a considerable
number of cases have been noted where infection with
various diseases came from these articles. Such baskets
may be so mixed at the market that when they are
returned those from one nursery are sent to another,
and so disease is spread. Baskets should not be taken
near to the growing plants for fear of introducing some
new trouble, and during the winter months all baskets
should be sterilized in readiness for the coming season.

(e) Nursery Workers.—Diseases of all kinds are readily
spread by the workers in the houses. Infection may be
carried on the hands, clothes, and boots, as well as on
pruning knives, and therefore assistants working in
diseased houses should not enter houses where healthy
plants are growing unless their hands have been washed
and their clothes either stoved or exposed to direct
sunlight. The careless visiting of nurseries by workers
from other places should be prevented, for cases are
known in which disease was spread in this manner.

Much of the existing knowledge of crop management
has arisen from skilled growers as the result of long
experience, and as most of it is intuitive it is difficult
to explain to others. Many a grower has learned how
to produce good, healthy crops on his own sample of
land and under conditions operative on his own nursery,
but can give no definite reason for many of his practices.
It is thus evident that the fundamentals concerned in
crop management should be exactly determined by careful
investigation, so that definite reasons for each operation
may be available, and the grower may be in a position

32 DISEASES OF GLASSHOUSE PLANTS

to apply the correct methods of treatment for the exist-
ing conditions or for new conditions. Such investigations
are in progress at the Experimental and Research
Station situated at Cheshunt, in the centre of the glass-
house industry in the Lea Valley, and their aim is to
supply a rational basis for existing practice and to
improve this practice.

CHAPTER II

DISEASED CONDITION OF PLANTS DUE TO
ENVIRONMENTAL FACTORS—LIGHT, HEAT,
HUMIDITY, SOIL, ETC.

THE various factors which comprise a plant’s environ-
ment have an important bearing upon its health; and
while certain conditions have a detrimental effect upon
its power of resistance to disease, others actually induce
a diseased condition.

Light

_ Itiscommon knowledge that plants grown in darkness
become elongated, slender, whitish in colour, and im-
mediately wilt when exposed to the heat and light of an
ordinary glasshouse.

Similar effects are produced in a lesser degree when
glasshouse plants are exposed to long periods of dull,
cloudy weather.

For instance, plants grown during November and
December possess imperfectly coloured weak leaves,
with slender petioles and badly developed stems. The
weak light is insufficient to mature the mechanical and
resistant tissues, and consequently such plants wilt in
the bright sun of early spring. This physiological
wilting is well known to growers of early cucumbers,
and indeed may appear later in the year if after long
periods of dull weather the plants are exposed to high
temperatures and bright light. Such plants recover
during the night, but wilt again during the day. The
trouble becomes accentuated in weak plants grown in

3 33

34 DISEASES OF GLASSHOUSE PLANTS

houses which are badly ventilated and too hot. It is
thus apparent that the physiological condition of a plant
may be adversely affected by light conditions.

As a general rule, insufficient light produces immature
tissues, which are more susceptible to disease than those
developed under well lighted conditions. Thus the
mildews of the grape, cucumber, and strawberry develop
most rapidly in shaded positions or in dull weather,
and may entirely disappear when exposed to strong
sunlight. ‘‘ Damping off” of seedlings also has been
found to increase most rapidly in the shade. De Bary
was able to induce susceptibility to attack by Botrytis
in petunias by placing them in the dark for some days,
and it is a matter of common observation that those
parts of the tomato which are shaded are the most
easily attacked by this fungus.

On the other hand, numerous observations have been
made which indicate that half shade has a beneficial
effect upon certain plant diseases. Thus, shaded celery
plants are generally free from early blight caused by
Cercospora apii, and asparagus rust caused by Puccinia
asparagi is reported to react in a similar fashion. Again,
observations at Cheshunt Experimental Station have
shown that shade has a beneficial effect upon tomato
plants attacked by Verticillium albo-atrum.

Sunlight which is too intense may have an effect upon
plants by burning them. Thus “tip burn” of lettuce
and tomato occurs under glass when brilliant sunshine
follows periods of dull weather. In lettuce plants the
outer leaves wither at the tips and the affected areas
curl backwards and turn brown. “Tip burn” of the
tomato consists of a withering, curling, and browning of
the tender tissues at the growing point. During the dull
days which precede the burning the plants have been
transpiring very little because of the high percentage of
humidity in the glasshouse atmosphere, and the tissues
become gorged with water. The sudden appearance of
brilliant sunshine produces a rise in temperature and a

_ DISEASED CONDITION OF PLANTS 35

decrease in the humidity. Consequently the plants
transpire more freely and the roots may be unable to
replace the water lost by the leaves. If this continues
for any length of time the more tender parts collapse,
and finally turn brown and wither. The practical
remedy is to damp the soil surface on bright days follow-
ing dull weather, and by thus keeping the air moist
prevent any sudden increase in the rate of transpiration
of the leaves.

Tomato fruits suffer from sunburn during prolonged
periods of intense sunshine. The tissues round the
stalk end of the fruit become dry, brown, and hard.
Small cracks appear, giving the surface a rough powdery
appearance. In most commercial nurseries sunburn
of the tomato fruit is comparatively rare, and the little
that does appear may be disregarded, but where it is
prevalent the houses should be shaded with limewash
and varieties with dense foliage should be grown.

Heat

The relation between temperature and disease must
be considered with regard to its effect upon the plant
complex and the pathogen complex, both singly and
combined.

The Plant Alone.—The heat requirements of glasshouse
plants vary considerably, for not only do different crops
require different temperatures for their maximum
development, but different temperatures are required
at different stages of growth. The minimum growth
temperature is just sufficient to prevent the ces-
sation of plant processes, which are speeded up as
the temperature rises, and attain perfection at the
_ optimum temperature. A further increase in temperature
upsets the balance of the processes and produces abnormal
plants. Maximum development, flower and fruit pro-
duction, and resistance to disease occur at optimum
temperatures, which do not necessarily coincide; but

36 DISEASES OF GLASSHOUSE PLANTS

any deviation from the particular optimum produces
an adverse effect proportional to the extent of deviation.

The temperature of the soil is just as important as
that of the atmosphere, and both must be taken into
account when considering temperature effects. Harmful
effects are produced in the plant when these two tempera-
tures do not approximate to one another, as instanced
by the wilting of cucumbers planted in cold beds in a
warm house. The chilled roots are unable to absorb
water from the soil with sufficient rapidity to replace
that evapora ted from the leaves. Similar adverse effects
upon the physiological processes, while not so obvious
as this, are produced whenever an appreciable difference
occurs between the two temperatures, the effects being
proportional to the difference between them. In con-
sequence the plant’s vitality is impaired and its power
to resist disease is reduced. Generally speaking,
temperatures between the optimum and maximum tend
to the development of soft, sappy growth and an increased
susceptibility to disease, while those below the optimum
encourage slow, hard growth accompanied by an in-
creased resistance to disease. Exceptions, however, are
found in respect of certain vascular diseases, to which
hard, slow growing plants with thin stems are more
susceptible than soft, rapidly growing plants with
thicker stems.

The Pathogen Alone.—The fungus or bacterial parasite
reacts to temperature in much the same way as the plant,
and possesses minimum, optimum, and maximum tem-—
peratures for growth. As a general rule fungi can
survive very low temperatures. Most fungi are uninjured
by exposure to a temperature of 0° C. and can be frozen
solid without undue harmful results. Fungus spores are
not destroyed to any useful extent by the normal winter
conditions which prevail in this country.

Similarly, many fungi and bacteria are able to survive
exposure to high temperatures. The thermal death
point of some fungi has been found to be as high

DISEASED CONDITION OF PLANTS 37

as 75°C., but usually few will grow at temperatures
above 40°C. The spores of certain bacteria are highly
resistant to high temperatures, and in some cases have
not been killed by boiling for two or three hours.

Comparatively little information is available con-
cerning the resistance of fungus spores to high tempera-
tures, but it would appear that some resting spores
and sclerotial bodies are capable of withstanding high
temperatures.

The optimum temperature for growth of many fungi
does not coincide with that for spore production, and it
is important that this should be taken into account
when studying plant diseases. Thus, at a certain
temperature the fungus will progress most rapidly
through the plant tissues, and at another temperature
most readily produce spores and increase the rate of
spread from plant to plant.

Plant and Pathogen Combined.—While the temperature
relation of many fungi in pure culture has been studied,
it is only within the last decade or so that any research
has been carried out on the relation of temperature to
the process of infection. This is largely the result of
investigations by the United States Department of
Agriculture and by Professor L. R. Jones and his col-
leagues at the University of Wisconsin.

The temperature of the soil has been shown to be a
limiting factor in certain diseases, among which may be
mentioned the Fusarium wilt of tomatoes. Edgerton (20)
has shown that infection takes place and the disease
_ develops most rapidly if the soil temperature is main-
tained around 29° C. Very little infection occurs if the
temperature is much below this for any length of time.
Investigations upon the Verticillium wilt of tomatoes
carried out at Cheshunt Experimental Station illustrate
the importance of air and soil temperatures in con-
ditioning the progress of disease (10). It was shown that
temperatures between 16° C. and 24° C., with an optimum
of 21°-22°C. are favourable to the rapid spread of the

38 DISEASES OF GLASSHOUSE PLANTS

Verticillium wilt, which below 16°C. and above 24°C.
is exceedingly slow. Plants badly wilted at a temperature
of 20°C. recovered when transferred to a temperature
of 25°C., and cultural methods based on these facts
were devised for controlling the disease. Recently
Johnson (26) has demonstrated that Mosaic disease of the
tobacco is most virulent between 28°-30°C., and that
above 36°-37°C. mosaic plants send out new leaves
showing no symptoms of the disease, and the chlorotic
leaves may recover their normal colour.

The value of such work will be appreciated by all
growers of glasshouse plants, who are more favourably
situated for controlling the temperature to which their
crops are exposed than those who cultivate outside
crops.

Water

(a) Humidity of the Atmosphere.—In a similar manner
the presence of moisture in the air is related to the
incidence of disease by reason of its effect upon the plant
and the pathogen, both separately and combined.

As is well known, different plants do not require the
same amount of atmospheric moisture for their maximum
development. Thus the atmosphere of a cucumber
house must contain a higher percentage of humidity
than that of a tomato house, otherwise the plants will
suffer. When plants are exposed to an atmospheric
humidity lower than that normally required they remain
under-developed and produce small, thin leaves, but are
generally more resistant to disease. With humidities
above the normal the rate of transpiration of water from
the leaves becomes reduced. In consequence, the plants
become gorged with water, the tissues become tender,
and sappy growths are produced, which are very sus-
ceptible to disease. Abundant moisture has an important |
effect upon the fungus parasites, for it increases spore

DISEASED CONDITION OF PLANTS 39

production and is essential to spore germination, which
-is the first step towards infection of the host.

Much general observation and experiment has demon-
strated the association of fungus and bacterial diseases
with a moist atmosphere, and many epidemics have
been intimately connected with abundant rainfall and
consequent high humidity. In the glasshouse industry
it is common experience that leaf diseases originate
during periods of dull, humid weather, when the atmos-
phere of the houses remains constantly moist. The
humidity of glasshouse atmospheres is thus important as
influencing both plant growth and resistance to disease.

(b) Soil Moisture—Water is essential to life, and
glasshouse soils, being untouched by the natural rainfall,
must be artificially watered.

~The most successful growers are those who have the
good fortune to possess the faculty of knowing when to
water and how much to apply at any given time.
Unfortunately, much of the process is performed without
an adequate knowledge of cause and effect, and much
experimental work is necessary to set the practice of
watering upon a sound scientific basis, for improper
methods are a fruitful source of harm to glasshouse crops.
~ An excess of water in the soil fills up all the spaces,
and there is less room for air in the soil; consequently
soils saturated with water contain practically no air and
the plant roots are suffocated. Under such conditions
the roots cease to function and the plant wilts. Examples
of this may be found in almost every nursery, where
plants growing in the sodden patches near to a standpipe
may be seen in a wilted condition. On digging up these
plants the roots will frequently be found to be brown
and decaying. Such dying roots provide a ready means
of entrance to soil fungi. Should the waterlogged
conditions persist for any length of time the plant will
die; but if the soil is allowed to dry out and become
aerated new roots will form and the plant recovers.
The dead roots still remain as places of entry for soil

40 DISEASES OF GLASSHOUSE PLANTS

fungi, and it will thus be apparent that waterlogged
conditions, although they may persist only for a short
time, have a deleterious effect upon plants and reduce
their resistance to disease.

The appearance of plants grown under waterlogged
soil conditions is admirably illustrated by an experiment
conducted with tomatoes. A metal trough ten feet long,
two feet wide, and two feet deep was placed on the
ground and one end tilted up to rest on a support a
foot high, as in Fig. 1. A perforated wooden board was

BER EAE a a dione to-etiorgsia br the fomato.
placed in the trough, resting on the bottom of the tilted
end, laid parallel to the ground, and supported on bricks
at the other end. The space between the board and the
trough bottom was filled with water and a siphon with
tap was fitted at the deeper end. Nine inches of compost
was placed on top of the board, and six tomato plants
(Kondine Red) were planted at equal distances.
The soil was well watered, and after a time the level of
the water was adjusted to the level of the board by
means of the siphon, and was kept at this level throughout
the experiment. The plants were numbered for purposes
of description, No. 1 being at the low end of the trough
and No. 6 at the tilted end. Within five days plant
No. 1 began to show signs of ill health. The lower leaves
turned yellow and withered and the plant showed no
new growth. The yellowing and the withering of the
leaves continued up the stem until in 17 days the plant
was dead. Plant No. 2 developed similar symptoms in
14 days and was dead in 43 days. In 31 days No. 3
began to show signs of yellowing and lost the five bottom

DISEASED CONDITION OF PLANTS 41

leaves. The top of the plant became a light green colour
and yellow blotches appeared on the leaves. This
gradually became worse, the yellow blotches dried up,
and the plant developed the typical symptoms of
“ chlorosis.” After 60 days the new growth was weak
and yellow and the flowers dropped without setting.
In 37 days plant No. 4 developed symptoms of chlorosis,
which are so familiar on some nurseries, but they were
not so serious as in plant No. 3. The leaves became a
light green colour mottled with yellow and the tops
were decidedly weak. Plants Nos. 5 and 6 developed in
the normal way and produced healthy leaves and large
trusses of fruit.

Stagnant water is a common cause of chlorosis of
tomatoes, and may be found on all lands where the
water table is near to the soil surface. Experiments
conducted on such lands have demonstrated the import-
ance of efficient drainage, which has produced satisfactory
results when all other efforts of soil amelioration have
failed. “Yellows” of many plants have been shown
to be directly connected with waterlogged soil conditions.
Thus ferns lose their green colour and turn white, the
leaves drop off, and all growth ceases. When such
plants are re-potted into fresh soil they quickly
outgrow their diseased condition. Cucumbers grown
in beds placed on excessively damp borders become
chlorotic when their roots reach the base. H the
beds and borders become waterlogged the plant
wilts, and will die if the water is not quickly drained
away.

On the other hand, a deficiency of water in the soil
has a harmful effect upon plant growth. Every plant
requires water to enable it to reach its maximum state
of development, and if the soil moisture is not sufficient
the growth is less than normal. Not only is the rate of
growth affected, but the leaves are small and flower and
fruit production limited. ‘‘ Blossom-end rot” or “ point
rot’ of the tomato, which is found wherever tomatoes

42 DISEASES OF GLASSHOUSE PLANTS

are grown, serves to illustrate the intimate relation
between plant disorders and an insufficiency of soil
water. The first sign of the disease is the appearance
of a small, bruised, water-soaked spot near the blossom
end of the fruit. The spot rapidly increases in size,
turns a brownish-black colour, and becomes hard and
leathery. The discoloured area extends deep into the
fruit, the affected tissue shrinks, and the fruit flattens.
Occasionally saprophytic organisms enter the affected
region and complicate matters by inducing a soft
rot. Careful investigations have shown that soil
moisture conditions affect the development of this
disease.

Many cases of this disease have come within the
author’s experience, and drought has been an important
factor in each of them. Perhaps the most striking is
that of a nursery situated at the top of a hill where
the only water obtainable was pumped by means of a
gas engine. Three times within one year the engine
broke down and no water was available for a week.
The plants wilted badly during the day, but recovered
at night, and from 25 to 30 days after each breakdown
“‘blossom-end rot” appeared on the plants on trusses
at the same level. Five tons of diseased tomatoes were
gathered from 34,000 plants during the three outbreaks,
but during the rest of the season, when water was given
at the rate of 35,000 gallons per acre per week, no disease
appeared. Plants growing rapidly and producing soft,
sappy shoots are most susceptible to a check caused by
drought, and are therefore more susceptible to the disease.
Heavy dressings of stable manure and ammonium com-
pounds favour the progress of the disease, while nitrate
of soda and lime appear to check it.

A disease of glasshouse plants which is the result of
a number of different factors is that termed ‘“ Dropsy ”
or “‘(@idema.” In this country it occurs mainly on the
tomato and geranium, and in the case of the former is
distinguished by a whitening or yellowing of the leaf

Figs. 2 and 2a. (£dema of the geranium, showing swellings on the
leaves and stem. [Facing page 42

: a 1

DISEASED CONDITION OF PLANTS 43

veins and adjacent tissues. This is easily noticeable
from the upper surface of the leaf, but an examination
of the lower surface reveals a peculiar powdery appear-
ance on the veins. These are seen to be much swollen,
and in places the swollen tissues have become ruptured.
Similar pimples appear on the lamina above small veins.
The lesions are seen to best advantage on the older leaves,
and show as translucent areas when held up to the
light. In bad cases the spots dry up and tiny holes are
produced in the leaves. The lesions on the geranium
become corky, and in bad cases the leaves and petioles
are covered with corky spots and ridges. Microscopic
examination shows that the cells have become much
swollen and distorted, and the chloroplasts are few in
number—the excessive enlargement of cells causing the
tissues literally to burst. The disease is favoured by
poor light, a high soil temperature, and over-watering.
Frequently the plant recovers when removed out of doors,
and control may be effected by lowering the soil tempera-
ture, reducing the water supply, and providing abundant
light and ventilation.

The Soil

The intimate relationship between soil conditions and
the growth and health of plants has been apparent to
growers from the earliest times, and successful cultivation
largely depends upon a knowledge of the effect of the
many factors which influence the conditions operative
about the plant roots. Many excellent treatises have
been written about the soil, and as these are available
to every one there is no need to discuss the problem in
any but the briefest way.

The physical conditions, chemical conditions, and
vast population of microscopic animals and plants in
the soil influence its fertility and freedom from disease,
and therefore are important factors in crop production
and health.

44 DISEASES OF GLASSHOUSE PLANTS

THE PHYSICAL CONDITION OF THE SOIL

The nature, size, and manner of arrangement of the
particles of the soil very largely determine its texture,
which is so intimately related to plant growth and
resistance to disease. Upon the texture of the soil
depends its water-holding capacity, food-holding capacity,
degree of aeration, suitability as an anchorage ground for
plant roots, and degree of compactness.

(a) Water-holding Capacity—The soil particles vary
in size from the very finest silt to coarse sand and stones.
The smaller the particles of soil the smaller the spaces
between them, and therefore the more retentive is the
soil of moisture. Thus soils containing a large percentage
of sand are more porous than those with a large proportion
of clay or silt. The addition of straw manure to a soil,
and the consequent increase in humus, increases its
water-holding capacity.

The quantity of water, the length of time it remains,
and the ease with which it leaves the soil are important
factors in glasshouse cultivation, with its specialized
water requirements. Different crops have different
requirements, but generally speaking the best soils
possess efficient drainage and are neither too wet nor too
dry. Perhaps the most important condition is that of
constancy of water content. Soils which are alternately
very wet and very dry produce unhealthy plants, which
are more susceptible to disease than those in uniformly
moist soils. Waterlogged soil conditions also induce an
increased susceptibility to disease.

(6) Food-holding Capacity.n—Porous soils are generally
deficient in plant foods. They allow the rapid drainage
of water through them, and soluble foods are washed
away very quickly. Bacterial, fungal, and algal growth is
at a low ebb, and consequently the rate at which these
organisms manufacture plant foods is greatly reduced.
Fertilizers, therefore, should be given in an organic form,
for these are not easily washed out of the soil and yield

DISEASED CONDITION OF PLANTS 45

only small quantities of available plant foods at a time
as the result of bacterial and fungal action.

(c) Aeration.—Plant roots require air for their success-
ful development. Artificial aeration experiments con-
ducted with tomatoes at Cheshunt, in which warmed
moist air was blown through a system of underground
pipes, indicated that increased aeration produces bene-
ficial results as regards crop yield and resistance to
disease. In India, Howard has also shown that artificial
aeration induces an increased resistance to disease.

(d) Anchorage.—This factor is important in certain
tender crops where in loose soil there is danger of rubbing
at the stem base, with the consequent bruising and
wounding of the tissues and the possible entrance of
disease organisms.

(e) Degree of Compactness.—This is important as
influencing root development and the diffusion of ferti-
lizers and sterilizing agents. Preliminary experiments
upon the effect of ramming the soil have indicated that
root development is improved by moderate ramming.
In loose soil and in very tight soil the roots are less
abundant than in moderately tight soil. It was found
that soft, sappy plants could be made to produce a
harder and more resistant type of growth by ramming
the soil round the roots. Also in soils containing large
lumps it is difficult to ensure a thorough dispersion of
fertilizers or sterilizing agents.

THE CHEMICAL CONDITION OF THE SOIL

Upon the chemical condition of a soil depends its
acidity, neutrality, alkalinity, and the balance of plant
foods, all of which may be limiting factors in plant
growth.

(a) Soil Reaction—Soils which deviate to an
appreciable extent from the neutral point are generally
unsuitable for the cultivation of crops, and many methods
have been devised for their amelioration. Fortunately,

46 DISEASES OF GLASSHOUSE PLANTS

in this country most soils are neutral or approximately
so; strongly acid soils are occasionally found, while
alkaline soils are comparatively rare. Acid soils result
from the decomposition of large masses of luxuriant
vegetation in the presence of water, and are found
chiefly in low marsh lands and on some upland moors.
In such soils most crops never reach their maximum
development, but are stunted in growth, turning yellow
and chlorotic, and their roots are slowly destroyed. In
alkaline soils the effect is much the same. Such ab-
normal conditions assist the incidence of disease by
reducing the vitality of the plant, by producing suitable
places of entrance for fungal or bacterial parasites
through injured roots, or by providing increasingly
favourable conditions for fungal or bacterial growth.

On the other hand, some diseases, such as potato
“scab,” may be prevented by making the soil slightly
acid.

(b) Balance of Plant Foods.—The amount and balance
of the three most important plant foods—nitrogen,
phosphates, and potash—bears an intimate relation to
crop production and disease resistance. An excess of
nitrogen induces a rapidly growing, soft, and sappy
plant, highly susceptible to disease, which condition may
be counteracted by suitable additions of phosphates and
potash. Potash is especially valuable by reason of its
hardening effect, which produces an increased resistance
to disease. This may be illustrated by results obtained
at Cheshunt in connexion with “ Stripe” disease of the
tomato, caused by Bacillus lathyri (36). The number
of striped tomato plants were counted on each of the
plots in the experimental houses. These plots were laid
down in 1915 and have received the same treatment
each year since that time.

Table 9, page 130, indicates that manurial treatment
has an important effect upon the disease, for plots
receiving no added potash yielded the greatest number
of striped plants, while on those receiving no added

DISEASED CONDITION OF PLANTS 47

nitrogen the number of striped plants was considerably
less. Further experiments were arranged in which
inoculated tomato plants were given increasing amounts
of potash and nitrogen. The progress of the disease was
estimated by measuring the distance up the stem from
the point of inoculation that the “stripe” lesions had
travelled in a given time. The results showed that the
lesions had travelled furthest where the largest amounts
of nitrogen were given, and as the nitrogen was gradually
reduced the height of the lesions became consistently
shorter. As the amount of potash was increased the
progress of the disease was reduced. Later experiments
on commercial nurseries confirmed these results, and it
is now common practice to induce tomato plants to grow
away from “ stripe”’ by suitable additions of potash.

An excess of nitrogen in the soil has long been known
to increase susceptibility to some diseases, and in this
respect the most easily assimilated forms, such as
nitrates, have the most marked effects. On the cele-
brated Broadbalk wheat plots at Rothamsted, those
receiving an excess of nitrogen show a high susceptibility
to rusts and Hpichle typhina, while the plots receiving
the normal balanced fertilizer mixture are free from
these diseases, although equally exposed to infection.
Similar results were observed in respect of mangold leaf
spot caused by Phoma bete.

The manurial trials with wheat and barley at Woburn
also indicated that the high nitrogen plots, especially
those receiving sodium nitrate, are more susceptible
to mildews than the normal plots. At Cheshunt the
tomato experiments have long indicated similar results
with regard to the incidence of some diseases. Thus
the plots receiving heavy dressings of straw manure
have consistently shown a greater number of casualties
from root diseases than the other plots. Observations
in the tomato and cucumber industry in the Lea Valley
have provided abundant proof that methods of cultiva-
tion favouring heavy feeding with nitrogenous manure

48 DISEASES OF GLASSHOUSE PLANTS

produce a soft, sappy type of plant predisposed to
disease.

On the other hand, all diseases are not affected in |
this way, as instanced by the Verticilliwum wilt of the
tomato, which is more destructive to hard, underfed
plants than to the softer rapid growing types produced
by abundant nitrogen.

Phosphates have the effect of inducing the production
of abundant fibrous roots, and are generally regarded
as causing an increased resistance to disease, but in this
respect they are not nearly so effective as potash.

Many cases might be quoted in which potash-starved
plants have shown a strong susceptibility to disease,
which has been readily corrected by suitable dressings
of potash salts. At Rothamsted the potash-starved
plots of wheat and mangels show a marked susceptibility
to rusts and other diseases, while in the glasshouse
tomato industry it has been found that potash manuring
is necessary to keep many fungus and bacterial diseases
in hand. As previously shown, tomato plants attacked
by “Stripe ” disease can be made to grow away clean by
feeding them with sulphate of potash.

Lime is used in cultivation to “sweeten” the soil
and is generally considered to be beneficial. It is
especially useful on acid soils and those rich in humus,
where its addition serves to neutralize the acidity. It is
a common belief that the addition of lime to em soil
assists in controlling disease organisms; and while this
is usually true, it should be borne in mind that some
diseases are increased by liming. Thus, before applying
this treatment for the purpose of disease control it is
necessary to know its effect upon the disease in question.
Soft rot of the calla lily caused by Bacillus carotovorus
is increased by treatment with lime or lime water, as is
‘“‘ damping-off ” of tomato seedlings due to Phytophthora
cryptogea or Ph. parasitica. On the other hand, both
the Fusarium and Verticiilium wilts of the tomato are
reduced by treatment with caustic lime in the winter.

DISEASED CONDITION OF PLANTS 49

An excess of lime in the soil has also been shown to
induce chlorosis of many plants.

(c) Soil Population.—The micro-organisms that in-
habit the soil bear an important relation to its fertility,
By feeding upon the organic matter and chemical salts
they produce important changes in the soil. These may
be classified into two groups: (1) Down grade and (2) up

ade.
ia Down Grade Changes.—These are concerned with
the decomposition of complex compounds and the
production from them of simple compounds, such as
carbon dioxide and ammonia. Among the different
kinds of organisms which are instrumental in these
changes the bacteria, fungi, protozoa, and alge are
important.

' Up Grade Changes.—These are concerned with the
building up or synthesis of valuable plant foods from the
simple compounds liberated by the down grade processes.
The bacteria are mainly responsible for these changes,
but in the light of recent knowledge it is probable that
the algz also are concerned. In fertile soils the down
grade and up grade changes are in equilibrium, but
this is easily disturbed by a change in soil conditions, as
these affect the growth and behaviour of soil micro-
organisms much in the same way as they do those of
plants.

While much information has been obtained regarding
the behaviour of micro-organisms present in agricultural
lands, but little is known about the population of the
soil under the abnormal conditions prevalent under glass.
Investigations of these problems are very desirable and
should elucidate many obscure problems connected with
the infertility of glasshouse soils.

Beside these organisms which assist in the main-
tenance of soil fertility there are to be found in the soil
organisms which are distinctly harmful, namely, those
which cause plant diseases. While most soils are com-
paratively free from these organisms, many become

4

50 DISEASES OF GLASSHOUSE PLANTS

contaminated and the infection spreads so rapidly that
susceptible plants cannot be grown. The disease-causing
organisms live upon the decaying organic matter of the
soil and attack the young plants at the most suitable
opportunity.

It is thus apparent that the soil cannot be regarded
as consisting of dead particles of rock, but rather as
teeming with life, having a determining effect upon soil
fertility and freedom from disease. Science has shown
that fertility remains so long as the beneficial organisms
are not swamped by those which are harmful. In the
latter case, especially if parasitic organisms are abundant,
the soil is said to be “sick” and many attempts have
been made to devise methods for bringing the soil back
to a healthy condition.

Malnutrition

Plants suffering from malnutrition have a stunted
appearance and a light yellowish-green foliage, which is
especially noticeable between the veins, which are
yellowish or brown in colour. The flowers and fruits
are small and misshapen ; the roots are ill developed and
the secondary ones die prematurely. Investigation has
shown that the disease is not attributable to parasitic
organisms, but is the result of unsuitable soil conditions,
generally set up by incorrect manuring.

Fresh cow or pig manure has been shown to cause
malnutrition of the tomato, and excess of acid fertilizers
has a similar effect. An excess of readily available
nitrogen in cucumber beds produces an incurling of the
leaves, the margins of which turn brown and wither.
On the other hand, cucumbers grown in frames and
supplied with an insufficiency of available nitrogen
become light in colour. Such fruits turn darker when
the plants are given a soluble nitrogenous fertilizer,

Growers of roses are familiar with the bronzing of
the leaves of certain varieties. This begins as a bronze

DISEASED CONDITION OF PLANTS 51

mottled patch, and the entire leaf may become bronze
in colour or may develop bronze spots surrounded by a
yellow zone. The affected leaves soon drop and spoil
the appearance and health of the plant. Generally the
disease appears on new branches below the place where
the blooms have been removed. It is most prevalent
on young plants, and especially those which have been
forced too rapidly. Over-feeding appears to be an
important factor in connexion with the disease.

The dropping of flower buds is an important malady
closely related to malnutrition. The flower bud appears
in due course, turns yellow, and drops off the peduncle
or stalk. In some cases the cause may be of fungoid
origin, but generally it is due to an unbalanced food
supply. Thus the tomato and sweet-pea both drop their
flowers when there is an excess of available nitrogen in
the soil, which causes them to grow too rapidly. The
sweet-pea also drops its bloom in starved soil, and hot,
dry weather may cause tomato flowers to drop.

An excess of nitrogen and potash has a distinctly
harmful effect upon carnations, as is well known to
growers of this plant. The former produces weak flowers
liable to scorch and attack by Botrytis. Such blossoms >
are malformed and frequently do not open. An excess
of potash salts produces a hardening and wrinkling of
the petals, and at times the margins turn brown and
wither. The buds do not open properly and are dis-
torted. The plants become stunted and the leaf tips
turn yellow and die. Occasionally carnation flowers
split prior to opening; and while the cause is yet
obscure, there are indications that unsuitable manuring
is an important factor.

If the tomato is overfed in the early stages of its
growth and starved at a later period it develops a hollow
stem, and becomes physiologically weak and liable to
disease. Similar symptoms develop in the tops of
tomato plants which have borne an exceptionally heavy
crop on the bottom trusses; such plants are physio-

52 DISEASES OF GLASSHOUSE PLANTS

logically weak and highly susceptible to disease. It is
of the greatest importance in the prevention and control
of plant diseases that the physiological strength be
maintained. In order to resist disease it is necessary
to pay careful attention to all environmental conditions
and feeding, for every check to which plants are submitted
reduce; their physiological strength.

Chlorosis 7

An excess of lime in the soil has been shown to
produce a chlorosis of many plants, and as these are
more susceptible to disease than normal plants this soil
factor is one to be avoided. Lack of iron in the soil is
said to be productive of chlorosis, which in this case may
be remedied by the application of small quantities of iron
sulphate.

Stigmonose

Aphides and other sucking insects puncture the
tender tissues of young leaves and produce minute lesions
which have frequently been attributed to bacterial
causes. Such spotting is called stigmonose. A suitable
example is furnished by the carnation. The spots are
at first pale yellow in colour and later turn a reddish-
purple. The surface tissue dries, the spots enlarge and
become sunken, the leaves turn yellow and wither, and
the whole plant may be stunted. The changes have
been traced to the injection of an irritant by the insect
into the plant cells and their consequent reaction.
Tomato fruits are attacked in a similar fashion, but the
spots are circular and resemble small white blisters
with minute dark purple centres. As the fruit colours, the
spots become indistinct and are practically unnoticeable.

Gas Injury |
The presence of poisonous gases in glasshouses
produces marked effects upon the plants. Thus fumiga-

DISEASED CONDITION OF PLANTS 53

tion with nicotine and hydrocyanic acid gas under
unsuitable conditions causes discoloration and scorch
of certain flowers and plants. The presence of such
vapours as paraffin and petrol produces similar injurious
effects.

Diseases of Unknown Origin

Several diseases of glasshouse plants exist which
cannot be attributed to definite causes. Among these
may be mentioned tulip blindness, or the failure of
apparently normal bulbs to produce flowers. Tulip
bulbs which have bloomed well one year may be blind
the next and bloom again the year after. The hyacinth
and other bulbous plants frequently develop a disease
called “ gummosis,” in which a white gum is produced
by the sub-epidermal tissues. The gum-bearing cells
swell, and finally the epidermis ruptures and the gum
exudes. The “ yellow stripe” disease of the daffodil is
another of the same category. In an advanced stage
the leaves develop parallel yellow bands coinciding with
the veins, and in bad cases the leaves wither and the
blossoms die prematurely. Another such disease is the
failure of rosebuds to open. The buds develop normally,
when suddenly the outer petals turn yellow and cease
to grow. Such buds may open partially but never reach
normal development, and are distorted.

CHAPTER III

DISEASES DUE TO FUNGI

THE fungi comprise a big group of plants which do not
possess the green colouring matter common to flowering
plants generally. They are thus unable to manufacture
the carbohydrate part of the food from the atmosphere,
and are dependent upon organic material for their food.
Some of them live upon dead vegetable matter and are
called saprophytes, but most of the plant disease organ-
isms are parasites and obtain their food from the living
plant. The most successful plant parasites, however,
are able to exist upon dead vegetable matter when no
living tissues are available. They thus live upon decaying
plant remains during the winter, but retain their ability
to attack and destroy the living plant when such is
available.

Instead of developing stem, leaves, and roots in the
way the higher plants do, the fungi produce tiny thread-
like structures called hyphe. The hyphe intertwine
and spread in all directions, and by continued growth
produce a weft or mat-like mass called the mycelium.
The hyphe penetrate the plant tissues, through which .
they ramify and produce the spots, cankers, or other
lesions typical of the particular disease. Certain threads
of the mycelium grow up clear of the substratum and
cut off small bodies called spores. These have thin walls, |
and being very light are suitable for quick distribution
and immediate infection of neighbouring plants. They
are unable to withstand abnormal conditions, and are
often called “summer ”’ spores. Frequently other spores
with thick, dark walls are produced, called chlamydo-

54

DISEASES DUE TO FUNGI 55

spores, which are highly resistant to external conditions
and carry over the disease from one season to another.
Sometimes the spores are enclosed in tiny flask-like
structures buried more or less deeply in the leaf, stem,
or fruit tissues, and being thus protected are able to

Bassss=

ayo |r

Fig. 3. Ae) Hyphae, (0) thin-walled ‘‘summer”’ spores, (c) thick-walled resting spore,
(d) a a pycnidium, (e) a perithecium, (f) ascus containing eight ascospores.

resist abnormal conditions. These bodies are of two
kinds—the pycnidia and perithecia. In the former the
spores are produced on small stalks which line the inside
of the vessel; in the latter case a number of small sacs
are produced within the vessel, each of which contains

56 DISEASES OF GLASSHOUSE PLANTS

eight tiny special spores or ascospores. The pycnidia
and perithecia do not open except when conditions are
favourable, and then there are often special devices for
ejecting the spores from the inside (Fig. 3).

t. Root Diseases

“DAMPING OFF”? OF TOMATO SEEDLINGS

Symptoms.—Typically the plants are attacked at the
soil level (Fig. 4), where the tissues are browned, become

Fig. 4. Tomato seedling attacked by ‘‘ damping off ”’ disease at a.

softened, and collapse, causing the plant to fall over.
The disease spreads rapidly from seedling to seedling in
the film of water covering the soil surface. ‘Thick
sowing is to be deprecated, for this assists the spread of
the disease, as shown by the following experiment.
Seeds were sown in infected soil in thicknesses varying
from 600 to 25 seeds per box, in sets of four boxes at
each degree of thickness. In half the boxes the seedlings

DISEASES DUE TO FUNGI 57

were removed as they became infected to eliminate the
factor of spread. The remainder were untouched and
indicated the rate of superficial spread of the fungus (5).

TABLE lI.
No. of Seeds Average per cent Diseased Seed- Average per cent Diseased
per Box. lings Removed when Attacked. Seedlings not Removed.
600 51 100
300 45 100
200 49 100
100 42 78
50 37 46
25 35 41

The second column, Table 1, shows the uniform
results obtained when the seedlings were removed as
they were attacked, and indicate that the number of
seedlings primarily attacked depends upon the number
of disease centres in the soil and not upon the closeness
of sowing when the factor of superficial spread of the
organism is eliminated. In the third column, where the
fungi were allowed to remain, the spread of the organism
is more rapid where the seeds are sown closely than
where they are sown thinly. In the closer sowings the
density of the plants increases the film of water adhering
to the seedlings and offers a ready means of spreading
the disease through the box. Sowing above fifty to the
box should be avoided, for this materially assists the
disease. A film of water over the surface of the soil is
necessary for the rapid spread of the disease, and water-
logged conditions are also in its favour. Moistening the
soil through capillary attraction by standing the boxes
in shallow trays of water is less favourable to the disease
than watering from the top. Temperature is important,
and relatively low temperatures—about 10° C.—are less
favourable to the disease than high temperatures of
23° C.-27° C. Thus summer sowings of tomato seeds are
more liable to “ damp off”’ than early spring sowings—

58 DISEASES OF GLASSHOUSE PLANTS

a fact that is well known to nurserymen. The disease
thrives upon rich composts, and such are therefore more
favourable for it than loam alone.

Causal Organisms.—Considerable experience of this
disease has shown that it is caused mainly by three fungi
—Rhizoctonia solani Kuhn, Phytophthora cryptogea Pethy-
bridge and Lafferty, and Phytophthora parasitica Dastur.
The first is less common than the latter two fungi,
which are consequently the most important cause of the
disease. Rhizoctonia solani is the
sterile stage of a spore-bearing
fungus, Corticium vagum B. & C.,
var. Solant Burt. The young
hyphe produce branches which
generally leave the mother fila-
ment at an acute angle and.
subsequently turn and lie parallel
to it. The branch is constricted
near the point of origin and a
cross wall is formed a little
beyond the constriction (Fig. 5).
In age, the angle of branching
becomes more nearly a_ right
angle, and the hyphe turn first
yellow and then a deep brown
colour. Resting bodies or sclero-
tia, capable of resisting abnormal
conditions, are produced, being at
first small, soft white masses, but later they increase in size
and turn darkand hard. TheCorticiumstage, which forms
later, shows as an ashy-grey tufted layer at the base of the
stem, from which innumerable small spores are produced.
These are easily carried by the wind, and so disseminate the
disease. This stage is not always produced, and indeed
requires special cool, moist conditions for development.
Rhizoctoma solani is commonly found in rich soils,
where it constitutes an important disease of many plants.
Phytophthora parasitica Dastur (16) was first described by

Fig. 5. Hyphe of Rhizoctonia
solani.

DISEASES DUE TO FUNGI 59

Dastur in India, during 1913, as causing a disease of the
castor bean. In 1917 Sherbakoff (43) described “‘ buckeye”’
rot of tomato fruits as caused by a new species of Phytoph-
thora—Ph. terrestria—which he also found attacking
citrus trees and lupins. In pure culture tests it appears
that Ph. terrestria is the same as Ph. parasiiica, the latter
name having priority. It is highly probable that Ph.
parasitica is a universal organism considerably more
important than is at present imagined. Phytophthora
cryptogea was first described by Pethybridge and Lafferty
(37) in 1919 as the cause of tomato foot rot. From
inoculation experiments they concluded that it produced
the same kind of disease in the potato, Giulia tricolor, and
elm seedlings. It was also found attacking the petunia,
cineraria, aster, and wallflower. The genus Phytophthora
develops non-septate thin-walled hyphe of a relatively
large diameter and characteristic appearance. It
produces egg-shaped conidia or sporangia, from the inside
of which are liberated a number of active zoospores, each
of which is supplied with two lash-like processes or
cilia, enabling it to swim away in a film of moisture.
Thick-walled vegetative cells, or chlamydospores, are also
produced as well as thick-walled oospores, the product
of sexual fusion. It is by means of the zoospores that
the disease is able to spread rapidly. The zoospores
settle down against the outside of the plant and produce
germ tubes which enter the tissues. The thick-walled
chlamydospores and oospores are able to resist abnormal
conditions and so tide over the barren season or other
adverse conditions. Ph. cryptogea and Ph. parasitica are
closely allied species, differing only in minor details.
Perhaps the best way of distinguishing between them
is provided by the different way in which the two species
produce sporangia. In Ph. cryptogea the stalk or spor-
angiophore grows up through the old sporangium after
the zoospores have been liberated, and produces a second
sporangia beyond ,the old one, and this process may
be repeated several times (Fig. 6). In Ph. parasitica

60 DISEASES OF GLASSHOUSE PLANTS

the sporangiophore does not do this, but the new
sporangium is produced on a side branch of the old
sporangiophore some distance below the old sporangium.

Sources of Infecttion.—The main sources of infection
are the soil and water supply, but experiment has shown
that the disease organisms are also carried over from
one season to the next by seed-boxes and pots. Cracked
pots are especially dangerous, and the discoloration and
destruction of plant roots have frequently been traced to
some crack or crevice which has harboured the resting
spores of the fungus. These
organisms, like many others,
spend part of their existence
in the living plant and the rest
in hibernation over the winter
in the decompésing soil humus.
Composts rich in humus, such
as old mushroom beds, are
relatively more suited to per-
meation by infection than poor
soils. The water supply and
drainage has proved to be a
potent source of infection, and
: preliminary experiments upon
Fig. 6. Fiytopithora eryptogea: this disease led to the examina-

(1) ye bodies—(a) con- ~~
gree) ZOURR Gree. tion of many nursery water

(2) Ty pical aetedeet ment of conidia,
showing how one conidium after

another. is produced on the SUpplies and the consequent

main jy youse conidia, “” proof of their powers of infection.
Surface drainage is also dangerous, and has been shown in
many cases to be the indirect cause of many epidemics
of this disease.

Control.—Sterilization of infected soil by steaming,
baking, or with a 2 per cent solution of formaldehyde
(1 gallon of commercial 40 per cent pure formaldehyde in
49 gallons of water) will completely rid it of the disease
organisms. Pots may be sterilized by boiling in water
or by treatment with formaldehyde, while boxes are best
sterilized with formaldehyde, as steam and hot water

DISEASES DUE TO FUNGI 61

soon destroy the wood. A contaminated source of water
is more difficult to purify, but the general method will be
treated in a later chapter. Frequently the propagating
soil becomes contaminated by a chance infection, which
is not realized until the disease reaches the epidemic stage.
Such cases have emphasized the necessity of devising
methods of checking the disease when it appears, and as
the result of investigations (6) at the Cheshunt Experi-
mental Station it was found that a solution of a mixture of
copper sulphate and ammonium carbonate will do this,
The mixture, which for convenience has been named
“Cheshunt Compound,” consists of two parts by weight
of copper sulphate and eleven parts ammonium carbonate.
The ammonium carbonate, which must be fresh, is reduced
to a fine powder by crushing out the lumps. It is then
thoroughly mixed with powdered copper sulphate in the
correct proportions, and stored for twenty-four hours in
a tightly corked glass or stone jar before using. The
solution is prepared by dissolving one ounce of the dry
mixture in a little hot water and adding to two gallons of
water. It should not be put into vessels of iron, tin, or
zinc, as it corrodes them and loses its strength, and
just as much as is required for immediate use should be
prepared. Plants may be watered with this solution
without injury, while the “ damping off” organisms in
the soil are destroyed, but infected plants receive no
benefit from the solution, for the fungus is already inside
the tissues where the liquid cannot reach it, and such plants
eventually die. The seed-boxes should be thoroughly
watered with the solution after sowing and covering the
the seeds. When potting up, the seedlings should be
watered first with the solution while still in the boxes,
to ensure that the roots are thoroughly wetted, and
after transplanting into the pots the solution should
again be applied. To enable sufficient solution to be
given to each plant the level of the soil should be one
inch below the level of the pot. The solution may be
used in the houses by adding a pint to each of the holes

62 DISEASES OF GLASSHOUSE PLANTS

prepared for the plants. In replacing diseased individuals
the method should be to remove the dead plant, water
the hole with a pint of solution, replant with a healthy
“individual, and again water with the solution. The
compound has been successfully employed on commercial
nurseries, and has a beneficial effect apart from killing
the disease organisms, for the nitrogen it contains imparts
a greater vigour to the plants.

During an epidemic at one nursery the grower
replanted two houses four times, and each time lost
practically the whole of his plants. When replanting
for the fifth time the soil was treated with “ Cheshunt
Compound,” with the result that only four plants were
lost out of two thousand.

The disease may be aggravated by certain cultural
conditions of moisture and temperature. Thus a rela-
tively high percentage of moisture in the soil and the air
is favourable to the disease organisms, and careful
regulation of the watering, so as to keep the seed-boxes
uniformly moist, combined with efficient ventilation of
the propagating houses to dry out the surface soil, will
help to keep the disease under control. The optimum
temperature for growth of Ph. parasitica is about 30° C.
(86° F.), and that of Ph. cryptogea and Rhizoctonia solant
about 25° C. (77° F.). Below 12° C. (54° F.) the growth
of all three species is very slow. When the disease has
started among the plants the grower should endeavour
to keep the temperature as low as possible without
impairing the health of his crop.

Diseased plants may be saved only by cutting away
the lower diseased portion and treating the healthy tops
as cuttings.

93

*“* DAMPING OFF’ OF CUCUMBER SEEDLINGS

Two fungi mainly concerned with “damping off”
of cucumber seedlings are Pythiwm de Baryanum Hesse
and Colletotrichum oligochetum Cav.

Fie. 7. Phytophthora foot rot of the tomato. [Facing page 62

) DISEASES DUE TO FUNGI 63

The former has been reported as a common cause of
‘damping off”’ in many plants, and probably has been
confused many times with various species of Phytophthora,
from which genus it differs but slightly. Ward reported
it as being very prevalent in the garden soils of Europe.
Colletotrichum oligochetum is fully described under
** Anthracnose of the Cucumber,” and is a common
cause of “ damping off”? of cucumbers in this country.

The methods of control recommended for “‘ damping
off’ of tomato seedlings apply equally well to that of
cucumber seedlings.

FOOT ROT AND COLLAR ROT OF THE TOMATO

These terms have been applied to diseases due to a
number of different fungi, and in order to avoid
confusion the name of the fungus concerned is here
placed before the general term. ‘The most important of
these diseases are Phytophthora foot rot, Botrytis foot
rot, Macrosporium foot rot, and Verticilliwm collar rot.
Typically these diseases appear about the soil level, and
while the disease symptoms may vary the final result is
the death of the plant.

Phytophthora Foot Rot.—Two species of Phytophthora
produce foot rot of the tomato, the most common
being that described by Pethybridge and Lafferty (37)
as Phytophthora cryptogea, which, as already stated, causes
“damping off” of tomato seedlings. The symptoms
of foot rot are identical with those of “‘ damping off,”
except that older plants are concerned (Fig. 7), and
there is a gradual transition from ‘“‘ damping off” to
“foot rot” in accordance with the age of the plant at
the time the disease appears. In some cases plants
eighteen inches high have been known to be attacked.
The disease is characterized by a dark brown or black
discoloration of the outside tissues of the stem. At
such parts the stem tissues shrink and collapse, causing

64 DISEASES OF GLASSHOUSE PLANTS

the top of the plant to fall over. The disease organisms
enter the plant via the roots or a leaf touching the soil.
In the former case the fungus passes rapidly up the stem,
and ultimately causes the collapse of the tissues at some
definite part. ‘This collapse usually occurs at or near to
the soil level, but many cases have been examined where
the constriction appeared at a height of twelve to sixteen
inches above ground, whereas in other cases the root is
destroyed and separated from the upper portion of
the plant. Generally speaking, this fungus rapidly kills
the plant, and the losses caused are considerable, not
infrequently 50,000 to 90,000 plants being destroyed in
two or three weeks on a single nursery.

Another species, Phytophthora parasitica, described —
previously as a cause of “damping off” of seedlings,
also produces foot rot. In this case the plant is attacked
in much the same way as by Ph. cryptogea, but death is
slower. Tomatoes planted out on March Ist may be
destroyed by Ph. cryptogea up to the end of April, while
it is not uncommon to find Ph. parasitica as the cause of
death in June and July.

The methods of controlling this disease are the same
as recommended for ‘“‘ damping off”? due to these two
species of Phytophthora.

Botrytis Foot Rot.—Symptoms.—tThe first sign of this
disease is the appearance of smooth, slightly sunken grey
patches on the stem just about the soil level. These
lesions enlarge, become dry and brown until finally they
encircle the stem, when the top of the plant wilts and may
fall over. Under moist conditions a grey fungal growth
of Botrytis appears over the diseased area (Figs. 8 and 9).
On cutting open the stem it can be seen that the fungus
has penetrated the cortex, vascular system, and pith, and
is travelling up the stem in the wood. The diseased
tissues are dark reddish-brown in colour. "

Causal Organism.—The cause of the disease is a
species of Botrytis, which is one of the most common

Fig. 8. Botrytis foot rot of the tomato.

Fie. 9. Botrytis sp. growing on tomato
stem. [Facing page 64

7

DISEASES DUE TO FUNGI 65

fungi found on vegetation. The fertile hyphz stand up
in dense grey, velvety tufts and masses, producing oval
conidia or spores on branched heads. These conidia are
extremely light, and, being carried long distances in
strong air currents, provide a most efficient method of
spreading the disease. Numerous hard, black sclerotia
are produced on the plant, and being highly resistant to
abnormal conditions enable the fungus to hibernate from
one season to the next.
Sources of Infec-
tion.—The infection is
present at the surface
of the soil, and may
have been introduced
by air currents, or
with water supply, or
manure,
Control.—Warm,
moist conditions at
the base of the plant
favour this disease and
therefore should be
avoided. Oncea plant
is attacked nothing
can save it, but the
fungal infection present
at the soil surface and
plant base may be de-
stroyed by spraying Fig. 10. Macrosporium foot rot of the tomato:
with a 9 per cent solu- (a) spores, (b) diseased stem. 5
tion of calcium bisulphite, which is specific for Botrytis.

Macrosporium Foot Rot.—Quite recently Rosenbaum
(42) has described a foot rot of the tomato due to
Macrosporium solani E and M.

Symptoms.—The symptoms are described as resem-
bling those of black-leg of potato stems. The stems
turn dark brown at the ground line, and the tissues

5

66 DISEASES OF GLASSHOUSE PLANTS

shrivel up and may break, causing the plant to fall over.
At times the lesions appear higher upon the stem, and the
entire shoot and blossoms may be affected. The brown
rot spreads deep into the tissues, both above and below
ground (Fig. 10). Rosenbaum states that in one case 40
per cent of the plants on a nursery were affected, which
indicates that the disease may at times be a serious one.

The Causal Organism.—acrosporium solani was first
described in 1882 as causing an early blight of potatoes,
and since then has been recorded on many plants. The
fungal hyphe vary from light brown to an olive colour,
and produce characteristic brown septate spores.

The disease has not yet been reported in England.

Verticillium Collar Rot.—Pritchard and Porte (38)
have described a collar rot of tomato seedlings caused
by anew species of Verticilliwm to which they have applied
the name V. lycopersict. Dark brown lesions, similar to
those produced by Macrosporium solani, appear girdling
the stem, mainly at the soil level. These enlarge and the
tissues become weak and brittle; but, unlike the Macro-
sporium disease, there is very little infection of the roots.
The authors found that diseased plants were readily
snapped off by the wind, and while some recovered by
forming a callus over the wound, these individuals were
seldom as productive as healthy plants. The disease is
chiefly one of the seed-bed, where the tender nature of
the plants renders them highly susceptible.

The fungus produces a similar disease of the potato
and horse nettle—Solanum carolinense L. As the disease
is typically one of the seed-box, sterilizing the propagating
soil is recommended as a means of control. Susceptible
hosts should not be allowed to grow in infected areas, as
these assist in the spread of the disease.

CROWN CANKER OF THE ROSE

This disease is commonly found under glass, and while
the percentage of plants actually killed is small, there is

DISEASES DUE TO FUNGI 67

no doubt that the loss of vigour caused by this fungus is
the cause of considerable financial loss. Infection takes
place at the crown or collar of the plant, frequently at
the graft. The disease appears as a slight discoloration
of the bark, which later becomes water-soaked and black.
Gradually the lesions girdle the stem and cracks appear
in the sunken infected area. The disease spreads down
into the roots and the diseased tissue becomes brown and
powdery. Such developments have a detrimental effect
upon the health of the plant, which becomes weak and
spindly and produces small, valueless flowers.

The disease is caused by Cylindrocladium scoparium
Morgan (34), which produces cylindrical, one-septate
spores in characteristic heads. Over-watering is highly
favourable to the disease, and alternating periods of
drought and excessive dampness should be avoided.
The disease organisms live in the soil and may be de-
stroyed by steam sterilization. To some extent it may
_ be controlled by planting grafted individuals, so that
the union of scion and stock is above the soil.

ROOT ROTS

The roots of most plants are susceptible to disease
when the soil conditions are wrong. The extent of root
rot may be slight and produce results equivalent to those
produced by root pruning. On the other hand, should
the rot get the upper hand, death of the plant is the
ultimate result. Numerous root-rotting fungi have been
recorded, and the list is by no means complete. An
excess of soil water, and consequently lack of aeration,
is a potent factor in inducing root rot, and beds rich in
organic matter, especially of a nitrogenous nature, favour
the disease. Generally speaking, root rots due to fungi
may be divided into two main classes—those caused by
wound parasites and those caused by fungi capable of
attacking healthy roots in the absence of wounds. Thus
soil insects, by opening up wounds in the roots and stem

68 DISEASES OF GLASSHOUSE PLANTS

base are the indirect cause of many root diseases. The
soil temperature is also important, and fungi which at
one temperature do little damage may rapidly destroy
the whole root system at a suitable temperature.

Tomato Root Rots.—Phytophthora cryptogea, Phytoph-
thora parasitica, and Rhizoctonia solani, as already
described, may produce a rapid root rot of the tomato.
Other root rots are due to various species of Fusarium |
and Sclerotiwm.

Fusarium Root Rot of the Tomato.—Various species of
Fusarium have been isolated from tomato roots in
England, and have been found invariably to follow
wounding by woodlice or wireworm. The fungus causes
a reddish-brown discoloration of the roots, which spreads
rapidly during the summer months when the soil tempera-
ture is high. The affected plant becomes stunted in
growth, the leaves turn yellow and wither, and finally
death ensues. When the plant is dead the fungus grows
out over the surface of the tissues and, unless the dead
plant is removed in time, produces an abundance of
spores, which rapidly spread the disease.

Sclerotewum Root Rot of the Tomato.—In America
Sclerotium rolfsit Sacc. has been described as producing
a root rot of the tomato (23), but this disease is as yet
unknown in England. Infection takes place at the
base of the plant in about the top inch of soil, and
becomes evident as dark brown lesions, at which stage
the plant wilts slightly. Later the lesions become covered
with a mat of white radiating hyphe, which surrounds
the base of the plant. The fungus works into the stem
tissues and the plant wilts and dies. The fungus does
not pass up the stem to any height, but works down
into the roots, especially those near the “surface. On
pulling up a dead plant the roots are seen to be covered

FI@. 11. Colletotrichum tabificum on tomato roots, showing typical minute black
sclerotia.

Fig. 12. Sclerotium disease of the tulip. [Facing page 68

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DISEASES DUE TO FUNGI 69

with a white fungal growth. If a diseased plant in the
soil is kept under moist conditions the fungus grows out
from the stem and spreads over the soil surface in
radiating fans. The disease is carried from season to
season by sclerotia as small as the smallest mustard seed.
At first they are milk white in colour, but on maturing
they turn a dark mahogany-red or black colour. Sclerotia
are not produced on the plant until the final stages of
the disease, and on dead plants may usually be found
in abundance. Rolfs, who first discovered the disease,
reports that it also attacks potatoes, cabbages, beets,
and melons, as well as Hydrangeas and Daphnes.
Control.—The disease grows rapidly in compost heaps,
and indeed in any soil rich in decaying vegetable matter.
Clean methods of culture and the omission of stable
manure from the soil will prevent the disease, while
efficient ventilation at the base of the plant and deep
cultivation are also beneficial. Spraying the soil surface
and base of the stem with a solution of “ Cheshunt
Compound” is recommended as a preventative.

Colletotrichwm Root Disease.—This disease has recently
been identified in England. The symptoms of the disease
are similar to those generally present when tomato roots
are slowly destroyed by a parasitic organism. Such
plants cease to develop normally, the lower leaves turn
yellow and wither prematurely, and the new growth is
thin and pale in colour. In many cases the stem becomes
hollow and the plant dies prematurely, being relatively
stunted in growth.

On pulling up a plant the roots offer no resistance,
for they are dry and shrivelled, and the decayed base
readily leaves the soil. The cortical tissues of the
roots readily come away from the centre core of wood,
and the distinctive feature of the disease is the presence
of innumerable minute sclerotia the size of begonia
seeds (Fig. 11). These are produced inside the woody
tissues as well as external to them. The affected wood

70 DISEASES OF GLASSHOUSE PLANTS

is only brown in colour, and somewhat darker than when
infected by Verticillium or Fusarium.

Infection takes place in June and July when the soil
temperature is high. At this time, although the fungus
is present in the roots, a well-grown plant may
show no signs of disease, but after it has produced
five or six trusses of fruit, and is consequently reduced
in vigour, signs of root trouble appear. Thus, unless
the conditions are exceptional, the outward signs of the
disease do not appear earlier than the middle of August.
Considerable financial losses, however, are incurred
through the disease, which prevents the plants from
continuing to bear fruit to the end of the season and
causes their premature death. It has been calculated
that between twenty and twenty-five per cent of the
total yield per plant is lost when it is attacked by this
fungus.

Causal Organism.—The disease is caused by a
fungus Colletotrichum tabificum Pethybridge. The actual
hyphe are difficult to distinguish on the plant, and
occasionally very small spores or conidia are produced.
Typical resting bodies or sclerotia are formed, which
are at first white in colour, but finally turn dark brown
or black and become covered with dark brown bristles
or setx, The fungus ramifies through the wood, and
produces sclerotia inside the large wood vessels, and in
any hollows inside the stem. Generally sclerotia are not
produced until the plant is almost dead.

Control.—The disease organisms develop most rapidly
in soils rich in organic matter, and thrive in manure
heaps and any straw or woody material. In the labora-
tory the fungus has been cultivated successfully on wood
and straw. It is thus obvious that clean cultivation by
removing such materials is an important factor in the
control of the disease.

Watering the soil round the plant roots with a solution
of ‘‘ Cheshunt Compound” has been found to prevent
the disease from spreading. Sterilization with steam or

DISEASES DUE TO FUNGI 71

by baking will destroy the disease organisms in the
soil.

Root Rot of the Carnation.—The first sign of this
disease is the yellowing of the leaves. Generally the
entire plant is affected, but in some cases only a shoot
or two may show the yellowing. Later, when the fungus
is firmly established in the plant, wilting during intervals
of intense sunlight may occur. The disease is caused
by Rhizoctonia solani Kuhn, which confines itself mainly
to the roots and lower parts of the plant, where a brown
rot may develop. High temperatures, over-watering,
excessive feeding, and deep planting favour the disease.
Control methods are similar to those described on page 60.

_ Root Rots of the Sweet-pea. Rhizoctonia Root Rot.—
This is much the same disease as already described for
the carnation, but it is more serious than in the latter.

BULB ROTS

Sclerotium Disease.—This disease, due to Sclerotium
tuliparum Klebahn, is found on tulip and other bulbs,
Towards the end of the flowering period the plants assume
an abnormally unhealthy appearance, and on lifting the
bulbs small black sclerotia may be seen in the scales,
and especially on the inner side of the outer ones.
Affected bulbs become desiccated and finally die
(Fig. 12).

The disease has been investigated by J. K. Rams-
bottom (40). Infected soil may be sterilized by means
of steam, while a dressing of flowers of sulphur has fre-
quently proved beneficial.

Botrytis Rot.—This disease, which shows itself by the
development of the typical grey Botrytis mould upon the
leaves and bulb scales (Fig. 13), and the consequent
rapid decay of the diseased tissue, is largely connected
with bad cultural conditions. Hot, moist conditions,

72 DISEASES OF GLASSHOUSE PLANTS

accompanied by imperfect circulation of the air and
injudicious watering, are favourable to the disease. For
a description of the fungus and method of control,
see page 64.

Fusarium and Penicillium Rots.—Certain species of
Fusarium and the common blue mould Penicilliwm produce
a bulb rot of the hyacinth, narcissus, and tulip. The
rot commences from the base of the bulb, and is typically
a storage rot. Unsuitable conditions at the time of
harvesting and imperfect drying of the bulbs are the
main factors connected with
the disease. Diseased bulbs
are frequently planted be-
cause external signs of rot are
absent. The disease organ-
isms attack the base of the
bulb and work up into the
centre, where the flowering
shoot may be destroyed.

Such bulbs rarely flower,

si
§ “— no new bulb is formed, and

the old one is converted into
a powdery mass.

Fig. 13. Botrytis disease of the tulip : Attention to the condi-
(a) bulb showing black sclerotia, . ° .
(o) fungal growth on the bulb. tions operative at harvesting

and during storage so as to ensure that the bulbs are
perfectly dry, and the elimination of rotten bulbs from
which infection may spread, are the best ways of
preventing the disease.

CHAPTER IV

DISEASES DUE TO FUNGI—Continued

2. Wilt Diseases

“ SLEEPY DisEAsE”’ or wilt disease is common to most
plants, and is indicated by a wilting of the leaves followed
by a yellowing and desiccation which usually ends in the
death of the individual. Obviously any root injury, if
sufficiently drastic, will cause such symptoms, but the
true wilt diseases are due to a reduced functioning or
complete stoppage of the vascular system, which com-
prises the water- and food-conducting tissues of the plant.

Massee (31) first described the Sleepy Disease of
tomatoes in Britain and attributed it to Fusarium
lycopersict Sacc., an organism with two developmental
- stages—the Diplocladium and Fusarium stages. In-
vestigations at the Cheshunt Experimental Station (10)
demonstrated that these two stages are separate fungus
species, each of which under definite conditions can
produce wilt. The Diplocladium form is Verticilliwm
albo-atrum Reinke and Berthold, and the pathogenic
Fusarium form is Fusarium lycopersici Sacc. There are
therefore two wilt diseases of the tomato in Britain—
the Verticillium wilt and the Fusariwm wilt—and other
workers have shown that many other cultivated plants
are susceptible to these two wilt diseases.

VERTICILLIUM WILT OF THE TOMATO

Verticillium wilt is widely distributed throughout the
_ British and Channel Islands where tomatoes are grown,
73

74 DISEASES OF GLASSHOUSE PLANTS

and is responsible for considerable financial losses to the
industry. In normal years it first appears about the
middle of April and increases in intensity up to the second
and third week in May, when it reaches its maximum.
In normal summer temperatures the attacks die down
during the second half of June, July, and August, but
reappear at the end of September, when the plants die
prematurely.

Disease Symptoms.—Diseased plants are usually
stunted, but not invariably so, while the internodes,
especially the younger ones, are badly developed. When
the conditions of temperature and light are most favour-
able to the fungus, the disease symptoms appear quite
suddenly and the plant wilts while the leaves are still
green. ‘The plants may recover their turgidity at night,
but wilt again as the morning advances. The leaves
wither from the base of the plant upwards, adventitious
roots emerge from the stem, and the plant dies. The
process of death is much slower when the conditions are
less favourable to the fungus: yellow blotches appear
on individual leaflets on the lower leaves, and these
leaflets either wilt and wither or wither without wilting. —
Under conditions least favourable to the fungus the
leaves do not wilt, but gradually desiccate from the base
of the plant upwards. Finally death ensues. On cutting
open the stem of a diseased plant the wood is seen to be
discoloured practically to the top of the plant—the colour
varying from light to dark brown. The discoloration of
the stem may be followed to the root, and the place of
entrance of the fungus into the plant may be distin-
guished by the intense browning at that point. The
disease-causing organisms hibernate in the soil or compost
from season to season and infect the young roots as they
develop. Entrance to the plant is assisted by wounds,
but experiment has shown that such are not essential for
infection, Verticillium having the power to invade roots
absolutely free from wounds. The fungus destroys the
cortex at the point of entrance, and, entering the wood,

a b
Fig. 14. Verticillium wilt Fig. 15. Verticillium wilt of the tomato: (a) Wilted plant six
of the tomato. Old weeks after inoculation with V. albo-atrum, (b) control plant
diseased tomato stem [Facing page 74

showing the fungal
outgrowth at the base.

’

DISEASES DUE TO FUNGI 75

passes up into the stem. Here it is found solely in the
woody parts. When the plant dies, the fungus leaves
the vessels and, penetrating the cortex, forms at the base
of the stem a white external growth (Fig. 14). This is
the active fruiting stage, and the spores are readily
blown about, enabling the fungus to spread rapidly.
Occasionally the fungus may penetrate the fruit, causing
a soft rot. Infection rarely takes place in the seed-boxes
or pots, but almost entirely after the plants have been
placed in the houses.

| 5 et ci 2q
ee : g eco | Be.
w 8 RY oa a AS Ass
oo o =~ =o rhe}
~~ a o =| sia ie
oe ce > ° S98 7s
Fo Ay Me 2 og oa
a as
Type of Plant Used. 2a on a 8 wae Zane
Am = e Og @ Oo oH
« ° o 5 Woe ws
ag ret i) 8 Of Sod
os by w Fi me
ice x < 2 5 e oA
Ba = poa | 234
q “43 avi

““Comet ” soft growth... |0-8 |12]| 6 | 4/4/20
Do. do. .. |1:05|12] 8 | 4/4/20

Do. do. .. | 1-20] 12 | 10 [18/4/20
“Comet ” hard growth... |0-45;12 | 6 | 4/4/20
Do. do. .. |o7 [12] 8 | 4/4/20
Do. do. .. |0-75/12/ 10 [18/4/20

“Comet ’ starved growth |0-45/12| 6 | 4/4/20

The Causal Organism.—Verticiluum albo-atrum, to
which this disease is due, produces its spores characteris-
tically at the tips of short branches arranged in whorls or
verticils around the main conidiophore. The spores,
which are produced in quantity at the base of every
plant killed by the fungus, are extremely light, and are
readily blown about in the air. They settle upon the
soil or plant debris and germinate, producing a white
mass of hyphe. If sufficient food is available, the
hyphe produce a crop of spores, and so infection is
abundant. After a time the mass of hyphe produces
innumerable small sclerotia or resting bodies, by means

76 DISEASES OF GLASSHOUSE PLANTS

of which the fungus tides over the winter or other adverse
conditions. Such sclerotia may be produced in the dead
remains of a diseased plant, or generally in the soil
humus. In the spring, the sclerotia germinate and pro-
duce hyphe able to infect young healthy plants. Suscepti-
bility to the disease is influenced by the character of the
plant, as indicated in Table 2 (p. 75), giving the results
of inoculating plants in different stages of growth and of
varying degrees of hardness and softness of growth.

It is a general experience with most diseases that
plants developing rapidly and producing quantities of
soft, sappy growth are the most susceptible to disease,
but in this case the above table indicates the reverse
effect, for plants of “ hard” growth with a thin stem, or
plants obviously starved, most readily succumb to the
disease.

TABLE 3.
f Days after
ee oa Tnoeuletign ba a soph Pathological Symptoms.
27/3/19 15 Complete wilt with slight
yellowing
26/4/19 14 Complete wilt, no yellowing
25/5/19 10 Do do

; 27/6/19 No wilt in 10 weeks | Lowest 9 leaves turned
yellow and _ partially

dried up.
25/7/19 Do. do | Lowest 3 leaves desiccated
25/8/19 53 Complete desiccation of
leaves from base upwards
22/9/19 40 Bottom 4 leaves desiccated
22/10/19 26 Complete wilt with practi-

cally no yellowing

Temperature Relations.—If different sets of tomato
plants are inoculated with V. albo-atrum each month
throughout the year, striking results are obtained. In
such an experiment twelve tomato plants were inoculated
each month, and were grown under similar conditions

DISEASES DUE TO FUNGI 77

throughout the year except for such alteration as was
caused by seasonal variation. The results given in
Table 3 (p. 76) indicate that the conditions during the
months of June, July, and August are unfavourable to
the rapid progress of the fungus in the plant.

These results indicate the probability that tempera-
ture is an important factor in relation to the disease.

As a series of glasshouses where different temperatures
could be constantly maintained was not available,
inoculated plants (hypocotyl stab) were placed in different

TABLE 4.

House. House.

| Tomato Cucumber

Average temperature °C. | 12-5° | 16-6° 20-0° 25°

Absolute minimum ° C. 5-6 11-1 12-2 20-6
Absolute maximum ° C. 20-6 22-2 27°8 33:3
Date of inoculation 14/4/20 | 14/4/20 | 14/4/20 | 14/4/20
No. of days after inocula-
mon... Ae “P 21 21 21 21
Ratio of wilted to total
leaves .. “s = 0:10 6:12 8:12 0:12
Height of discoloured
wood above stab... | 15cm. | 26cm. | 28cm. | 9 cm.
No. of days from inocula-
tion to complete wilt 49 28 28 No wilt
after
80 days

positions in the experimental houses, corridors, etc.,
under different average temperature conditions. Twelve
plants were placed in each position, and the average
temperatures were calculated from readings taken twice
daily from maximum and minimum thermometers placed
beside the plants. The final observations, shown in the
Tables 4 to 7, were taken twenty-one days after
inoculation, and where figures are given they represent
the average obtained from twelve plants.

While the results obtained are open to criticism
because of the wide range of temperature to which the

78 DISEASES OF GLASSHOUSE PLANTS

plants were submitted in any one position, certain facts
emerge which have been fully confirmed by observations
on commercial nurseries. Chief among these are the bene-
ficial effects which shade and temperatures above 24:0° C.
have upon plants suffering from wilt. Table 4 (p. 77)
shows that average temperatures of 16-6° C. and 20-0° C.
are favourable to the rapid progress of the disease, that
of 12-5° C. is unfavourable, while that of 25° C. practically
inhibits it. It will be seen that the organism has travelled
most rapidly up the stem, as indicated by the browning
of the wood, at 16:6° C. and 20° C. and at these tempera-

TABLE 5.

TOMATO HOUSE. CUCUMBER HOUSE.

SSS Frame. 7 us
D- n-
shaded. | Shaded. | ghadea. | Shaded.

Date of inoculation | 14/4/20 | 14/4/20 | 14/4/20 | 14/4/20 | 14/4/20
No of days from

inoculation ‘Me 21 21 21 21 21
Average tempera-

ture ° C. ou 17 22 20 26:3 25
Ratio of wilted to

total leaves .. 6:10 8:10 1:10 0:10 0:10

tures, also, complete wilt occurred most rapidly. The
results shown in Table 5, while confirming the tempera-
ture relations shown in the preceding table, also show the
beneficial effect of shade. While plants in the unshaded
house readily wilted, those in the shaded house did not
wilt although the temperature was favourable to the
disease.

General observations have also shown that tempera-
tures between 15-6° C. and 24° C., with an optimum of
21-1°-22-8° C. are favourable to the rapid progress of
Verticillium wilt, which below 15-6° C. and above 24° C.
is exceedingly slow, while suitable shading counteracts
the effects of low temperatures.

A series of experiments were next arranged in which

/
.
Fig. 16. This photograph shows FIG. 17. Sweet pea, showing right-hand branch wilted as the result
the wilted plant in Fig. 15 after of inoculation with a pure culture of V. albo-atrum.

being submitted to shade and
an average temperature of 25°C
for thirty days. The wilted
leaves have fallen off, but the
plant has recovered and made
good growth in the top.

[Facing page 78

DISEASES DUE TO FUNGI 1?

wilted plants were transferred to high temperatures to
ascertain if they would recover and if such a recovery
would continue when the plants were returned to lower
temperatures. The results which are shown in Table 6
indicate that wilted plants recover when the average

TABLE 6.
Length of Time in Length of Time after
No. of Wilted | Shaded Cucumber Effect of High Returning to Average
Plants. House. Average Temperature. Temp. 20°C. before
Temp. 25° C, Plants again Wilted.
12 1 day Recovered 15 hours
12 2 days Do. 15 hours
12 7 days Do. 2 days
12 14 days Do. 8 days
12 30 days Do. 16 days
12 75 days Do. 30 days

—d

temperature is raised to 25° C. and they are shaded
(Figs. 15 and 16). When such a temperature is
operative for a short time the effect is not a lasting one
for the plants rapidly wilt again when the temperature

TABLE 7.

No. of No. of Days Wilt has Per Cent Recovered Per Cent Recovered

ae : : in Shaded Cucumber |in Unshaded Cucumber
Plants, | "°°"experiment. | House. Average | House. Average

20 2 100 100

20 Z 100 100

20 14 100 100

20 21 100 90

20 30 100 80 |

is lowered. Longer exposure to the high temperature
produces a more lasting result, for after 75 days at 25° C.
the plants remained turgid for 30 days at a temperature
favourable to wilt. Table 7 compares the percentage of
wilted plants which recover when transferred to a shaded
house at an average temperature of 25° C. with that of

80 DISEASES OF GLASSHOUSE PLANTS

similar plants transferred to an unshaded house at the
same average temperature.

Plants in different stages of wilt were used, from a
series where the wilt was just appearing to a series in
an advanced stage after thirty days’ wilting. All the
plants recovered in the shaded house, but only a per-
centage recovered in that which was not shaded. The
plants which did not recover in the unshaded house,
being badly wilted ones, were probably desiccated before
they had a chance to recover.

These observations appear to justify the conclusion
that temperature is a most important factor, while
shading is valuable because it assists the plant by re-
ducing transpiration. The minimum, optimum, and
maximum temperature for growth in pure culture of the
strains of Verticillium albo-atrum utilized for the inocula-
tions were 4:4° C., 23-3° C., and 30° C. respectively. It
will be seen, therefore, that the optimum temperature
for infection coincides approximately with the optimum
temperature for growth in pure culture. Verticillium
wilt is distinctly a disease of low temperature, and is most
severe in spring and autumn.

Shade.—Shade, as we have seen, has a_ beneficial
effect upon the resistance of the host to the disease.
Probably this is due to retarded transpiration and conse-
quently to the decreased rate of conduction of the water
in the vessels, so that the toxic products excreted by the
fungus are not carried up the plant in such large amounts.

Sow Factors.—Experiments carried out with soils of
different types show that Verticilliwm wilt is not restricted
to any particular soil.

Generally speaking, plants on soils which contain a
large amount of humus yield a greater amount of disease
than those growing on soils of a poorer nature.

Clay soils, in virtue of their greater water-holding
capacity, are cooler than sandy soils, and plants grown
upon them are more prone to wilt than those grown on
the latter.

DISEASES DUE TO FUNGI 81

Control—Cultural Methods.

Cultural methods for the control of the wilt have been
devised and have been tested with promising results.

It is commonly held by pathologists that plants
exhibiting hard growth are more resistant to disease than
the more succulent types, but observations on wilt disease
show in this case the reverse to be true, the harder
growing varieties succumbing more readily than the
more succulent ones. The only variety, Manx Marvel,
which so far has proved highly resistant to Verticalliwm
shows a distinctly free growth with thick stems and large
leaves.

It has also been specially noted that plants, starved
in the early stages, or having suffered from a severe
check, are highly susceptible. It has been mentioned
previously that the average temperature of the air and
soil are limiting factors in the incidence of the disease.
The disease is first seen in spring, when the temperature
is low, but with the coming of the higher summer tem-
perature the wilted plants recover, and the percentage
mortality of the plants infected in the spring depends
upon the length of time the cold weather lasts.

The incubation period for the disease varies from
eight to twenty days under favourable conditions, and
complete wilt and death occurs in from six to eight weeks
after the first symptoms appear, if conditions are favour-
able for the fungus. Should the temperature be suffi-
ciently raised before the plant dies it will recover and
produce a satisfactory crop during such time as the
temperature remains high. Once the temperature drops
in the autumn wilt reappears and the plant dies pre-
maturely. Early summer temperatures, therefore, enable
the plants to resist the fungus. It will at once be evident
that good results may be obtained by artificially shorten-
ing the period of low temperature, and in glasshouses
this may be done by increasing the boiler heat and
closing down the houses in the middle of the day.

The following cultural methods for controlling the
c

82 DISEASES OF GLASSHOUSE PLANTS

disease have yielded satisfactory results. As soon as the
wilt appears and it is proved that Verticillium albo-atrum
is the pathogen, the average temperature of the houses
should be raised above 25° C. by suitably increasing the
boiler heat, regulating the ventilation, and by closing
down the houses from two to four hours in the middle of
the day. A light dressing of whitewash on the glass
makes the conditions still more favourable for the plants.
As little water as possible should be given to the roots,
as heavy watering merely aggravates the wilting, but a
light overhead damping helps the wilted plants to
recover. The plants should be encouraged to make
fresh roots above the original diseased ones by placing
fresh soil round the base of the plant.

In one case 68 per cent of the plants in a nursery
were showing symptoms of wilt disease before the above
methods were enforced, but a fortnight later only 10
per cent remained wilted. In view of the fact that low
spring temperatures are favourable to infection by
Verticillium, some advantage might be gained by plant-
ing later than normally, so that the higher summer
temperatures may arrive by the time the plants have
reached a stage suitable for infection.

Examination of the effect of soil type upon the
incidence of the disease has shown that soils rich in
humus yield a higher percentage of diseased plants than
those of a poorer nature. Results obtained on the experi-
mental plots at the Experimental Station, Cheshunt,
confirm this, the greatest percentage of Veriecilliwm wilt
occurring on the plots receiving complete artificials with
large amounts of dung.

Soils infected with Verticilliwm may be cleansed by
sterilization with steam or formaldehyde.

VERTICILLIUM WILT OF THE CUCUMBER AND MELON

This disease is in every way similar to that of the
tomato. The first symptoms are the wilting, yellowing,

DISEASES DUE TO FUNGI 83

and desiccation of the lowest leaves, followed by similar
effects upon successive leaves from the base of the plant
to the top. Finally the whole plant hangs limp and dies.
On cutting open the stem the wood is seen to be a
yellowish-brown, which discoloration extends up the
plant practically to the growing point, and Verticillium
albo-atrum is readily isolated.

The disease is one of early spring and autumn, when
the temperature is low, and is most destructive in cold
houses and beds. Recommendations for the control of
Verticillium wilt of the tomato are equally effective in
the case of the cucumber and melon.

VERTICILLIUM WILT OF THE SWEET-PEA

During recent years growers who cultivate winter
sweet-peas as a catch crop between the tomato seasons
have been troubled by a wilt attributable to Verticilliwm
albo-atrum. As in the case of other plants, the lowest
leaves are first attacked—the fungus working rapidly up
the plant, until the entire leaf surface is in a state of wilt,
yellowing, and desiccation (Fig. 17). The wood is
browned and the roots are in most cases entirely de-
stroyed. The disease spreads rapidly, and the entire
stock of plants may be destroyed within three weeks of
the first appearance of the disease. In this case the
disease progresses so rapidly in the plant, and the con-
ditions of flowering are so restricted, that, except in
special cases, the cultural methods of control advised for
the tomato may have no value.

When the disease is slight it may frequently be

~ checked by an application of ‘‘ Cheshunt Compound,”
but in bad cases the soil should be sterilized by steam
before planting.

Other Hosts of Verticilliwm albo-atrum.—Recent work
has shown that V. albo-atrum attacks a wide range of
plants, many of which are of interest to the commercial
glasshouse grower. The writer has isolated this fungus

84 DISEASES OF GLASSHOUSE PLANTS

from the following plants during the last three years:
potato, tomato, cucumber, melon, sweet-pea, antirrhinum,
and elm. Cross inoculations have demonstrated that
the fungus from any one of these plants will infect all the
others. Inoculations of cotton, egg-plant, capsicum,
and sycamore with V. albo-atrum from the tomato have
yielded positive results.

It is thus apparent that many external sources of
infection exist, and these must be carefully guarded
against if glasshouse crops are to remain healthy.

FUSARIUM WILT OF THE TOMATO.

While a number of soil Fusaria are frequently found
causing rotting of tomato roots, having gained entrance
through wounds made by soil insects and small animals,
the true Fusarium wilt, due to Fusariwm lycopersici Sace.
is comparatively rare in England. It occurs only at the
height of the summer, when the soil temperature is
sufficiently high to enable the parasite to function. The
relatively low temperature of glasshouse soils in this
country is suited to development of Verticillium albo-
airum, which grows best at a mean temperature of
21°-22° C., but is too low for F. lycopersici, which requires
a mean temperature of at least 28° C. before it can do
much damage. In America, however, the soil tempera-
ture in the chief tomato-producing regions is generally
too high for Verticillium wilt, but is suitable for Fusariwm
wilt. American reports show that Fusarium wilt is well
distributed over the southern states, extending up to
Illinois, Indiana, Ohio, and New Jersey. North of this
it is found only occasionally.

Symptoms of the Disease-—The external symptoms of
Fusarium wilt are indistinguishable from those of
Verticillium wilt. There is the same wilting, yellowing,
and desiccation of the lower leaves, and the progression
of these symptoms from the base of the plant upwards.
Diseased plants have a stunted appearance, but the

DISEASES DUE TO FUNGI 85

shortening of the internodes is more marked at. the top
of the plant than at the base, presumably because the
low temperature which prevailed while the lower parts
were developing kept the fungus in check. The root of
the plant is affected by Fusarium wilt to a much greater
degree than is the case with Verticillium wilt, and
during the last stages of the disease the roots may be
completely destroyed. Another distinguishing feature
between this and Verticilliwm wilt is the colour of the
infected wood inside the plant. In Verticillium wilt the
light brown colour of the affected wood extends practi-
cally to the growing point, while in

Fusarium wilt, the colour usually (\
ends at least a foot below, and in a.
most cases does not extend so far. 0
The colour of the wood in Fusarium

wilt is a much darker brown, and in

the root is almost black. The fungus Os
passes from the main stem, through

the leaf traces into the leaves, and Ps

occasionally reaches the fruit. No

fungus growth appears outside the c.
stem until the plant is dead, when

a pink growth forms round the
basal parts. A similar outgrowth

: ais Fic. 18. Spores of Fusarium
appears in the case of Verticilliwm —easinfectum: = (a)_ micro-

wilt, but the colour is white at first. itsaeaea
It is not always possible to distinguish between the two
diseases in this way, as saprophytic F'usaria often produce
a pink growth over that of Verticilliwm.

Causal Organism.—The disease is due to Fusarium
lycopersict Sacc., which name was first given by Saccardo
to a fungus he found growing upon decaying tomato
fruits. The genus Fusarium produces three kinds of
spores or conidia (Fig. 18). One kind are very small
microspores, others are larger and sickle-shaped with
three or four cross walls, while the third type consists
of thick-walled chlamydospores, or resting spores,

86 DISEASES OF GLASSHOUSE PLANTS

which enable the fungus to exist through unfavourable
conditions.

Fusarium lycopersici is able to live as a saprophyte
in the soil, and while it is known to have remained there
for three or four years in the absence of a tomato crop,
there is no reliable data as to limit of its saprophytic
existence. It has long been known that heavy dressings
of caustic lime tend to produce a considerable reduction
in Fusarium wilt.

Temperature Relations.—Inoculation experiments con-
ducted under different temperature conditions have
demonstrated that temperature is a limiting factor in this
disease, just as it is in the case of Verticilliwm wilt. |
Infection is most successful, and the disease develops
most rapidly at a soil temperature of 29° C., while little
infection occurs if the temperature remains constantly
much below this.

Control of the Disease-—In a climate like ours, which
is unfavourable to the disease, it may be controlled by
reducing the temperature of the soil and the air, but in
favourable climates control is dependent upon efficient
soil sterilization by heat or formaldehyde or the raising
of resistant varieties of tomato plants.

FUSARIUM WILT OF THE CUCUMBER AND MELON

The temperatures of the soil and atmosphere in which
these plants are cultivated are more favourable to Fusarium
wilt than those of a tomato house. In consequence this
disease is not uncommon in this country.

The symptoms are much the same as in the previously
described wilts. In slight cases of the disease, when the
plant possesses high resistance, the symptoms may be
that of stunted growth and a reluctance “to break” on
the part of the laterals; but as these symptoms can be
due to unsuitable physical conditions of the bed, this
stage of the disease may frequently be overlooked.
Later, the lowest leaves become affected and soon the

DISEASES DUE TO FUNGI 87

entire plant wilts. On cutting open the stem the wood
is seen to be of a reddish-brown colour.

The disease is due to Fusarium vasinfectum Atk., var.
niveum Sm. (4). At the present time no treatment is
known whereby infected plants may be induced to
recover their turgidity. Elimination of infection is im-
portant if the disease is to be controlled, and infected soil
should not be planted again unless it has been sterilized
by heat or with formaldehyde. In cases where occasional
plants are attacked they should be removed immediately,
taking up at the same time all the root and a ball of soil
a foot in diameter. The hole should be filled with a
mixture of five parts soil and one part lime and re-
planted. Watering with “ Cheshunt Compound ” at the
time of planting is an extra precaution which has proved
beneficial.

CHAPTER V

DISEASES DUE TO FUNGI (Continued)

3. Stem, Leaf, and Fruit Diseases

(a) STEM DISEASES

Sclerotinia Stem Rot.—While this disease appears regu-
larly every year, it ranks as one of the least important of
the diseases of glasshouse plants. Sclerotinia sclerotiorum
Massee (30) is the cause of the disease, and attacks a
wide range of host plants, including beans, cabbages,
carrots, chrysanthemums, cucumbers, melons, petunias,
potatoes, sweet-peas, turnips, and zinnias. Plants are
generally attacked at the soil level, where a white mould
develops on the stem and also spreads into the soil.
The fungal hyphe grow rapidly into the plant, choking
up the conducting vessels and disorganizing the softer
tissues. The pith is destroyed and a hollow cavity pro-
duced. Rotting of the stem progresses steadily up the
plant until some six inches may be involved. Diseased
plants show all the common symptoms of wilt, and soon
die. Following the decay of the stem, small, soft, white
bodies may be found embedded in the white mould.
These vary in size from a sixteenth of an inch to a quarter
of an inch in diameter, and eventually dry, becoming
hard and black (Fig. 19). Such bodies, which are the
sclerotia or resting forms of the fungus, are found within
the hollow stem as well as on the outside. Tomato and
cucumber stems are frequently attacked high up from
the ground, especially on the angles between the leaves
and the stem, which hold moisture, and so provide
88

Fig. 19. Sclerotinia sclerotiorum on the cucumber, showing typical
sclerotia. [Facing page 88

Ms

eM ae |

DISEASES DUE TO FUNGI 89

suitable conditions for the growth of fungus repro-
ductive bodies which may settle there. All diseased
plants should be removed and burned, for if they are
allowed to lie and decay in a heap or on the ground the
sclerotia will fall out of the stems into the soil. In the
following spring the sclerotia give rise to several brown
funnel-shaped structures with long slender stems. These
ascophores, as they are called, produce innumerable
spores, which if carried by the wind to a susceptible host
plant will cause disease. The cucumber is more suscep-
tible to this disease than is the tomato, and in the case
of the latter crop a virulent outbreak is a sign that some-
thing is wrong with the physical factors in the plant’s
environment. In bad cases sterilization of the soil will
destroy the disease organisms, but generally it is sufficient
to water the soil with a solution of “ Cheshunt Com-
pound ”’ before planting. When the disease appears the
base of the affected plants should be watered with this
solution, as also should the neighbouring healthy plants.

Stem “‘ Canker” of the Tomato.—This disease became
of serious importance to the tomato industry during the
early part of the present century. It was at first attri-
buted to a fungus, Mycospherella citrullina Grossenb.,
described as the cause of a stem disease of the melon in
America. Brooks and Searle have recently shown that
the fungus was incorrectly identified and is really Diplo-
dina lycopersici Cooke, Holtos emend., Brooks and
Searle (13).

About 1909 the disease had reached its maximum
virulence, and was extremely dangerous, but since then
it has decreased in power, until at the present time it
exists only in isolated places. Attention is first drawn
to the disease by the wilting of the affected plant.
Examination of the base, where the fungus infection
commences, shows the tissues to be waterlogged and pale
in colour. Later the affected portions become grey and
finally black. The diseased stem becomes shrunken,

90 DISEASES OF GLASSHOUSE PLANTS

cracked, and generally exhibits a canker-like appearance
(Fig. 20). Innumerable minute black pimples may be
seen on the surface of the lesions. These are flask-like
vessels called pycnidia, which contain many tiny spores,
by means of which the disease is able to spread.

During its most destructive years the disease was
extremely virulent. Affected plants died quickly and
infection spread with alarming rapidity. Diseased plants
should be at once removed and burned, and of recent
years a solution of ‘‘ Cheshunt Compound ”’ has proved
useful in preventing the spread of the disease from plant
to plant. Sterilization of the soil by means of steam has
proved successful in eliminating the disease from infected
nurseries.

A similar disease of the cucumber and vegetable
marrow has been reported in this country, and Massee
demonstrated that the fungus from the tomato was able
to infect these plants also.

Botrytis Stem Rot.—This disease is common to many
plants grown under glass, and is especially important in
the case of the tomato. Under conditions of excessive
humidity Botrytis sp. attacks badly pruned leaf bases of
the tomato and, gaining strength by feeding on this half
dead matter, works into the healthy stem tissues, de-
stroying the cortex, vascular ring, and pith in turn, and
by cutting off the water supply to that part of the plant
above the point of attack causes it to wilt.

The first signs of disease are the appearance of grey,
sunken areas on the stem, generally round a ragged
wound or half dead leaf base. The lesion slowly enlarges
and may finally extend from six to twelve inches along
the stem and girdling it (Fig. 21). Once the stem is
attacked the plant can be saved, but only by cutting out
all the browned tissue and rubbing the wound thus made
with a moistened lump of liver of sulphur or a crystal of
copper sulphate.

The disease may be prevented by careful pruning,

Fig. 20. Tomato stems showing “ cankers’’ made by Diplodina lycopersici.

Fig. 21. Botrytis stem rot of the tomato, showing typical lesions at a and b.
[Facing page 90

DISEASES DUE TO FUNGI 91

defoliation, and regulation of the atmospheric humidity
by efficient ventilation. When removing side shoots and
unwanted leaves, the cuts should be clean, and made
close to the main stem, when they will rapidly heal over.
If stumps are left, these do not heal over, but shrivel and
die, affording suitable places of entrance to the fungus.
When the fruit has been picked from the lower trusses
the plants should be defoliated from the base to the
lowest unpicked truss to allow the air to circulate freely
through the plants. In bad cases it may be necessary
to spray the stems with a 2 per cent solution of calcium
bisulphite to destroy the spores of the fungus and so
lessen the risk of infection. Carnations, chrysanthemums,
and roses under glass are subject to this disease, the
conditions of infection and control being the same as in
the case of the tomato.

Stem Canker of the Rose.—The symptoms of this
disease are large, cankered wounds frequently found on
rose stems. The lesions are brown in the centre, with a
black border, usually surrounded by a reddish zone.
The disease, caused by Coniothyrium Fuckelii Sacc., first
appears as small, reddish patches on the young wood.
Later, cracks develop at the infected points, and increase
in size, becoming extended and distorted by the produc-
tion of callus until the typical canker is produced.
Examination with a lens readily shows the presence of
minute black bodies scattered over the surface of the
canker; these are the pycnidia or fruit bodies of the
fungus. Infection takes place on the tender shoots or
through wounds made in older parts of the plant. All
infected parts should be removed and burnt at once if
the disease is to ke controlled.

Rose Graft Disease.—Recently a rose graft disease,
caused by Coniothyrium rosarium, has been described by
Vogel (51) in America. The disease attacks the scion at
the union, and speedily causes wilt and death. The

92 DISEASES OF GLASSHOUSE PLANTS

lesions are light yellow water-soaked areas at first, turn-
ing dark brown later. The epidermis becomes loosened
and changes from dark brown to a light brown colour.
Minute black pycnidia are produced on the light brown
areas. In some cases only one side of the graft is attacked
and the plants do not appear to have suffered, but when
taken into rose houses they are instrumental in spreading
the disease. If possible, resistant varieties should be
grown. |

(b) LEAF DISEASES

Leaf Diseases of the Cucumber.—Under the name
‘* Spot Disease ” the commercial cucumber grower groups
all the leaf diseases of that plant. The most important
of these in this country are those caused by Cercospora
melonis Cke., and Colletotrichum oligochetum Cav., while
under conditions of abnormally high humidity Clad-
sporium cucumerinum KEK and A may be a destructive
parasite. In spring Alternaria Brassice, var. nigrescens,
may damage the lowest leaves, and Hrysiphe polygons
D.C. may be destructive at low temperatures.

Cercospora Leaf Spot.—During the years 1896 to 1907
Cercospora melonis appeared in the Lea Valley in this
country and caused considerable destruction, but the
advent of the immune variety, Butcher’s Disease Resister
in 1903, and the application of methods of soil steriliza-
tion, led to its ultimate disappearance. At the present
time the Cercospora disease exists only in isolated parts
of Great Britain.

Instances are recorded of the entire crop in houses
one hundred feet long containing ninety-six plants per
house being completely destroyed in four to six days
after the disease made its first appearance.

The first sign of the disease is the appearance on the
upper surface of the leaves of tiny pale green, water-
soaked spots. These rapidly increase in size and number
and coalesce, turning grey at first and afterwards becom-

DISEASES DUE TO FUNGI 93

ing an ochreous-brown colour. The spots are definite in
outline, irregular in shape, and may be distinguished
readily from other leaf spot diseases. Frequently the
entire leaf withers within the space of forty-eight hours
after infection.

An examination with a pocket lens of the upper
surface of a “spot ’”’ shows the presence of brown erect
fungal filaments, bearing conidia at their tips. These
conidia are comparatively large, cylindrical in shape, but
narrowed towards one end,
and each possesses seven to
nine cross walls (Fig. 22).
Infection spreads rapidly, the
conidia being easily blown
about the house, as well as
being transported by the
workers, insects, and by the
process of overhead damping.

The disease can only
attain epidemic proportions
under conditions of high
temperature and humidity,
but rapidly growing, sappy
plants are more susceptible
to attack than slow growing
plants with harder tissues. It
has been observed that the
tops of badly diseased plants Fic. 22. Spores of Cercospora melonis.
. grow away clean if by any chance they pass through the
ventilators into the open air. Similarly, cucumbers grown
under frames or in the open air are not attacked by this
disease.

The control of the disease depends very largely upon
the humidity of the glasshouse atmosphere, and efficient
ventilation readily checks it by drying out the moisture
from the house. All diseased leaves should be at once
removed and burnt, for if these fall upon the damp
earth they become covered with a fungal growth bearing

94 DISEASES OF GLASSHOUSE PLANTS

innumerable spores, and so constitute a constant danger
as a source of infection. Spraying with a solution of
liver of sulphur and flour paste is recommended as a
means of control in severe cases.

Colletotrichum Leaf Spot or Anthracnose.—In its
commonest form this disease first attacks the leaves, and
may appear at any time during the life of the plant.
It has occasionally been observed during the propagating
period, but generally it does not appear until March or
April, when the plants are well established in the house
and some fruits have been cut. The time at which the
plants are attacked bears no relation to their age, but is
determined by the presence of suitable sources of infec-
tion. On the leaves the spots are washed into the
hollows of the leaf surface by the process of overhead
damping. The lesions commence as pale green water-
soaked spots, barely distinguishable by the untrained
eye, but quickly assume a characteristic appearance,
becoming dry and reddish-brown in the centre with a
yellowish water-soaked surrounding zone (Fig. 23). The
lesions vary in shape from an almost circular spot in
areas untouched by any large vein to irregular amceboid
patches where they form over a vein. The spots fre-
quently crack in the centre, and the desiccated tissue
may not infrequently be beaten out by the daily overhead
damping. The spots increase rapidly in size, become
more circular and blotch-like, finally coalescing, and the
leaf dies. At the final stage the leaves have a scorched
appearance and are covered with spots. Microscopic
examination shows the presence of numerous minute
acervuli bearing spores and setz on the upper surface of
the leaf.

As the disease advances lesions develop on the leaf
petioles and stems, showing as sunken, water-soaked areas
at first, but rapidly becoming dry and powdery. At
first they are usually linear in shape, but may spread
round the stem, and under glasshouse conditions it is not

Fig. 23. Cucumber “leaf spot’’ caused by Colletotrichum
oligochaetum.

Fig. 24. Colletotrichum oligochetum growing on cotton-wool, straw,
and wood. [Facing page 94

DISEASES DUE TO FUNGI 95

uncommon to see the soft tissues of the stem completely
destroyed, leaving the vascular bundles exposed as dry
fibres, and causing the death of the plant above the point
of attack. On stem and petiole lesions, sporulation is
abundant, a pinkish colour being produced, which turns
black with age.

The lesions on the fruits appear as pale green water-
soaked, sunken areas, the surface of which, owing to
abundant spore production, become pink in colour and
finally black. The tissues under the lesion are destroyed
and a cavity is produced, and this is exposed by the
cracking of the surface above. When the leaves are
attacked the health of the plant is impaired only by the
serious reduction of ieaf area, but lesions on the stem are
more serious and may cause the rapid death of the plant
by destroying the tissues.

The cause of the disease is a fungus, Colletotrichum
oligochetum Cav., which has been shown by experiment
to thrive upon such materials as new and rotten wood,
straw, cotton wool, and paper, provided these are kept
suitably moist (Fig. 24). Further tests have proved
that following a severe outbreak of the disease the causal
organism may tide over the winter by a saprophytic
existence in various parts of the glasshouses. As the
result of investigation (7), the following main conclusions
have been arrived at:

(1) The present methods of cleansing glasshouses
during the winter months are not sufficient to exterminate
centres of infection of Colletotrichum oligochetum, which
may exist from a previous diseased crop.

(2) Infection is more abundant immediately after the
diseased crop has been removed than after the period of
winter rest, but sufficient survives to carry the disease
over from one season to another.

(3) The fungus may live occasionally in the debris
which collects in the overlap between two panes of glass,
but except in old houses this does not form an important
source of infection.

96 DISEASES OF GLASSHOUSE PLANTS

(4) The fungus may carry on a saprophytic existence
in rotten wood in the house and in paper used for blocking
holes, and these constitute important sources of infection.

(5) Straw manure removed from beds in infected
houses was found invariably to harbour the parasite, and
when allowed to remain unburnt in a heap outside the
houses is a centre for the spread of the fungus.

(6) The examination of “ flats’ was unsuccessful in
obtaining positive evidence of their transmission of this
fungus, but observations upon the incidence of this
disease in commercial nurseries indicate the probability
that it may frequently be carried in this way.

(7) Manure fresh from country farms has proved to
be free from infection with this organism.

(8) Heaps of manure placed adjacent to those of decay-
ing cucumber remains have been found to be infected.

(9) Manure direct from town stables has in some cases
been found to be infected.

Other important sources of infection are the water
supply and the clothes of workers in the infected nurseries,
the latter having been found to be a most important
method of disease transmission in the Lea Valley.

As in the case of the Cercospora disease, humidity of
the atmosphere has proved to be an important factor in
the progress of the disease, which is considerably more
rapid at humidities about 90 per cent than at those below
60 per cent.

During the fallow season the fungus is known to
hibernate in various parts of the houses. This being so,
the efficient cleansing of the houses forms an important
part of the control of the disease. Fumigation by burn-
ing sulphur has not proved efficient in this respect, and
it is necessary to wash down the structure with oe
cresylic acid, as explained in Chapter IX.

Of the many spray fluids tested, those of liver of
sulphur or lime sulphur have yielded the most promising
results. One essential quality of first-class fruit is the
presence of a perfect ‘“‘ bloom” on the surface, and in

DISEASES DUE TO FUNGI 97

consequence commercial growers hesitate to use any
spray which destroys this “‘ bloom ” or spots the fruit.

Copper compounds have a tendency to do this, and
cannot be recommended.

The foliage of the cucumber is difficult to wet
thoroughly with ordinary aqueous solutions, and the
addition of an efficient spreader is necessary. Flour
paste has proved satisfactory in this respect and is there-
fore recommended. Liver of sulphur and flour paste or
lime sulphur and flour paste, the preparation of which
is described in Chapter IX, have proved satisfactory
for checking the spread of the disease during the growing
season. ‘To be quite effective they should be used in the
early stages of the disease, before the fungus has attacked
the succulent petiole and stem tissues. Generally one or
two plants are first attacked, and it is better to sacrifice
these than to endanger the rest of the plants by allowing
the diseased individuals to remain untreated.

When the disease first appears the plants should be
thoroughly sprayed with either of the mixtures recom-
mended, and on the next day every “spotted” leaf
should be cut out and burned. This process of spraying
and removing the diseased leaves should be repeated
again at weekly intervals, but generally two applications
are enough if the fungus has not entered the petioles or
stems. Spraying should be carried out only in the cool
of the evening, and the next morning the plants should
be thoroughly sprayed with water to remove any surplus
spray liquid that may have remained on them, and
a little ventilation should be allowed. Care should be
taken to see that the houses are well shaded, as after
this treatment direct sunlight may give rise to scorching.
The effect of the spray on cucumber plants is slight if
careful attention is given to the above precautions.
Occasionally the spray burns a newly opened leaf or
tendril but rarely does any appreciable damage. If
the fungus is allowed to get a strong hold upon the
petiole or stem tissues it is increasingly difficult to check

7

98 DISEASES OF GLASSHOUSE PLANTS

the disease permanently. Its spread may be stopped for
a time, but as only the spores and spore-bearing hyphze
on the outside of the plant are killed the hyphe within
the stem grow out in time and produce masses of spores
which are rapidly carried about the house and the
disease again appears. In these cases it is advisable to
take out the diseased individuals and replant the house
after thoroughly cleaning it.

The latter may be done by means of a cresylic acid
emulsion, after which cleansing planting must be deferred
for a fortnight; or else by a solution of liver of sulphur
at the rate of 6 Ibs. in 100 gallons of water, or lime
sulphur at the rate of 3 pints per 100 gallons. When
liver of sulphur or lime sulphur is used the house may be
replanted in twenty-four hours.

Dusting with sulphur powders has been proved to
check the disease, but a complete control has not been
obtained by this means.

Much can be done to prevent and control the disease
by providing the best cultural conditions for the plants.
The disease assumes its worst form, and spreads most
rapidly, when the atmosphere of the house is badly
ventilated and saturated with moisture, and also when
there is a marked difference between the day and night
temperatures in the houses. The conditions which best
enable the plants to resist the disease may be summarized
as follows : .

Plants should be grown steadily from the beginning,
without any attempt at forcing, and a little air should
be given whenever outside conditions will allow. The
atmosphere of the houses should never be stagnant or
saturated with moisture for long periods, and efficient
circulation of air should be encouraged by suitable
ventilation. The beds should never be cold or sour, and
careful attention should be paid to the maintenance of
constant day and night temperature.

Cladosporiwm Leaf Spot.—Cladosporium cucumerinum

DISEASES DUE TO FUNGI 99

E and A., described later (page 117) as the cause of a disease
of cucumber fruits, has been found to cause a spotting of
the leaves under conditions of exceptionally high humidi-
ties constantly maintained for long periods. Innumerable,
small, light-brown, irregular spots are produced on the
leaves, which are ultimately destroyed.

Usually the disease does not attack the leaves, but on
the rare occasions when it is present the attack may be
checked rapidly by reducing the amount of moisture in
the atmosphere.

Alternaria Leaf Spot.—This disease, caused by Alter-
naria Brassice, var. nigrescens, not infrequently appears
during the early part of the season. The lowest leaves
are attacked, and dead, reddish-brown spots, circular in
shape, are produced on the leaves. Such spots fre-
quently possess faint concentric growth rings, and are
not unlike the spots produced by Colletotrichum.

Powdery Mildew.—This disease, caused by Erysiphe
polygont D.C., is not infrequently found in cucumber
houses, but it rarely reaches a serious stage. It appears
as small, greyish, water-soaked spots, on the upper surface
of which fungal growths soon appear, giving them a
white, mealy appearance (Fig. 25). Under suitable con-
ditions the isolated spots rapidly extend until the whole
surface of the plant may be covered with a white
powdery, fungal growth. Excessive watering, insufficient
ventilation, and irregular temperatures, combined with
insufficient light, provide favourable conditions for the
disease. In this country the disease appears chiefly in
winter and may be controlled by providing suitable
ventilation so as to dry out the atmosphere of the house,
reducing the water supply, and improving the conditions
of lighting. Dusting with sulphur powders is of con-
siderable assistance in controlling the disease.

Downy Mildew.—This disease, at present unknown in

100 DISEASES OF GLASSHOUSE PLANTS

this country but important in America, is caused by
Pseudoperonospora cubensis (B and C) Rost. When the
air is very moist and warm the disease appears on the
leaves as ill-defined yellow spots, which rapidly run
together and cause the leaves to turn yellow and die.
At low temperatures the progress of the disease is con-
siderably less rapid. The disease attacks the old leaves
first, but rapidly spreads over the entire plant, which
produces only a few small, misshapen fruit.

When the disease appears all overhead damping
should cease and the foliage should be kept dry. Spraying
with Bordeaux Mixture is recommended in America as
being an efficient means of control.

Tomato Leaf Mould.—The most important leaf disease
of the tomato in this country is that caused by Clado-
sporium fulvuum Cke., and variously named “ mildew,”
“rust ’ and “leaf mould” (Fig. 26). The term “ mil-
dew,’ which is most commonly used, is unfortunate, as
most mildews are whitish in appearance, and the most
suitable name would seem to be “leaf mould,” for
diseased leaves possess a typical mouldy appearance.

The first sign of the disease is the appearance of a
pale olive-buff, downy growth in local spots on the under
surface of the leaf. A little later the top surface of the
leaf immediately above the diseased spot turns a pale
yellow colour which merges gradually into the green
colour round the spot. As the disease progresses, the
yellow colour of the top surface turns a deep ochre-yellow,
finally becoming reddish-brown when the leaf tissue
is killed.

The fungus growth on the lower surface of the leaf
changes colour as the disease progresses. From the
original pale olive-buff it becomes tawny-olive and finally
purple when the leaf tissues are dead. The infected
areas soon begin to die, and the fungus growth covering
the dead tissues assumes a violet-purple colour.

As the first infected parts of the leaf are the first to

Fig. 25. Powdery mildew of the cucumber.

Fie. 26. Tomato ‘* mildew ” caused by Cladosporium fulcrum, showing the fungal
masses on the underside of the leaf. [Facing page roo

DISEASES DUE TO FUNGI 101

die a very pretty effect is frequently seen on the under
surface of the leaves where violet-purple areas are sur-
rounded by the tawny-olive zones of the later infections.
The fungus spreads rapidly over the leaf, which soon
shrivels up and hangs as dead tissue covered with a
fungus growth producing innumerable spores. In bad
attacks the spores are so numerous and so easily dis-
lodged from the leaves that by shaking the plants the
air may be filled with them. It will readily be seen that
infection may be both abundant and rapid. Generally
the disease does not appear until July or August, but
under specially favourable conditions, which are largely
climatic in origin, it has been observed as early as April.
Under normal conditions the older and more succulent
leaves are first attacked, the disease passing from the
lower parts of the plant to the top, but in severe attacks
even the young leaves are attacked almost as soon as
they are unfolded. The fungus occasionally attacks the
flowers and young fruit, and while large fruit are un-
injured those below the size of a pea have frequently
been found to contain the filaments of C. fuluum. In
severe cases the entire flower may be attacked and
destroyed. Infection takes place by means of the
stomata, through which the germ tube from the spore
passes into the interior of the leaf.

The effect of the disease upon the plant varies with
the extent of the disease, which is again dependent upon
the atmospheric conditions of the glasshouse. In slight
attacks the old leaves only are attacked, and as these are
being continually removed in the cultural process of
defoliation, little injury is done to the plant. In severe
cases the reduction in leaf area resulting from the action
of the parasite is so great that the plants are weakened
to a considerable degree, fruit production ceases, and the
plants die prematurely.

The extent and progress of the disease varies with
the conditions of light, humidity, and temperature of the
glasshouses. Light is unfavourable to the rapid develop-

102 . DISEASES OF GLASSHOUSE PLANTS

ment of C. fulvum, laboratory experiments having shown
that the rate of growth is much slower in well-lighted
conditions than in the dark. The minimum growth
temperature is approximately 9° C., the maximum
temperature 30° C., while the most favourable tempera-
ture lies between 20° C. and 25° C. The fungus also
grows best when the atmosphere is rich in moisture.
Such facts are confirmed by observations in commercial
nurseries, where outbreaks have generally occurred after
periods of dull, humid weather. Also, those parts of a
house or block of houses where the heat and water vapour
tend to accumulate are notable as places where the
disease first appears and spreads most rapidly. Thus a
block of houses without dividing partitions, if built on a
severe slope, develops leaf mould first at the highest
level, where the heat and moisture accumulate. This
can be obviated by interposing dividing partitions at
intervals through the block. The disease also appears
first at any spot in a house where the air tends to be
stagnant and moisture accumulates. Any departure
from a uniform level of the soil surface by the formation
of hollows and depressions provides sufficiently moist
conditions for C. julvum to develop.

The disease is best controlled by providing a dry
atmosphere for the plants, efficient ventilation and
circulation of the air being important factors in this
respect. It is equally important to trim the plants so
as to allow the air to circulate freely between the leaves.
Spraying with liver of sulphur and lime sulphur have
been recommended as a means of control, but under
commercial conditions it is practically impossible to do.
much good by spraying, for the dense foliage and height
of the plants render a complete wetting of the plant
surface an almost impossible process.

Dusting with sulphur powders helps to keep the
disease in check, but is by no means a cure for it. Under
commercial conditions the most convenient method of
checking the progress of “leaf mould” is to vaporize

DISEASES DUE TO FUNGI 103

with sulphur at regular intervals by means of a mechanical
vaporizer and to provide efficient ventilation.

Leaf Spot Disease of the Tomato.—This disease, due
to Septoria lycopersict Speg., is described as being a
serious one in America, but fortunately it has not yet
appeared in this country. It can easily be recognized
by the appearance of numerous spots with light grey
centres and dark margins. Levin (28), who carried out
a careful investigation of this disease, describes the
symptoms as follows:

“The earliest indication of the disease is a water-
soaked spot which can be distinguished with a hand lens
on the underside of the leaf. There is no noticeable
discoloration of the tissue at the outset. As the spot
grows larger it becomes more or less circular in outline
and shows a definite margin. The affected tissue darkens,
becomes shrunken, and later appears hard and dry. The
colour of this spot may vary from black to greyish-white.
The spots may vary in shape and size from a small
circular spot of pin-head size to a large irregular spot of
about 2 cm. in diameter. Not infrequently the spots
coalesce. While the tissue is shrinking three to ten
small, black, glistening pycnidia appear in the spot.
Finally, yellowish mucilaginous masses can be seen
exuding from the pycnidia. Upon microscopic examina-
tion these are found to be masses of long, filiform spores.
The number of pycnidia are well defined, visible with the
naked eye, and separate. Pycnidia may occur on the
underside of the leaf; usually, however, they occur on
the upper side. At this point it must be noted that not
all spots contain pycnidia when the leaf dies. This point
will be taken up in detail later.

“* About the time of spore exudation the green tissue
of the leaflet contiguous to the fungous spot begins to
turn yellow. This yellowing increases, eventually in-
volving the entire leaflet. ‘Then the fungous spots which
have been so far pliable become dry and brittle. The

‘10 eee
/

104 DISEASES OF GLASSHOUSE PLANTS

leaflets gradually droop and dry on the stalk, which later
also shrivels up but remains attached to the stem until
broken off by a slight jar. On tomatoes which are staked
the disease is sometimes confined to the lower leaves.
Where the plants are allowed to trail at will the disease
may cause almost complete defoliation of the plant, the
small tufts of young terminal leaves alone escaping.

““'The disease is commonly found on the stems. It is
manifested by small, slightly elongated, dark spots con-
taining pycnidia. These spots are not so clearly defined
as those on the leaf. The damage to the stem is slight ;
these spots do not enlarge to form cankers and are not
serious except in so far as they produce spores for further
infection.

“Small spots, more or less elongated, occur on the
calyx, and take a form intermediate between those on
the leaves and stems.”

It may be remarked that the difference in form of the
spot are doubtless due to the texture of the host.

A moist atmosphere and temperatures of 25° C. to
30° C. are favourable to the disease. The fungus will not
grow at a temperature of 374° C., and ten days’ exposure
to this temperature has been found to prevent any further
growth even when the temperature has been reduced to
the optimum. Control measures recommended include
efficient ventilation, spraying with Bordeaux Mixture,
and the enforcement of strict sanitary conditions.

Carnation Rust.—This disease, which is probably one
of the commonest diseases of the carnation, is caused by
a fungus, Uromyces caryophyllinus (Schrank) Wint., and
has been known in Europe for more than a century.
Rust is easily recognized by the blisters or sori which are
produced on the leaves and stem (Fig. 27). These are
at first covered by the epidermis of the leaf, but this is
eventually ruptured, and masses of brown spores are
exposed. The disease begins at the lowest leaves, but
under favourable conditions it spreads rapidly over the

DISEASES DUE TO FUNGI 105

entire plant. Affected leaves turn yellow round the
diseased spots and ultimately curl and die. While few
glasshouses are entirely free from rust it only becomes a
serious trouble when the temperature is excessively high
and the plants are over-watered. When the leaves at
the base of the plant are continually moist ideal condi-
tions for spreading the disease are produced. Thus

Fig. 27. (a) Rust of the carnation, (b) uredo-spore, (c) teleuto-spore, (d) spore mass,
(e) stigmonose of the carnation.

where rust is prevalent it is advisable to keep the lower
foliage off the ground by wire netting supports, bent in
the shape of an inverted V, placed between the rows.
Rust is spread each year by means of cuttings taken
from diseased plants, and in consequence it is necessary
to propagate from healthy plants only. Sponging the
leaves with a solution of potassium permanganate has
been recommended as a means of checking the disease,
while spraying with liver of sulphur and flour paste solu-
tion has proved useful in severe cases. Careful attention

106 DISEASES OF GLASSHOUSE PLANTS

to cultural conditions and an avoidance of high tempera-
tures and excessive watering, combined with careful
selection of cuttings, are usually sufficient to prevent the
disease from reaching abnormal conditions, so that
spraying should not be necessary.

Septoria Leaf Spot of the Carnation.—This disease is
caused by Septoria dianthi Desm., which produces light
brown areas on the leaves and stem of the carnation.
Most commonly the lower portion of the leaves, especially
the sheathing base, is attacked, and this causes the leaves
to bend downwards. Diseased leaves also frequently
become much shorter and curl longitudinally. Diseased
areas on the stems are generally found between the nodes.
When the tissue has died as the result of the fungus
attack, numerous tiny black spots may be seen scattered
over the surface. These are the pycnidia or fruiting
bodies, which contain masses of long, narrow, septate
spores.

Under normal glasshouse conditions the disease is
rarely serious, being checked by efficient ventilation and
careful watering. Spraying with liver of sulphur or dust-
ing with sulphur powders is recommended as a means of
control. .

Leaf Mould of the Carnation.—The symptoms of the
disease, which is caused by Heterosporium echinulatum
Berk., are recognized by the production on the leaves of
small, pale grey spots about an eighth of an inch in
diameter. The spots become covered with a dense mat
of fungal growth, which assumes a grey and finally a light
brown colour. Numerous olive-coloured spores covered
with tiny warts are produced on the spots. The method
of control recommended is the same as for “ leaf spot.”

Macrosporium Leaf Spot of the Carnation.—Some
varieties of carnations suffer from a disease of the leaves
and stems due to Macrosporium diantha Stevens and Hall.
Pale grey, circular or elongated spots, the centres of which

DISEASES DUE TO FUNGI 107

are covered with a black fungal growth, are produced,
and may cause the death of the leaves and even the stem,
but usually only the leaves are attacked.

Low temperatures and an excess of moisture in the
air should be avoided. All diseased leaves should be
removed and burned. Spraying with Bordeaux or
Burgundy Mixture is recommended as a means of
control.

Powdery Mildew of the Carnation.—Occasionally this
disease, due to a species of Ozdiwm, is found on carnations
in this country. White, powdery patches appear on the
leaves, also the calyx and corolla of the flower. Good
cultural conditions are sufficient to control this disease
in the early stages, but in severe cases dusting with
sulphur powders or spraying with liver of sulphur may
be necessary. Mercer (34) reports Alington, Bridesmaid,
and British Triumph as being specially susceptible
varieties.

“ Die Back”’ of the Carnation.—Frequently badly cut
stems and flower stalks do not heal but die from the cut
end towards the main stem (Fig.36, facing p.132). Affected
portions first turn a pale yellowish-green, then become
yellow and die. A fungus belonging to the genus Fusarium
is responsible for the disease, having gained entrance
through the wound. Soft, rapid growth ; overcrowding ;
and high temperatures favour the disease. Care should
be taken to make clean cuts, and overhead spraying
should be performed in the morning so that the foliage
may be dry before night. All diseased portions should
be cut away at a point in the healthy tissue at least
two inches away from the apparent dead parts. The
removed pieces should be burned immediately.

Rust of Chrysanthemums.—This is a very common
trouble with chrysanthemums, and is similar in nature to
the rust of carnations. Small blisters are produced

108 DISEASES OF GLASSHOUSE PLANTS

beneath the epidermis of the leaf, and with age the
epidermis bursts and breaks away leaving a mass of tiny
rust-brown spores (Figs. 28, 29). Thus typically infected
leaves are densely spotted with tiny rusty pustules, the
majority of which are formed on the underside of the
leaf. At one time it was thought that the rust of
chrysanthemums was the same as that which attacks a
large number of plants of the same family, but Arthur
has shown that it is not so, and that the chrysanthemum
rust will not attack any other plant. This disease is
caused by Puccinia chrysanthemi Roze.

It is carried from season to season by means of
cuttings and old stock plants, and it is important that
cuttings should be made solely from healthy stock.
Dusting with sulphur powders is generally practised as a
means of keeping the disease in check. Under glass it is
rarely serious, and in fact need not be feared by careful
growers, as attention to ventilation, careful watering, and
selection of stock is sufficient to effect a control. The
disease is frequently imported on new stock, which should
be carefully examined for signs of it. Miss Clay Frick
(white), Niveus (white), W. Duckham (pink), and W. H.
Lincoln (yellow) have proved to be highly susceptible to
rust. Cheshunt White, Heston White, Ivy Gay (mauve-
pink), Kathleen May (red), Mrs. Henneage (yellow),
Nagoya (yellow), Mlle. R. Pancouche (white), Romance
(yellow), and Winter Cheer (pink) are moderately suscep-
tible. Baldock’s Crimson, Gladys Lane (bronze), Hortus
Tolosanus (bronze), Mme. R. Oberthur (white), Tuxedo
(bronze), The Favourite (white), and Winter Gem (yellow-
bronze) have proved highly resistant.

Chrysanthemum Leaf Blight.—This disease, caused by
Cylindrosporium chrysanthemi K. and D., makes rapid
progress once a plant is attacked. It is not common in
this country, but when it occurs serious damage results.
Large dark brown patches are formed on the leaves.
The tissues round the patches turn yellow and the leaves

aoe SEE

e 3

Chrysanthemum rust, showing the dark spore masses.

FIGs. 28 and 29.

[Facing page 108

DISEASES DUE TO FUNGI 109

soon die and hang down the stem. Tiny heaps of spores
are produced on the diseased areas on both sides of the
leaf. Control is a difficult matter, and infected plants
should be destroyed immediately. Spraying with a
copper fungicide is recommended as a means of protecting
the healthy plants.

Powdery Mildew of the Chrysanthemum.—Chrysan-
themums grown under glass frequently suffer from
this disease, due to Oidium chrysanthemi Robh. The
leaves become covered with
a white, powdery, fungal
growth. The presence of the
disease is conditioned by a
moist atmosphere, and atten- «@@&
tion to ventilation and the 3
massing of the plants so as to
encourage an efficient circula-
tion of air will do much to
limit its development. Dust-
ing with sulphur powders or
spraying with liver of sulphur
and flour paste has proved
successful in checking its
rapid spread.

Chrysanthemum Leaf Spot. rie. 30. Septoria leat spot of the
—Septoria chrysanthemella teat, () a pyeutdiun, (0) spores.
Cav. (Sacc.) causes a leaf spot disease of the chrysanthe-
mum (Fig. 30) which is distinguished by the presence
of small dark brown spots, bearing pycnidia or fruiting
bodies which show as minute black points. The spots
grow rapidly and coalesce, causing the leaves to curl at
the edges and to fall prematurely. Leaf spot is not a
common disease in this country, although it has been
recorded several times. In America and Europe it is well
known to growers. Diseased leaves should be removed
and burned, and the plants sprayed with liver of sulphur.

110 DISEASES OF GLASSHOUSE PLANTS

Downy Mildew of the Rose—This disease, while
common on glasshouse roses, is somewhat difficult to
detect. Frequently young plants appear to lack vigour
for no obvious reason, but a careful examination of the
leaves reveals the presence of minute fungal filaments.
The causal organism, Peronospora sparsa Berk., is capable

Fiq. 31. ape J mildew of the rose: (a) Diseased leaves, (b) summer spores,
(c) a perithecium, (d) ascus containing eight ascospores.

of attacking all leaves and young shoots. The first
symptom of the disease is the sudden flagging of young,
vigorous leaves, which readily fall off the stem if it is
shaken gently. The shoot itself droops and dies.
Diseased leaves and shoots possess reddish-purple patches
bearing a fungus growth. Gradually the disease develops
until all the green tissues are destroyed.

DISEASES DUE TO FUNGI 111

All infected material should be removed at once and
burned. Spraying with liver of sulphur and flour paste
has yielded promising results and vaporizing with
sulphur is also recommended.

Powdery Mildew of the Rose.—This disease, caused
by Spherotheca pannosa (Wallr.) Lév., may cause serious
trouble under glasshouse conditions.

On the leaves appear white powdery patches consist-
ing of innumerable spores (Fig. 31), while dense white
growths, producing comparatively few spores, develop
on the fruits and shoots of the plant. These latter
patches give rise to the perithecia or resistant fruiting
bodies which carry the disease over the adverse conditions
of winter. The mildew covers the leaves, especially of
the young shoots, causing the former to fall, and fre-
quently deforming the more sappy stems. Usually the
disease appears in two distinct attacks. One occurs in
spring, shortly after the leaves have unfolded, while the
second occurs towards the beginning of July. The
former attack injures the plant, but does nothing towards
producing resistant bodies capable of perpetuating the
disease over the winter. These are, however, produced
during the second attack, which is therefore the more
serious. Some roses are more susceptible to mildew than
others, but in every case moist, crowded conditions are
most favourable to the disease. Careful attention to
temperature and ventilation are therefore necessary.
Dusting with flowers of sulphur at intervals of ten days
is generally sufficient to control the disease. Liver oi
sulphur and ammoniacal copper carbonate are also
effective, while vaporizing sulphur in the houses for two
hours twice a week is also recommended.

Rose Leaf Blotch.—This disease, caused by Actinonema
rose Lib., is second only to powdery mildew in
destructiveness and frequency. More or less irregular
purplish spots with a characteristic fringed border are

112 DISEASES OF GLASSHOUSE PLANTS

produced on the upper surface of the leaves, varying in
diameter from an eighth of an inch to patches covering
half the leaf. (Fig. 32). The tissue round the spots turns
yellow, and, as the spots become old, minute dark pycnidia
or fruiting bodies are distributed over the surface. Ulti-
mately the leaves fall, and as the diseased plants lose
their foliage they become weakened and stunted blooms
are produced.

On the diseased areas conidia are produced in great
abundance during the sum-
mer and autumn months,
and in consequence any
new foliage is quickly in-
fected. Infected leaves
may remain green through
the winter and so carry the
infection over to the next
season. Mrs. Alcock (1)
has shown also that in this
country the fungus may
hibernate on the young
, wood of the previous
season. On such parts she
found discoloured areas,
which on examination
proved to contain abund-

ant mycelium, and compact
Fie. 32. Rose leaf blotch: (a) Diseased b ‘
leaf, (b) spore cluster, (¢) spores. fungal masses bearing in-
numerable spores of the leaf blotch parasite.

This discovery is of considerable importance as
affecting methods of control for this disease. Not only
must all infected leaves be removed from the tree or
collected from the ground at the base and burned, but
attention must be paid to the fungal patches occurring
on the new wood. ‘These should be removed by pruning,
but the operation must be conducted judiciously, in
accordance with the requirements of the variety. So
far as is known, the old pustules on two-year-old wood

DISEASES DUE TO FUNGI 113

do not bear spores, and therefore attention need be paid
only to the wood of the previous season.

To be effective, spraying or dusting should be con-
ducted early in the season, before the fungus has gained
entrance to the leaves. In America repeated dusting
with a powder composed of 90 parts finely ground sulphur
and 10 parts powdered arsenate of lead has proved
effective in controlling the disease.

Spraying with Bordeaux Mixture or lime sulphur
(1 in 50) has also proved effective, but both these sprays
disfigured the trees, and therefore they can only be
employed before the blossoming period begins.

Downy Mildew of the Sweet-pea.—This is practically
the only important leaf disease of the sweet-pea under
glass. It is caused by Peronospora trifoliorum De Bary.
The disease first appears when the young plants are
barely three inches high, but it is not uncommon to see
older plants affected. As in the case of the rose, but
little is seen of the causal organism, typical signs of the
disease being the yellowing and dying of the foliage.
The delicate fungal filaments may be seen readily,
when diseased leaves are submitted to microscopical
examination. Under exceptionally moist conditions,
however, a delicate grey mould develops. Attention
to cultural conditions constitutes the best way of con-
trolling the disease. All diseased parts of plants should
be at once destroyed to prevent further spreading of the
trouble.

(C) FRUIT DISEASES

“ Buckeye” Rot of Tomato Fruits.——This is mainly
a disease of the first or lowest truss of fruit of the tomato,
but it has been found occasionally on the second, where it
has come into contact with a diseased truss below. It is
known by different names, such as ‘ bad-penny,”’
“black rot,” “ water rot’”’ or “ buckeye rot.” Affected
8

114 DISEASES OF GLASSHOUSE PLANTS

fruit may be recognized by the appearance of discoloured
patches, varying from grey to reddish-brown in colour,
frequently arranged in alternating zones of colour so
that the whole lesion resembles the eye of a large animal
(Fig. 33). Thus the term “ buckeye rot” aptly describes
the appearance of the disease.

The main cause of the disease is a fungus, Phytoph-
thora parasitica Dastur., which is synonymous with
Ph. terrestria Sherbakoff, but occasionally Ph. cryptogea
Pethybridge and Lafferty has been found as the cause.

Fruits become infected by touching the soil in which
the disease organisms live, or, on the other hand, careless
watering may splash infected particles of soil on to the
truss as it hangs near the ground. The infection is held
in a film of water between the fruits, and readily attacks
them. The cold, moist conditions operative in the
neighbourhood of the bottom trusses are especially |
favourable to this disease, and if the fruit becomes
infected the disease makes rapid progress and spreads
from fruit to fruit through the entire truss. Finally
the fungus may work back along the truss and attack
the main stem, when the entire plant dies rapidly. The
diseased fruits remain quite hard and firm except when
bacteria are present, when a soft watery rot ensues.
Under moist conditions the fungus grows out into the
air and the fruits become covered with a dense white
fungal growth. Innumerable spores are produced and
the infection spreads rapidly in the process of watering.

The disease may be prevented by early mulching
with straw to cover up the infection and prevent it being
splashed on to the fruit. In this respect, however,
the mulch must not be applied until the soil has become
properly warmed, as the mulch, placed over cold soil,
prevents it from becoming warm, and so the rate of growth
of the plant is much reduced. Where the disease has
been abundant in previous years the soil may be watered
after planting with a solution of ‘‘ Cheshunt Compound,”
applied at the rate of four to eight pints per square yard.

Fie. 33. ‘* Buckeye ”’ rot of tomato fruits.

Fig. 33a. ‘* Foot rot” of the melon due to Bacillus carotovorus : (a) inoculated
plant, (6) control. [facing page 114

DISEASES DUE TO FUNGI 115

Spraying the bottom trusses and soil with this compound
may also be performed if the disease becomes serious.
Trusses touching the soil should be staked or tied up
- go as to keep them as far off the ground as possible.
It is also advisable to remove some of the bottom leaves
from all plants to allow the air to circulate freely and so
keep the fruit as dry as possible. All diseased fruit
should be removed at once and burned.

As the infection may be introduced by a contaminated
water supply, only clean water should be used.

Rhizoctonia Fruit Rot of the Tomato.—This rot, caused
by Rhizoctonia solani, is similar in appearance to “‘ Buck-
eye rot,” but the zone lines are much closer together,
and the affected portions are sunken. It is by no means
as common as “buckeye rot,” and can be controlled
by similar methods.

Botrytis Rot of Tomato Fruits—Towards the end of
the season, especially in unheated houses, which lie cold
and damp in consequence, a soft rot of green tomato
fruits due to a species of Botrytis is frequently found.
Such fruits become soft water-bags, which for a time show
no signs of fungal growth and no apparent wound through
which infection has taken place. The first sign of the
disease is the appearance of a pale, water-soaked area,
which is not confined to any particular part of the
surface, although frequently it occurs at the blossom
end, and indicates a probable infection through the
style in such cases. The affected area increases in size
until the whole fruit is involved, and ultimately the fungus
grows out into the air, when the fruits become covered
with a typical grey, velvety mould of Botrytis. Not
infrequently infection takes place at the calyx end,
when the ensuing rot causes the fruit to fall to the ground.

Infection is easily produced by means of wounds,
but the fact that a diseased fruit will infect an adjacent
healthy fruit which it touches, indicates that under

116 DISEASES OF GLASSHOUSE PLANTS

suitable conditions a wound is not necessary for
infection.

The rot may be prevented by careful attention to
the heating and ventilation of the houses, to ensure that _
the fruit do not remain covered with moisture for long
periods.

Careful pruning of the foliage to expose the fruit to
the sun and air is also advisable. It is probable that
sucking and biting insects carry the disease to healthy
fruits, and these should be eliminated as much as possible.

A similar disease of the cucumber appears under
abnormally moist conditions. Infection occurs at the
flower end of the fruit, and an abundant growth of
Botrytis develops, which works rapidly over the entire
fruit. Control measures are the same as for the tomato
disease.

Rhizopus Rot of Tomato Fruits.—A soft rot of minor
importance caused by Rhizopus nigricans Khrenbg. is
found occasionally towards the end of the season.

Fruits may be attacked while still on the plant in
the green condition, and in the first stages of infection
soft, watery patches resembling bruises are noticeable.
This rot develops rapidly, the epidermis bursts, and the
fungus grows out.

The fungal growth, which appears on the outside of
the fruit, is very marked in character, showing as a dense
mass of strong, erect filaments, each of which bears a
tiny black sphere at the end. These tiny spheres burst
and liberate numerous spores whereby infection is spread.

Experiments have shown that the fungus is a wound
parasite and cannot penetrate the uninjured skin of a
healthy tomato. Wounds made by insects, or the
natural splitting of the fruits under unsuitable conditions,
assist the disease, while warm, moist conditions favour
its progress. Diseased fruits should be destroyed at
once, as these become soft and are easily broken, scatter-
ing infection over the plant.

DISEASES DUE TO FUNGI 117

Penicillium and Fusarium Fruit Rots of the Tomato.—
Frequently various species of Penicillium and Fusarium
are found in tomato fruits, showing hard, brown diseased
patches. These have been isolated and shown to produce
a rot of healthy fruits.

In nature, however, attacks by these fungi generally
follow “‘ blossom end” rot which is not due to parasites.
The lesions of this disorder are very attractive to insects,
and it is probable that spores of Penicillium sp. and
Fusarium sp. are introduced into the affected tissues by
this means.

The control of the rot due to these fungi is largely
to be obtained by eliminating the insect carriers.

_ Other Tomato Fruit Rots.—Besides the rots previously
discussed a number of others have been reported in this
country, but as they are of minor importance and exist
mainly on tomatoes grown in the open, it is unnecessary
to describe them. The fungi causing these rots include
species of Phoma, Gleosporium, COolletotrichum, and
Diplodina.

Other fruit rots caused by Macrosporium solani,
M. tomato, and Phytophthora infestans are discussed later.

“ Gummosis”’ of Cucumber Fruits.—This is a common
disease of cucumbers under glass, caused by Cladosporiwm
cucumerinum. Commonly it appears towards the end
of the season, but under favourable conditions it may
appear much earlier. The first sign of the disease is the
appearance of small, sunken spots, mainly on the concave
side of the fruit. These rapidly extend and the skin
ruptures, when a small drop of amber-coloured, gummy
liquid exudes. This hardens and has the appearance
of a small globule of gum-arabic. Finally the fungus
grows out and the whole lesion becomes covered with a
dark olive-green velvety growth bearing numerous spores
(Fig. 34). The lesions frequently crack, exposing the
white flesh.

118 DISEASES OF GLASSHOUSE PLANTS

The malady is most destructive under excessively
moist conditions, and may be checked by drying the
atmosphere by means of suitable ventilation.

Dusting with sulphur powders, accompanied by
efficient ventilation, will generally effect a control. In

Fia. 34. Gummosis of the cucumber caused by Cladosporium cucumerinum: (a) Leaf
lesions, (b) fruit lesions, (ce) hyphx and spores, (d) spores.

bad cases spraying with liver of sulphur and flour paste
is recommended, and in any case diseased fruits should
be removed and burned.

GENERAL SURFACE DISEASES

Potato Blight of Tomatoes.—The disease of tomatoes
caused by Phytophthora infestans (Mont.) De Bary,
universally known as the cause of the late blight of the
potato, rarely attacks tomatoes grown in glasshouses
in this country. It is, however, a common disease of
outdoor tomatoes, and one may successfully predict its
appearance if abundant rain falls in July or August.
Indeed, so serious is this disease in wet seasons that the
outside tomato crop is frequently ruined, unless regular

DISEASES DUE TO FUNGI 119

spraying with Bordeaux or Burgundy Mixture is adopted.
Thus under cool, moist conditions the disease assumes
considerable importance. The first symptoms of the
disease is the appearance of black, water-soaked spots
on the stem and mature leaves. The lesions rapidly
increase in size, causing the death of the leaves and a
rotting of the stem tissues. The malady can easily be
distinguished from other tomato diseases, for the entire
plant appears to be suffering from the effect of frost
and in bad cases turns completely black and dies. Dis-
coloured sunken areas varying from grey to black are pro-
duced on the fruit. Such lesions are mainly produced at
the stem end of the fruit, where the water drips collect.

An examination of the under surface of diseased
leaves reveals a white, downy fungal growth, similar
in appearance to that present on diseased potato leaves.
This is the spore-bearing portion of the fungus, which
enables its rapid spread.

Under normal glasshouse conditions the disease is
unimportant, and where it has appeared a ready control
has been effected by increasing the temperature and
reducing the amount of moisture in the atmosphere.
In old, leaky houses and out of doors continuous spraying
with either Bordeaux or Burgundy Mixture is necessary
to check the disease.

The Macrosporium Disease of the Tomato.—As in the
case of the preceding disease, that due to Macrosporium
solani, which also causes the early blight of the potato,
is confined mainly to outside tomatoes or to those grown
in cold, leaky houses, although it is occasionally found
on the last fruits of the season, when these are picked
green from the plants and laid on benches to ripen.

The fungus attacks the leaves, producing brownish-
black angular spots and blotches, but is chiefly found
on the fruits, where sunken brownish-black areas are
produced chiefly at the stalk end. These lesions enlarge
and become covered by a black, velvety mass of spores.

120 DISEASES OF GLASSHOUSE PLANTS

Under glass this disease is rarely of sufficient im-
portance to warrant special precautions being taken to
combat it, and can be controlled readily by keeping a
normal temperature and regulating the moisture con-
ditions. In the case of outdoor tomatoes spraying with
Bordeaux or Burgundy Mixture may be necessary to
effect a control.

“* Nailhead”’? Spot of Tomatoes.—Another Macro-
sporium disease of the tomato, caused by Macrosporiuwm
tomato Cke., is that commonly known as “ Nailhead
Spot.” This disease, while common in America, is
unknown as yet in this country. It receives its name
from the typical more or less circular, reddish-brown
spots, resembling the head of a nail, which occur on the
leaves, fruit, and stems of tomato plants suffering from
the disease. Sometimes the spots possess peculiar ridged
lines which give them a target-like effect. Spraying
with Bordeaux Mixture is recommended as a means of
control.

CHAPTER VI

DISEASES DUE TO BACTERIA

ANOTHER class of micro-organisms which cause plant
diseases are the bacteria. Plant bacteria are generally
rod-like in form (Fig. 35), varying from one thirty-
thousandth to one ten-thousandth part of an inch in
length, and about a third of that in diameter. They
multiply by dividing across the middle, each half
becoming a separate bacterium.
This process takes place so fre- gts
quently that in a short time
many millions of bacteria are pr See by:
produced from one original
parent. Certain bacteria may
divide once every’ twenty
minutes, and at this rate the
progeny of one bacterium
would exceed sixty-four thou-
sand million in twelve hours,
were all the ofispring to Fie. 35. Typical appearance of
remain alive. Most disease- bacteria pathogenic to plants.
producing bacteria possess a number of tiny “ tails” or
flagella, by means of which they are able to swim in water.
Some types of bacteria produce within their bodies
hard, resting spores, capable of withstanding abnormal
conditions, but bacteria causing diseases of plants are
not known to do this.

Bacteria are carried from plant to plant by wind,
rain, insects, and animals, including man. Once on the
plant they may enter the inner tissues by means of the

stomata, or by wounds, and in this process they are
12!

122 DISEASES OF GLASSHOUSE PLANTS

assisted by a film of water on the surface of the plant.
Unless they gain a speedy admittance the rays of the sun
and dryness of the atmosphere will soon kill them.
Frequently bacteria are carried on the mouthpieces or
suckers of insects, and by these are placed in direct
contact with wounded tissues, into which they rapidly
spread.

Once inside the plant tissues the bacteria feed and
rapidly multiply, producing millions of new organisms
in an incredibly short space of time.

Wilt Disease of the Cucumber

The symptoms of the bacterial wilt disease of the
cucumber are very similar to those produced by fungi.
Plants moderately resistant to the disease, or those in
very favourable conditions, may wilt during the day and
recover at night, and such plants are frequently stunted
in comparison to their healthy neighbours. As a general
rule, however, cucumbers are so susceptible to the
disease that death is exceedingly rapid, and usually
occurs within two or three weeks after the appearance
of first symptoms. A yellowing of the foliage may
accompany the wilting, but generally this is not so, the
leaf-laminz wilting while still green. This takes place
quite suddenly, and the plant presents a curious appear-
ance of having its leaf surfaces wilted and hanging limp
from turgid petioles and stems. The leaves soon wither
up and die, being followed in this respect by the petioles
and stems, until the whole plant hangs limp. Frequently
no destruction of the root is evident, but upon cutting
across a diseased stem a white bacterial ooze emerges
from the cut ends of the vascular bundles. This is so
sticky that if the finger is pressed against the cut surface
and slowly taken away the bacterial ooze sticks and may
be drawn out in fine cobweb-like strings. The oozing
of the bacterial mass is most evident if cut pieces of
stem are placed with one end in a basin of water under

DISEASES DUE TO BACTERIA 123

a glass jar. Microscopic examination of diseased stems
shows the large vessels of the wood to be completely
filled with an incalculable number of bacteria, and the
wilting is thus brought about by the choking of the
conducting system.

The causal organism was first described by Dr. E. F.
Smith (46), who named it Bacillus tracheiphilus.

The disease is by no means common in this country,
but spasmodic outbreaks have occurred with serious
results. In America the disease is widespread, and
attacks cantaloupes, cucumbers, pumpkins, and squashes,
but not water melons, the cucumber being by far the most
susceptible of these plants. The disease has been found
to occur in varying intensity from season to season, at
times only an occasional wilted plant being reported,
while at others 70 to 95 per cent of the crop has been
destroyed. Rands and Enlows (41) found that the
percentage infection and the rate of progress of the
disease were not determined by weather conditions,
but were related intimately to the vigour of the plant
and to the prevalence of cucumber beetles present at
the time. They demonstrated that the bacteria do not
winter in the soil in America, and found no evidence
that they were carried by the seed. Infection does not
take place through the stomata or breathing pores of
the plant, but depends upon the introduction of the
causal bacteria into the inside of the plant by some
suitable agency, which might well be of the nature of
an insect.

In America the striped cucumber beetle (Diabrotica
vittata) and the twelve-spotted cucumber beetle (D.
duodecimpunctata) have been shown to carry the disease
from plant to plant during the growing season, and are
probably the only means by which the disease is spread
under natural conditions. Under glasshouse conditions
in this country it is probable that the cucumber wood-
louse is instrumental in disseminating the disease. It
has invariably been found that plants which have received

124 DISEASES OF GLASSHOUSE PLANTS

a check either by reason of a prolonged spell of cold
damp weather or some fault in crop management are the
most susceptible to the disease, but otherwise temperature
or shade have apparently no determining effect upon the
incidence or progress of the disease. Control measures,
thus, must be related to the growing of plants physio-
logically robust, and to the control of biting insects
which transmit the disease from plant to plant. The
bacteria are readily carried on the hands and tools of
the workers. After cutting a diseased plant, tools
should be sterilized by wiping ona rag dipped in a two
per cent solution of lysol or other disinfectant. Bacterial
wilt is most serious at low temperatures, and raising the
temperature of the houses until the average of day and
night approximates to 90° F. has been found to check
the disease.

Foot Rot of the Cucumber and Melon

A common disease of melons and cucumbers is that
which is commonly called ‘canker’ by the practical
man. It appears at the soil level, and is typically a
rotting of the outer tissues of the stem at this part.
These outer tissues soften, turn brown, and a rot begins,
which spreads down into the roots as well as upwards
along the main stem. Mainly it is confined to a region not
more than six inches above soil level. The rot extends
deep into the stem tissues and the disease organisms
enter the wood vessels, ultimately causing the death of
the plant (Fig. 33a), Under suitable conditions melons
and cucumbers are so susceptible to the disease that the
whole crop may be imperilled. Usually the disease does
not show until the plants are eight to ten weeks old and
are fruiting, but occasionally quite young plants are
affected.

The causal organism has been isolated and found to
be Bacillus carotovorus, which causes a soft rot of many
plants.

DISEASES DUE TO BACTERIA 125

It has been ascertained that the disease is intimately
connected with moisture conditions operative round the
base of the plant. When the soil is kept uniformly
damp at the surface, the disease may readily be induced
by spraying the base of the plant with a water suspension
of the organism. Ifthesoil is dry at this part the bacteria
may be pricked into the stem tissues, without causing
serious trouble. It is thus apparent that any serious
development of this disease is the result of very wet
conditions of the soil at the stem base. Alternate
periods of dryness and extreme dampness are still more
conducive to the rot, while plants, especially the
cucumber, which are physiologically weakened through
bearing a heavy crop of fruit, are most susceptible.

. The control] of the disease is a simple matter if the
correct conditions are imposed. It is essential to keep
the base of the plant as dry as possible, and the indis-
criminate pouring of water directly on to the stem must
be avoided. The young plants should be planted out
in the beds in metal collars about the size of a forty-eight
pot or else in pots with the bottoms knocked out. When
the plants are watered no water should be allowed
inside the collar, but it should be applied to the bed
around it. The roots readily grow out through the
bottom of the collar into the beds, and thus gain access
to moisture, but the base of the plant remains dry.
Under these conditions the disease makes no progress.
lf the disease has commenced under normal conditions
of cultivation, but has not gone too far, it may be checked
by dusting the base of the plant with a mixture of lime,
copper sulphate, and sulphur. Ten parts of dry slaked
lime, two or three parts of finely ground copper sulphate,
and two or three parts of flowers of sulphur are intimately
mixed and may be applied from a tin box with a
perforated lid, or simply by sprinkling by hand. It is
sometimes necessary to continue the treatment through
the season, and in any case fresh applications should be
made when top-dressing the beds. Growers of melons

126 DISEASES OF GLASSHOUSE PLANTS

need no convincing as to the importance of the disease,
but in the case of the cucumber it is not so obvious. At
the same time those who cultivate the cucumber must
have noticed each year that the base of the stem is the
weak part of the plant towards the end of the season,
and that given a healthy base there is no reason why the
plants should not continue to produce fruit for a longer
period than they do.

Angular Leaf Spot of the Cucumber

This disease, due to Pseudomonas lachrymans Sm.
and Bry. (47), is characterized by brown, angular spots
on the leaves. These begin as dark, watery spots, and
examination in the early morning shows the presence of
a watery bacterial exudate which collects in drops on the
lower surface of the spots. Later, this watery material
dries up and leaves a white chalky deposit. The spots,
which are rarely more than a quarter of an inch in
diameter, dry up and, becoming brittle, the centre falls
out, leaving a ragged hole in the leaf. In consequence,
affected leaves possess a very ragged appearance. The
disease rarely attacks the fruit, but where it does the outer
skin is ruptured and an amber gummy exudate comes
out, which dries up and becomes white. Secondary
infection by soft rot producing organisms frequently
occurs and spreads deep into the centre, which in a
short time becomes soft and watery. This gummosis
should not be confused with that caused by a fungus,
Cladosporium cucumerinum, in which the gummy lesions
become covered with a velvety olive growth.

Young plants have. been observed to be so badly
injured by the disease that they have become much
stunted in growth, and consequently the yield is much
reduced. In this country the disease is rarely serious
in normal, well-conducted nurseries, but in America it
ranks as one of the most important diseases of the
cucumber. The organism has been shown to enter the

DISEASES DUE TO BACTERIA 127

inner tissues of the leaf by means of the stomata, and as
they are open during the day and generally closed at
night, the greater part of infection occurs during the day.
Much the same process takes place on the fruit, and the
small, circular spots turn white in the centre and crack.
The lesions themselves are quite shallow, but the cracking
of the surface opens up wounds for the entrance of other
organisms, and a rapid soft rot frequently occurs. The
disease organisms are disseminated by the workers,
currents of air, overhead damping, and probably by
insects. Conditions of high humidity above 80 per cent,
such as occur during the early morning, are conducive
to the rapid progress of the disease, and under these
conditions the bacterial exudate beneath the spots
becomes abundant. Carsner reports that there is sub-
stantial evidence that the disease may be transmitted by
the seed ; and while the shallow nature of the fruit lesions
would indicate that seed is rarely infected under natural
conditions, the present-day methods of seed extraction en-
courage such infection. No one variety of cucumber has
proved more resistant to the disease than any other.
Control.—The causal organisms have been found upon
the seed, sheltering chiefly in the micropylar opening.
Carsner recommends sterilizing the seed in 1-1,000
mercuric chloride for five minutes as the best method
of control.
_ Inthis country dusting with sulphur powders or spray-
ing with liver of sulphur and flour paste has provided a con-
venient method of checking the rapid spread of the disease.

‘¢ Stripe ’’ Disease of the Tomato

The disease of tomatoes known to the nurseryman
as “stripe ’’ (see Frontispiece) is characterized by brown,
longitudinal markings or stripes on the stem, by shrivelling
of the leaves, and by sunken, irregularly shaped pits
usually of a brown colour on the fruit (36). It is a

specific communicable disease due to a bacillus which

128 DISEASES OF GLASSHOUSE PLANTS

has previously been described as the cause of a very
similar disease of the sweet-pea.

So far little is known concerning the geographical
distribution of the disease, but it is very common on
tomatoes grown under glass in this country and in the
Channel Islands.

In many cases the disease is not of a very serious
nature. By most nurserymen it is regarded rather as a
nuisance than as a disaster, for, with care, a moderate
crop of fruit can be obtained from plants which have been
attacked. At times, however, the disease may be so
prevalent as to ruin the whole crop. Sometimes, during
the first years of a nursery, plants have shown a con-
siderable amount of disease, but this has gradually
diminished until only a small percentage of the plants
has been attacked in the later years. Conversely, cases
are known where “ stripe”’ has appeared and gradually
increased after some ten or twelve years, during which
time no sign of the disease has been observed.

The disease may appear in the seed-boxes, producing
rapid destruction of the young plants and compelling
fresh sowings to be made; it is not uncommon to find
the first symptoms of the disease while the plants are
still in the small pots (sixties), or again after these have
been planted out in the houses for a fortnight or so.
Usually, however, the disease first appears about May,
when the earliest fruit is ready for picking, but frequently
no signs appear until the tops are allowed to develop,
when these often become badly attacked.

- There is a distinct connexion between soft and
rapid growth and the incidence of disease; plants
growing rapidly in the early stages are more liable to
“ stripe’ than others of a hardier nature. In one case
observed at Cheshunt the plants were so badly attacked
that it seemed impossible for the crop to recover. The
conditions were then altered so as to induce a slower
rate of growth, with the result that the plants completely
“orew out” of the “striped” condition, showed per-

DISEASES DUE TO BACTERIA 129

fectly clean tops, and yielded a good crop of sound
fruit.

During previous seasons observations have been made
in the houses at Cheshunt Experimental Station to
ascertain the relation between manurial treatment and
crop management to the incidence of the disease. The
results are indicated below in Tables 8, 9 and 10.

TABLE 8

The Effect of Different Manurial Treatments on the
Incidence of “ Stripe” Disease.

DISHASED PLANTS.

Total
Treatment. No. of 1919.

Variety. Pinus
Dts.

No. |Per Cent.| No. |Per Cent.

Comet 1C.A. without potash 120 | 78 | 65 88
Do. Untreated ee Be 42 57
Do. 1C.A. with dung » | 45] 388 42
Do. 1C.A. without phos-

| phates » | 41] 34 43
Do. 1 AS - 40 33 34
Do. 1C.A. without nitrogen » | o4 28 42
Do. 1Double C.A. > ee 28 28
Kondine | 1C.A. without potash 120 | 83 | 28 80
Red
Do. Untreated oo p80 Foss 40
Do. 1C.A. with dung oe [asf 38 82
Do. 1C.A. without phos-
phates » | 28} 23 26
Do. 1C.A. without nitrogen wee |S 23
Do. 1Double C.A. og ~ ta 12 26
Do. 1C,A. a pee BE 26

These tables show clearly the differences in the
relative susceptibility of the varieties tested, and point
to the selection or breeding of a resistant variety as one
means of controlling the disease. They also show that

1 Complete Artificials.
Q

130 DISEASES OF GLASSHOUSE PLANTS

a soft, rapid growth, such as is produced by liberal
dressings of nitrogenous fertilizers accompanied by high
temperatures, renders the plant more susceptible to
“stripe” than does a hard, slow growth accompanied
by suitable additions of potash. Overhead damping

TABLE 9.
The Effect of * Damping” on the Incidence of the Disease.

DAMPED. Not DAMPED.

Variety.

Total Plants. No. Striped. Total Plants. No. Striped.

{Comet 260 124 260 110
Fillbasket 260 112 260 95
Kondine Red 260 112 260 82
Ailsa Craig 260 95 260 34

TABLE 10.

The Effect of Forced and Slow Growth on the Incidence of
the Disease.

FORCED GROWTH. SLOW GROWTH.

Variety.
Total No. of No. of Diseased Total No. of No. of Diseased
Plants. Piants. Plants. Plants.
Fillbasket 100 44
Comet 100 88 26
Kondine Red 100 24 14
Ailsa Craig 100 16

also favours the disease. These observations are fully
confirmed by inoculation tests carried out on plants
grown in pots. Increased applications of nitrogen were
found to favour the disease, and suitable dressings of
potash to reduce it.

As stated above, the most usual mode of infection
would appear to take place underground, young attacked
plants showing on examination a brown discoloration
of the root cortex. The occurrence of disease in

DISEASES DUE TO BACTERIA 131

the seed-bed suggests that the seed from infected fruit
may also be infected or carry the causal organism
externally, though so far, no actual proof of this has been
obtained. Observations in nurseries and large scale
experiments have shown that the disease may sometimes
spread downwards; successful ‘“ prick” inoculations
have been made on the upper parts of plants and indicate
that insects may produce infection of these parts. It is
also fairly certain that the pruning knife is a potent
factor in spreading the disease. In one house it was
observed that this disease had spread from one end on both
sides of the house, while in another it had spread a certain
distance down one side only. In the former case it was
found that the pruning had been across the house from
left to right, while in the latter the pruning had been
down one side and up the other.

Symptoms of the Disease.-—The stem of an attacked
plant shows the earliest symptoms of disease as light or
dark brown to black, sunken patches of irregular shape,
varying from small spots to long furrows and “ blazes.”
The blazes are often three or more inches long, and
frequently extend over the entire length of an internode,
In slight cases these markings occur intermittently along
the stem, while in bad cases the typical furrows can be
found throughout the whole length of the stem and on
the leaf and truss stalks.

On the leaf the disease first appears as yellow blotches
near the mid-rib and the main veins. Later, these turn
brown and extend so that finally the greater part of the
surface becomes browned and much distorted by the
shrivelling of the diseased areas.

The fruits show light or dark brown sunken patches
with round or irregular outline. They vary from a
few spots developed near the calyx to many scattered
promiscuously over the surface of the fruit. They
greatly reduce the market value, and in bad cases the
fruit is rendered quite unsaleable, being almost covered
with these pock-like depressions.

132 DISEASES OF GLASSHOUSE PLANTS

Attacked plants become very brittle and are easily
damaged by the workers. In the worst cases the
whole plant becomes covered with lesions and finally
dies.

Lesions in the pith and cortex are the characteristic
internal features of the disease ; the walls of the attacked
tissues are strongly browned, so that the patches are
readily seen on splitting the stem with a knife. In older
stems which have become hollow, small brown patches
occur in the remains of the pith and in the cortex, but
at the nodes, where the pith is close and moist, the
patches are much larger.

The roots are often found to be diseased only in the
upper portion, and infection can usually be traced to
some wound or insect puncture just below the ground
level; the tissues of the lower roots are in these cases
white and apparently healthy. In some cases, however,
no wound can be discovered but the cortex is found
to be browned to a considerable depth below the soil
level, and microscopic examination shows the presence
of the bacillus in the tissue. It would seem either that
penetration of the root may occur without the aid of a
wound or that the latter has escaped observation, or,
on the other hand, that the disease has spread down to
the root from an aerial infection.

The disease is due to Bacillus lathyri Manns and
Taubenhaus, described as the cause of “ streak ”’ disease
of the sweet-pea, red clover, soya bean, etc. It has been
shown that the organism from the tomato can cause a
number of “stripe” or “streak” diseases of other
plants. Positive results have been obtained with the
following plants: sweet-pea, garden pea (Fig. 37), red
clover, broad bean, lucerne, lupin, vetch, sainfoin, and
potato. The symptoms of the disease upon these plants
bear a strong resemblance to those on the tomato. The
lesions are in the form of dry brown or black sunken
spots, blotches, or furrows.

The knowledge that “streak” disease of the above-

Fig. 36. ‘* Dieback ”’ of the carnation caused by Fusarium sp.

Fie. 37. ‘‘ Streak ’”’ disease of the pea.
[Facing page 132

DISEASES DUE TO BACTERIA 133

mentioned plants and “stripe” disease of the tomato
are caused by the same organism is of considerable
importance to commercial growers. It is now possible
to explain why certain nurseries have been badly attacked
by “stripe” in the first year of their existence—a fact
which has hitherto been inexplicable. It is highly
probable that the land had previously held a leguminous
flora suffering from “streak” disease which had been
overlooked. Already we have proved this to be true in
a number of nurseries where “stripe”? has just started.
The previous diseased crop was clover in three cases and
vetches in two. It is advisable, therefore, when choosing
a site for a new nursery, to have the advice of a plant
pathologist to ascertain if the existing crop is free from
““ streak” disease.

Control.—As the disease is intimately connected with
soft, rapid growth, induced by an excess of nitrogen and
a deficiency of potash in the soil, manuring with the
latter has proved an efficient method of combat-
ing it.

Affected plants will grow away clean if top-dressed
with sulphate of potash. A suitable dressing is five
hundredweights to the acre which in severe cases may be
repeated after an interval of a month to six weeks.
Generally the most convenient method of treatment is
to dissolve half a pound of sulphate of potash in fifty
gallons of water, and apply the solution at the rate of a
pint per plant to the diseased plants each week until
they show a clean new growth. The cutting away of a
diseased stem and allowing a lateral to develop will
often give a clean plant. As the disease is spread from
plant to plant by means of the pruning knife, workers
should be taught to leave the diseased plants until
all the healthy ones have been attended to, and
before passing to a healthy plant, after pruning a
diseased one, the knife should be cleaned by wiping the
blade with a rag soaked in two per cent lysol or other
disinfectant.

134 DISEASES OF GLASSHOUSE PLANTS

Wilt of the Tomato

The brown rot or wilt of the tomato is a disease
common to many members of the important plant
group, the Solanacee. As is the case in other wilt
diseases, the plant may wilt suddenly while the leaves
are still green, or the leaves may turn yellow and slowly
wither unaccompanied by wilting. Young plants show
the former symptoms and old woody ones the latter.
Finally the stems droop and the softer parts of the plant
collapse and shrivel. On cutting across a diseased
stem the vascular system is seen to be stained brown,
and a greyish-white or brownish-white bacterial slime
oozes out of the bundles. Unlike the cucumber wilt,
this slime is not sticky and cannot be drawn out in
fine strings. The brown discoloration is not confined
entirely to the wood, but may be seen in the pith and
sometimes in the bark. In the young, rapidly growing
shoots the discoloration may frequently be seen through
the more translucent tissues. Under favourable con-
ditions the interior of the pith is reduced to a soft, watery
mass teeming with bacteria. Generally no lesions can
be seen on the exterior of the plant to account for the
wilt, and it is necessary to cut open the stem. The
disease, due to Pseudomonas solanacearum KE. F.S8. has
been submitted to an exhaustive investigation by
Dr. Erwin F. Smith (44), to whom we are indebted for
the larger portion of our knowledge of bacterial diseases
of plants. It attacks a very large number of plants,
among which may be mentioned the potato (Solanum
tuberosum), tobacco (Nicotiana tobacum), egg-plant (So-
lanum melongena), black nightshade (Solanum nigrum),
thorn apple (Datura stramonium), D. metelloides, Physalis
crusstfolia, P. philadelphica, Petuniasp., and Nasturiivumsp.

While there is little doubt that this disease occurs in
England, it is probable that the temperature conditions
are not favourable to it, and no serious case has come
within the author’s experience. It is a serious disease,

DISEASES DUE TO BACTERIA 135

however, in the United States and causes much
destruction annually.

Control.—As there is evidence to show that the
organism enters the plant by means of wounds, every
care should be taken to prevent injury to the roots,
and in this respect the possible effects of attacks by
wireworms, woodlice, and eelworms should be borne in
mind. Excessive moisture, both of the air and soil,
favours the disease and should be avoided. Diseased
plants should be removed and burned immediately they
are noticed. In badly diseased land, sterilization should
be resorted to, and reinfection from susceptible host
plants should be prevented.

Grand Rapid’s Disease of the Tomato

This disease was first described by Dr. E. F. Smith
(45), who attributed it to a micro-organism, Aplanobacter
michiganense EK. F. 8.

It has been confused with the wilt or brown rot of
the tomato, although differing from it in a number of
ways. The symptoms described by Smith are as follows :
The leaflets do not wilt, but slowly shrivel one after
another, and longitudinal cankers are produced on the
stem petioles, through which the bacteria come to the
surface. In consequence there is an abundance of
infective slime on the outside of the diseased plants.
The disease spreads rapidly, and plants may be in-
fected through the stomata, wounds, or broken roots.
There are also indications that it may be seed-borne.
The tissues within the plant become disorganized and -
brown, and the entire pith may be hollowed. The
disease has not yet been reported in this country.

A Tomato Canker

Miss Doidge (18) has described a disease of the
tomato, to which she applies the term “‘ tomato canker,”

136 DISEASES OF GLASSHOUSE PLANTS

due to Bacterium vesicatorium Doidge. Numerous dark
green, semi-translucent, water-soaked spots appear on
the under surface of the leaves. These increase in size,
until they become 2 mm. in diameter, and may be either
round or irregular in shape. They are slightly sunken
and frequently coalesce to produce discoloured streaks.
Finally the affected portions dry up and break away
from the rest of the leaf. Affected leaves become
twisted and distorted. Similar spots appear on the
calyces, laterals, etc. Cankers are produced on the
stems, especially the older parts, where the tissues have
been injured by friction. These are corky in appearance,
“ raised and roughened with numerous small, longitudinal
cracks.”’ The disease is apparently superficial and does
not penetrate into the wood. On the fruit a “ minute
green or brownish blister is the first indication of
infection.”’ Later the spots become “ hard and scabby
in texture and usually slightly convex.” ‘‘ The epidermis
ruptures in the centre, showing whitish-brown over the
discoloured tissues, like the broken edges of a blister.”
Rarely are the individual scabs more than 5 mm. in
diameter, but by coalescence, large, scabby areas
several centimetres in extent may be produced. The
cracks produced in the scabs permit secondary infection,
and a soft rot may take place. The disease is not con-
sidered serious, but by disfiguring the fruit its market
value is reduced.

Tomato Fruit Diseases

Two of these are attributable to bacteria, namely,
a soit rot and a brown rot.

(a) Soft Rot.—This is typically a disease of green
tomato fruits, and usually appears towards the end of
the season. A pale green, water-soaked spot can be
noticed at first, which enlarges rapidly, while the tissues
beneath collapse and the surface of the fruit becomes
flattened. Finally the entire flesh of the tomato is

DISEASES DUE TO BACTERIA 137

converted into a soft, watery mass, held by the tough
skin. These water bags may hang for a time, but soon
burst and scatter the bacteria-infested contents over
the leaves below. The watery mass is very attrac-
tive to insects, which are instrumental in spreading the
disease.

The disease is caused by the common soft rot-pro-
ducing organism, Bacillus carotovorus Jones.

Investigations undertaken by the author indicated
that the disease may be controlled most efficiently by
limiting insect life about the plants. At one nursery
where the disease attained epidemic proportions a deep
ditch, filled with vegetation and eminently suitable as a
breeding place for insects, ran alongside the houses.
Sucking insects allied to the mosquito group abounded
in the houses, and were observed to feed on the disrupted
rotten tomatoes. The ditch was cleaned out and covered.
in, and with the consequent disappearance of the insects
soft rot of the fruits ceased to spread and finally
disappeared also.

(6) Brown Rot.—This disease may be identified by
the appearance of hard, dark brown circular patches,
smooth at first, but afterwards becoming wrinkled.
Later the tissues become disorganized and a soft rot
results, the fruits becoming mere water bags. This rot,
due to an unnamed bacillus, may be distinguished from
that caused by B. carotovorus by the brown colour which
is assumed by the affected tissues. The methods of
transmission and control are similar to those described
in the disease caused by B. carotovorus. The organism
has frequently been isolated from “‘ blossom end rot”
lesions, which appear to be very attractive to insects,
which cause a secondary infection by introducing the
bacillus in question.

Soft Rot of the Arum
For a great many years cultivators of the arum or

138 DISEASES OF GLASSHOUSE PLANTS

calla lily (Richardia) have been troubled by a disease
which threatened to exterminate their entire stock.

On some nurseries its hold became so strong that it
was impossible to grow arums, and the whole of the stock
had to be burnt. Only after an interval of several
years was it possible to cultivate this plant again. The
disease generally shows itself about January, after the
first lot of blooms has been gathered. First symptoms
consist of a withering of the leaf apex and a desiccation
of the entire margin. The desiccation spreads as blotches
from the margin and the diseased
leaves die prematurely. The plants
come to a standstill, do not readily
produce more flowers, and do not
react to feeding. j

An examination of the roots
shows them to be brown, soft, and
watery. The rot spreads from the
roots back into the pseudo-corm
(Fig. 38), becoming worse as the
season advances. Quite commonly
the foliage appears to recover, but
soon goes back to the diseased ap-
pearance. This recovery corresponds
to the production of clean roots, but
as these are attacked in turn so the
Mia, 3: gacllue carotovorus LOliage suffers.

oo The disease is of bacterial causa-
tion, and isolation has yielded a bacillus indistinguishable
from B. carotovorus Jones. Townsend has also described
a soft rot of the calla in America due to B. aroidew, which
is closely related to B. carotovorus, but our isolations
appear to differ from B. aroidece Towns.

Certain methods of control have been devised which
up to the present have yielded satisfactory results (7).
The general method is as follows: After the plants have
been allowed to “dry off” they are shaken out of the
pots and all soil removed. The adventitious roots are

DISEASES DUE TO BACTERIA 139

scraped off, all decayed parts cut out, and the corms
well scrubbed with water, using a fairly strong brush.
The prepared corms are next steeped, taking care to
keep as much of the foliage out of the liquid as possible,
in a 2 per cent solution of formaldehyde (1 part 40 per
cent formaldehyde in 49 parts of water) for four hours.
After this time they are removed and immediately potted
up in disease-free soil and sterilized pots.

The treated corms rapidly produce a supply of clean,
white roots, and have every appearance of health, while
the untreated controls quickly show all the symptoms of
the disease. This method has been tested on a large
scale at a commercial nursery, and so far the plants
have remained healthy.

_ The chance of secondary infection in nurseries where
the disease has been epidemic is exceedingly great, and
special care must be taken to prevent it. Disease-free
soil and pots are essential, and the treated corms must
not be replaced in infected houses unless these have been
sterilized previously. Should the well from which the
water supply is drawn be a shallow surface one with
faulty brickwork, there is every possibility that it has
been infected by surface drainage. Expert advice should
be obtained on this point, for a clean water supply is
imperative.

CHAPTER VII

MOSAIC DISEASES

In previous chapters an account has been given of two
big groups of diseases caused by fungi and bacteria
respectively. Considerable progress has been made in
the study of these, but there is a third large group of
diseases the cause and nature of which, although studied
by investigators in all parts of the world, remains largely
unknown. This is the group of Mosaic Diseases, or
Virus Diseases, as they have latterly been named.
Of profound interest, and of great and perhaps in-
creasing economic importance, the solution of their
main problems would probably constitute one of the
most important events in the study of plant diseases of
modern times.

The name “ Mosaic Disease” (Mosaikkrankheit) was
first applied by Mayer (33) in 1886 to a disease of the
tobacco in which the leaves assumed a mottled, dis-
torted appearance. To-day the name is applied to several
diseases the cause of which is unknown, but which all
possess two very definite characters. The first is a
mottling of the plant, in which light yellow patches
alternate with others of a dark green colour. Also
the various parts of the plant may be distorted, giving
rise to blistered, irregular leaves and flowers. The
second character is the highly infectious nature of the
plant juices, even after filtration through porcelain
vessels,

Practically one hundred different species belonging
to widely separated genera are known to be susceptible

140

MOSAIC DISEASES 141

hosts, and as these include many of the food plants of
the world, the economic importance of the disease cannot
be over-estimated. Growers of glasshouse crops are
mainly interested in the fact that tomatoes and cucumbers
are susceptible to this disease, but it must be very
clearly realized that infection may be introduced from
diseased plants of other species, such as certain common
hedge and field weeds, and therefore that a wide know-
ledge of the disease as a whole must be obtained if
complete control is to be effected.

General Symptoms of the Disease

The chief symptoms of mosaic disease consist of a
mottling and abnormal development of the leaves,
flowers, fruit, etc. While diseased plants generally
possess both symptoms, they may have one or the
other separately, or indeed both may appear on the
same plant at different times in accordance with the
conditions to which it is exposed.

Mottling—Mottled leaves develop irregular, pale
green or yellow patches alternating with patches of dark
green. The mottling may be so slight as to be almost
indistinguishable, or it may be strikingly evident. Under
certain conditions the light areas turn brown and wither,
but frequently the tissues remain alive. Mottling of
stems, flowers, and fruits also occurs.

Abnormality—This may include curling, blistering,
and distortion of the leaves, flowers, and fruits, and a
dwarfing of the stem. Curling of the margin of the leaf
is a common occurrence in the case of the clover,
cucumber, lettuce, potato, raspberry, tobacco, and
tomato. Where blistering occurs, small, raised islands
or blisters of dark green show up in striking relief to
the rest of the surface, which is flat and light green
in colour. ‘this type of symptom is especially notice-
able in the case of the cucumber, petunia, tobacco, and
tomato. —

142 DISEASES OF GLASSHOUSE PLANTS

Distortion.-—The effect of the disease upon leaf growth
also shows itself by a reduction in size of the leaf blade,
with a consequent alteration in the position of the veins,
and the assumption by the edge of the leaf of a fantastic
form. This is especially noticeable in the case of the
tomato, where filiform and fern-like leaves are common.
Among growers, plants affected in this way are said to
be suffering from “‘ sweet-pea disease,” because the leaf
blades are often reduced to such an extent that only
the mid-ribs remain, and the resemblance to sweet-pea
tendrils is obvious.

Similar effects have been noticed in the case of the
petunia and tobacco. Distortion of the symmetry of
the leaf is a common symptom, produced by retarded
growth at certain parts of the margin. In some cases
as many as 43 per cent of the tomato flowers have been
observed to fall from diseased plants, the fall from healthy
plants under the same conditions being 2 per cent.
In other cases abnormal fruits have been observed, and
a considerable reduction in the number of viable seeds
in each fruit.

Other symptoms connected with mosaic disease in
general are a dwarfing of the plants and a general paling
of the foliage. Many investigators have examined the
roots of diseased plants, but have been unable to
determine any definite root symptoms.

Symptoms of Tomato Mosaic Disease

The appearance of tomato plants infected with mosaic
disease varies considerably, according to infection having
taken place in the early or late stages. Plants infected
when quite young develop typical symptoms in all parts.
They are generally stunted in growth and paler in colour
than normal plants. Those infected later in life show
typical symptoms only on those parts which develop
after infection.

In respect of the foliage, five main types of symptoms

FIG. 39. Mosaic disease of the tomato.

Fig. 40. Mosaic disease of the cucumber, showing the distorted leaves.
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MOSAIC DISEASES 143

exist, and may be found singly on individual plants or
together on the same plant. The first type consists of
a simple mottling of the foliage without distortion of any
kind. Pale green areas appear between the veins, and
in process of time these turn yellow, with an indistinct
margin. ‘The second type resembles the first somewhat,
but the spots produced appear very quickly without any
gradual transition from pale green, and are distinct in
outline and deep yellow in colour, like the variegations of
Aucuba Japonica. This type is probably similar to the
aucuba-mosaic of the potato described by Quanjer (39).
The third type shows a distorted leaf margin, but without
blistering or mottling.

In the fourth type blisters are produced on the leaf
surface, and the leaf margin is more or less distorted.
There is no mottling (Fig. 39). The fifth type, or tendril
type, is distinguished by a reduction of the lamina and the
production of fern-like or tendril-like leaves. Mottling is
absent.

While a considerable amount of investigation is
necessary before the full facts are available, it would
appear that these symptoms are manifestations of the
same disease, their appearance being governed by a
number of factors, chief of which are the environmental
conditions under which the host plant is grown and the
degree of resistance exhibited by it. Thus, most varieties
of tomato plants inoculated in February and March
develop distorted, blistered leaves without mottling, but
as the season advances these symptoms are gradually
replaced by mottling and distortion. Towards the end
of the year the new leaves develop blisters and become
distorted, but mottling is rare.

The type of symptom exhibited by the host may
vary with the variety, even when the conditions are
uniform. Thus in the case of Manx Marvel and In-
vincible mottling of the foliage occurs later in the year
than it does in the case of other varieties like Kondine
Red, Blaby, and Comet. The flowers of tomato plants

144 DISEASES OF GLASSHOUSE PLANTS

suffering from mosaic disease are frequently abnormal,
cohesion and twisting of the various parts being fairly
common. The anthers are sometimes quite small and
sterile, and many flowers drop from the pedicels at an
early stage. Some workers claim that the fruits are
frequently mottled, but this symptom has not proved to
be common in this country. The stems are often marked
with streaks of a paler colour, and occasionally small,
thin, brown streaks are present.

The total effect on the plant of all these abnormalities
is to reduce the weight and quality of fruit produced. —
Plants physiologically weak are more severely attacked
than those more robust, but in commercial nurseries
the worst results are seen after the plants have been
‘“‘ stopped.” “Stopping” consists in cutting away the
growing point of the main stem just above the leaf above
the fifth truss, in order to encourage the fruit to mature.
Side shoots are then allowed to develop and produce what
are known as the “tops.” If an exceptionally heavy
crop has been produced by the lower five trusses the
“tops”? may grow away weak and spindly, and if in-
fected with mosaic disease at that stage the effect is
extremely bad. Mottling, distortion, and desiccation
of the leaves follow rapidly and the yield becomes
negligible.

Symptoms of Cucumber Mosaic Disease

Mosaic disease of the cucumber may easily be recog-
nized, and two types have been observed in this country.
One resembles the aucuba type of the potato and tomato,
and is marked by localized yellow patches of distinct
outline. Curling and distortion of the foliage is rare in
this type, which apparently does little damage to the plant.
The second type is characterized by mottling, wrinkling,
and blistering of the leaves and dwarfing of the plant
(Fig. 40). Infection takes place at an early stage and may
lead to complete “‘ blindness’ of the growing point. As

MOSAIC DISEASES 145

a general rule only a small proportion of infected plants
go “blind,” the majority continuing to produce foliage
bearing typical lesions throughout the season. It has
been observed that infected plants may produce normal
foliage after a time, but they cannot be said to have re-
covered because their juices are still infectious. Infected
plants continue to bear fruit during the season, but the
crop is considerably less than that from healthy plants.
The flowers of diseased plants are frequently smaller and
paler in colour than normal flowers, and mottling of the
fruits may occur in advanced stages of the disease.

As in the case of the tomato, the resistance of the host
plant and its environmental conditions are important
factors in determining the severity of the external
symptoms. Thus the variety Butcher’s Disease Resister
grown in this country shows only slight symptoms in
comparison with other varieties commonly cultivated.
The temperature of the soil and air, as will be shown later,
are also important factors influencing the progress of the
disease.

Pathological Anatomy of Diseased Plants

A critical microscopic examination of the tissues of
plants suffering from mosaic disease reveals several
interesting facts. Woods (50) in 1902 was the first to
observe structural differences between the light and dark
green parts of mosaic-infected leaves. He found that
the palisade cells of the lighter areas did not develop
normally but were cuboidal in shape, being only a third
or half as long as the palisade cells from the dark areas.
Also, the cells from the lighter areas contained an abnormal
amount of starch. Recently Dickson (17), working with
rapidly growing leaves of highly infected plants of
different varieties, obtained interesting results. He
found that the thickness of the light areas of the leaf was
in the majority of cases only two-thirds that of the dark

areas, whilst the dark green areas were rarely thicker
10

146 DISEASES OF GLASSHOUSE PLANTS

than normal leaves. He confirmed the previous findings
concerning the abnormality of the palisade cells, and
noted the degeneration of cell contents. Slightly affected
cells differed from healthy ones in that the chloroplasts
were a paler green than usual. Increased effects were
accompanied by a reduction in the number and in the
depth of colour of the chloroplasts, and in severe cases
they coalesced to form irregular green masses. Finally,
the latter broke up into small hyaline bodies, at which
stage the cell contents were observed to move very
rapidly, and he concluded that the rapidity of movement
of the granules was greater than could be accounted for
by ordinary protoplasmic streaming. In the dark green
areas the cells were generally larger than usual and con-
tained more chloroplasts, which were of a darker green
than is normally the case.

The Infectious Nature of the Disease

In certain plants, more especially the tomato, a
mottling or variegation of the leaves frequently occurs,
and this apparently results from malnutrition induced by
unsuitable soil conditions. This state, which is generally
known as “ chlorosis,’ may be confused with true mosaic
if the basis of comparison is solely a macroscopic one, but
the two disorders differ materially in one important
respect, namely, the infectiousness of the plant juices.
The investigations of ITwanowski in 1892 led to the dis-
covery that the extracted juices of a tobacco plant
infected with mosaic disease are capable of infecting a
healthy plant when pricked into the tissues, even after
passage through a Chamberland filter. His results were
confirmed by Beijerinck (11), in 1898.

This fact has now come to be recognized as a critical
character of mosaic disease, and distinguishes it from
non-infectious disorders of the “‘ chlorosis”’ type. True
mosaic disease, therefore, is of an infectious nature, and
is readily spread from plant to plant by natural and

MOSAIC DISEASES 147

artificial agencies. The expressed juices have been care-
fully examined for the presence of a micro-organism
likely to cause the disease, but without success.

In the past many theories have been propounded to
account for the disease. The first suggested that it was
of a bacterial nature, and this was supported by Mayer
(33), Iwanowski (25), Hunger (24), and Boncquet (12).
Iwanowski states that in the cells of tobacco leaves from
a diseased plant he found bacteria, amceba-like bodies,
colourless lamelle, and waxy crystalline deposits.

The enzymatic theory next found favour with
investigators, among whom may be mentioned Woods
(49), Chapman (14), and Freiberg (22). They considered
the cause of the disease to be an excessive development
of oxidizing enzymes, which prevented the production
of the green colouring matter. In this respect the later
work of Allard in 1916 is interesting. He showed that it
is possible to destroy the oxidase by hydrogen peroxide
without destroying the infectiousness of the juice, and
that on destroying the virus without changing the oxidase
no infection is obtained.

The virus theory, which is most generally accepted
to-day, is the outcome of Beijerinck’s theory (8) of a
contagium vivum fluidum. Although he first favoured
the bacterial theory, he later suggested that the infective
principle is soluble in water. The term “ virus”’ as used
to-day includes the existence of an ultra-microscopic
organism or an infective principle of an unknown type
in the expressed juices of an infected plant.

Of late years a new theory—the amceba theory—has
been expounded by Matz (32) and Kunkel (27). In their
published results they suggest the presence of amceba-
like bodies as the cause of mosaic disease of the sugar-cane
and maize.

Many attempts have been made to isolate the
causal organism in pure culture on artificial media
by methods well known to bacteriologists, but without
success,

148 DISEASES OF GLASSHOUSE PLANTS

Properties of the Virus

The infectious virus has been studied critically by
Allard (2) and others, who have made important deduc-
tions. It was found that the virus of tobacco mosaic
retained its infectivity after passing through Berkefeld,
Chamberland, and Kitasato filters, but was non-infec-
tious after filtration through filters with fine pores, such
as the Livingstone atmometer cup or a layer of powdered
talc seven-eighths of an inch in thickness. Thus 91 per
cent of the plants inoculated became infected after the
virus had been passed through a Chamberland filter,
63 per cent when a Berkefeld filter was used, and 40 per
cent with the Kitasato filter. The cucumber mosaic
virus has been found to pass a Berkefeld filter but not a
Chamberland. The virus may be destroyed rapidly by
treatment with 80 per cent alcohol or 4 per cent formalde-
hyde. The virus is destroyed in thirty-one days when
treated with formaldehyde of a strength greater than 1 in —
800. Carbolic acid and mercuric chloride have little
effect on the infectivity of the virus. When heated to
boiling point the infective principle is quickly destroyed,
but lower temperatures are only doubtfully effective.
The virus may be diluted to 1 in 1,000 without losing its
infectivity, and it is claimed that infection has been
obtained with dilutions as low as 1 in 10,000. ‘The juice
has been kept for fifteen months and has retained its
infectivity, although in an advanced state of putrefaction.

Experiments with the virus of cucumber mosaic have
shown it to be less resistant to killing agents than is that
from the tobacco. Heating to a temperature of 70° C. and
treatment with 0-5 per cent solution of copper sulphate,
formaldehyde, or phenol has been found to destroy its in-
fectivity, as has a 0-5 per cent solution of mercuric chloride.

Transmission of the Disease

The researches of previous workers have shown that
once the infectious principle has been introduced into a

MOSAIC DISEASES 149

susceptible host the disease spreads rapidly through the
plant, and juice taken from all parts is found to be in-
fective if introduced into a healthy plant. It has been
shown also that the disease is very easily spread, it being
sufficient to transfer a minute quantity of juice from an
infected plant to the wounded tissues of a healthy one to
render it diseased. The infection has been transmitted
by such delicate operations as cutting the leaf hairs of a
diseased plant with a pair of scissors and then cutting
those of a healthy plant.

Under conditions of cultivation the disease may be
spread by such processes as defoliation, picking, pruning,
“stopping,” and tying. Transmission by means of
insects has been proved to take place in many instances
and is now an accepted fact. In this respect aphides are
_ especially dangerous, and experiments conducted by the
author indicate that the white fly (Alewrodes vapora-
riorum) is an important factor in the spread of mosaic
disease of the tomato under glass.

Experiments with tobacco mosaic were conducted by
Allard to determine if the disease is transmitted in the
soil, and his results lead him to conclude that infection
does not take place in this way, but there is evidence that
the active virus may be carried over the winter in the
roots of susceptible perennials.

The importance of seed transmission has attracted
much attention to this phase of the problem, but except
for isolated cases it has not been proved to occur. Dick-
son (17) claims that seed transmission occurs in the case
of the pea (Pisum sativum), certain clovers (Trifolium
pratense and 7’. hybridum), and the sweet clover (Melilotus
alba). Doolittle has found also that it takes place in the
cucumber. No records of seed transmission are known for
the petunia, potato, tobacco, and tomato.

Cross Inoculations

The question of the host range of any disease is
always important to its control, and experiments with

150 DISEASES OF GLASSHOUSE PLANTS.

mosaic disease have shown that it contains a number of
separate diseases, each of which has a limited host range.
Thus tomato mosaic may readily be transmitted to the
petunia, tobacco, bittersweet, and black nightshade, and
with difficulty to the potato. It is also possible to cross-
inoculate any of these from the others. The mosaic
disease of the cucumber has not been transmitted experi-
mentally to. any of the above plants.

Carrier Plants

Inoculation experiments conducted with large num-
bers of plants have added facts of considerable value to
our knowledge of this disease. It is frequently observed
that a limited number of plants remain apparently
healthy. They do not develop characteristic lesions, and
one might expect them to be healthy. When, however,
the juice from such plants is extracted and pricked into
healthy plants the latter frequently become diseased.
This phenomenon has been examined critically, and it has
been shown that plants may become infected without
showing any outward signs of the disease. In other
words, “‘ carrier” plants exist—a fact which has also
been observed in susceptible weeds, such as the black
nightshade. The identification of sources of infection
thus become increasingly difficult, and their complete
elimination impossible.

The Effect of Environmental Conditions

The influence of environmental conditions upon the
incidence of other disease has proved to be considerable,
and it is not strange to find that the same occurs in the case
of mosaic disease. The temperature of the air is im-
portant in this respect, the development of the symptoms
being retarded by low temperatures and increased by
higher ones, in proportion to the extent to which the rate
of growth is decreased or increased. Thus in the case of

MOSAIC DISEASES 151

the tomato the development of the symptoms is slow
during the spring, when low temperatures prevail and
plant growth is retarded, but rapid during the higher
summer temperatures, when the plant is growing quickly.
Indeed, as noted before (page 143) the character of
mosaic symptoms at these two periods may vary
considerably.

Similar facts have been proved for the tobacco, where
the incubation period for the disease is shortest at
temperatures between 28° C. and 30° C. The optimum
temperature for cucumber mosaic has proved to be 30° C.,
infection failing below 20° C.

Light may also have an effect upon the development
of symptoms, as shown by the researches of Lodewijks
(29) and Chapman (15). The latter showed that when
tobacco plants inoculated with the mosaic virus were
exposed to blue light they developed slight symptoms of
the disease, although the expressed juice was infectious.
Dickson (17) has shown that this effect is due to the
modified conditions of growth occurring in plants exposed
to this light.

The Control of the Disease

Mosaic disease appears to be attaining serious pro-
portions, and its control becomes of great economic
importance.

Four main lines of control may be considered, as
follows :

(1) The determination and elimination of infection
centres.

(2) The determination and elimination of agents by ©
which the disease is transmitted.

(3) The determination of the environmental conditions
which will induce a high resistance in the host plant.

(4) The breeding of immune varieties.

The Determination and Elimination of Infection Centres.
—This involves a knowledge of all host plants of the

152 DISEASES OF GLASSHOUSE PLANTS

particular strain of mosaic. disease to be combated.
Neighbouring weeds as well as cultivated plants must be
taken into consideration. Growers should make a careful
examination of weeds and other plants growing in the
immediate vicinity of the nurseries, and any plants bear-
ing symptoms suggestive of mosaic disease should be dug
up and burned. No suspected plant should be given the
benefit of the doubt. In relation to tomato mosaic it is
now known that certain solanaceous weeds like the
black nightshade (Solanum nigrum) and the bittersweet
(Solanum dulcamara) are susceptible hosts, and should
be eliminated from the neighbourhood of tomato
nurseries, as they may act as centres for the spread of
the disease. Similar dangers exist in the petunia and
potato, and manure harbouring potato tubers should not
be used, as, if these contain the virus of mosaic disease,
infection may spread from the diseased shoots when
these develop. .

Alternative hosts for cucumber mosaic in this country
have not been discovered, and it is somewhat difficult to
understand from whence the infection comes, unless one
accepts the statement which observation on commercial
nurseries seems to support—that the disease is trans-
mitted in the seed. There are no indications that tomato
mosaic is carried in the same way.

In the case of sporadic infection of plants in a nursery
during the early days of the season, care should be taken
to remove immediately the affected plants before the
disease can spread. In the case of mid-season infection,
no useful purpose would appear to be served by removing
infected plants. The difficulty which presents itself
when an attempt is made to effect a control by eliminating
centres of infection is that these are not always easy to
identify when the symptoms are slight, and in the case
of “carrier” plants showing no outward signs of the
disease identification by simple means is impossible.

The Determination and Elimination of Agents by which
the Disease is Spread.—The chief means by which the

MOSAIC DISEASES 153

disease is spread from plant to plant are by insects and
by the workers.

The complete elimination of insect carriers is almost
impossible, but much can be done by reducing the number
to a minimum. The white fly pest of tomato houses
may be effectively controlled by hydrocyanic acid gas or
tetrachlorethane, while aphides, when present, are easily
controllable by many simple insecticidal fumigants.

By careful education of the workers, the rate of spread
of the disease by this means can be reduced. They
should be made to understand that merely crushing a
diseased leaf with their fingers and then bruising a
healthy plant, thus conveying infected juices to it, is
sufficient to transmit the disease. It is imperative in
cultural operations that all diseased plants should be left
alone until the healthy plants have been treated, and in
no case should a healthy plant be touched immediately
after a diseased one. It is claimed that infection is
spread by the clothes rubbing against a diseased plant
and then a healthy one, but this does not appear to
have been confirmed experimentally. After handling a
diseased plant it has been shown that the infection can
be removed from the hands by washing with soap and
water. Pruning knives, after contact with a diseased
plant, should be wiped on a rag moistened with some
disinfectant like lysol.

The Determination of Cultural Conditions necessary to
Increase the Resistance of Susceptible Plants.—As yet but
little information in this respect is available, and investi-
gations should yield important results.

The Breeding of Immune Varieties——Many attempts
have been made to breed varieties immune to this
disease, but while satisfactory results have not been
obtained, there are indications that success will eventually
be forthcoming. In the case of cucumber mosaic the
variety Butcher’s Disease Resister is highly resistant.

CHAPTER VIII

GENERAL REFLECTIONS AND CON SIDERATIONS
ON DISEASE TREATMENT

Soil and Water Sterilization

THE practice of soil sterilization is now an accepted
method of increasing the fertility of infertile soils and of
ridding them of diseases and pests.

When soils are heated, they undergo a process of change,
the extent and form of which varies with the increase of
temperature and length of time it is operative. Even
slight increases of temperature are important as affecting
the functions of roots and the behaviour of soil organisms
as well as the chemical reactions occurring in the soil.
When, however, soils are heated until the temperature
becomes approximately 97° C. (207° F.) important
changes take place and the soil becomes “ partially
sterilized.” Partial sterilization has a beneficial effect
upon soils, which is most marked in infertile or “ sick”
soils. Complex chemical compounds, unavailable as
plant foods, are split up by the heat, and the simple com-
pounds thus set free are either directly available as plant
foods or rendered so in a short time by the bacteria
present in the soil. The high temperature affects the
living organisms of the soil in a differential manner,
those which possess thin walls and are not specially
adapted to resist abnormal conditions being killed, while
others survive in virtue of possessing resistant spores and
other reproductive bodies. Thus the non-sporing bacteria
are destroyed, while those possessing spores which are
resistant to abnormal temperature conditions are able

154

REFLECTIONS ON DISEASE TREATMENT — 155

to survive. The result is a specially clear field of action
for those organisms which remain because of the destruc-
tion of enemies which in normal soils restrict their
development. Among the spore producing bacteria
which remain after partial sterilization are the ammonia
producing organisms, which are thus enabled to spread
rapidly and carry out unrestricted operations. After
partial sterilization there is an immediate drop in the
number of bacteria present in the soils, when there is a
rapid increase which continues for several days, after
which the number decreases slightly and then remains
constant for a considerable period. Similar effects are
shown in the amount of ammonia produced. This ceases
immediately after heating, but increases suddenly, drops
slightly, and remains approximately constant following a
curve similar to that of the bacterial numbers. As
might be expected this increased ammonia production has
a beneficial effect upon plants, as is shown by a com-
parison of those grown in sterilized and unsterilized soil.

Partial sterilization effects a reduction in the extent
of disease, but this varies with the nature of the disease
organisms. The vegetative hyphe and _ thin-walled
spores are readily destroyed, but not so the thick-walled
resistant spores and other bodies which, like the bacterial
spores, are able to withstand such conditions. For the
purpose of eliminating disease the soil must be heated to
a greater extent than is required for increasing fertility.
Generally, however, a temperature of 200-205° F., main-
tained for an hour, is sufficient to eliminate most fungus
diseases. Complete sterilization or over-sterilization is a
possible contingency which must be guarded against, for
when this takes place the soil is rendered unable to main-
tain plant growth until reinoculated with fresh soil and
sufficient time allowed for the introduced organisms to
permeate the original soil and restore fertility.

In practice, the aim of sterilization is to restore soil
fertility and to effect the complete eradication of harmful
soil organisms, including fungi, bacteria, amcebe, insects,

156 DISEASES OF GLASSHOUSE PLANTS

and small animals. It is achieved by steaming or baking
the soil, or treating it with some special chemical com-
pound.

Steam Sterilization

This is effected by continuously passing into the soil
large quantities of steam at a high pressure, until the soil
temperature becomes sufficiently high, when it is main-
tained there for a definite time.

(a) “ Box and Grid” Method.—A trench about six
feet wide and one foot deep is made across one end
of the house, and the soil thus removed is taken to the
other end. The subsoil at the bottom of the trench is
forked up and if necessary a light dressing of lime may
be given. Four boxes, six feet square, or of some size
to suit the position of pipes in the house, and without
bottom or top, are placed in position over the trench,
the house being 27 feet wide. One box fits between the
wall flowpipe and the return on each side of the house,
and two in the middle of the house. The boxes average
5 feet 6 inches square at the top and 6 feet square at the
bottom, the sloping side being about 18 inches in depth.
A “ grid” for distributing the steam is passed under one
side of each box and laid on the subsoil. The grid
resembles a large digging fork, and is made of perforated
hollow pipes (Fig. 41). Two grids are connected, by
means of a steam barrel T-piece fitted with two control
valves, to a length of best flexible hose, which in turn is
connected to the steam barrel leading to the steam
boiler. When the boxes and grids are in position
another trench 6 feet wide and 1 foot deep, adjacent to
the line of boxes, is dug and the soil thrown into the
boxes. It is important here to fill the boxes evenly, and
in no case must the soil be firmer in one part than another,
for steam always takes the line of least resistance, and
the result will be that it will rush through the loose parts

REFLECTIONS ON DISEASE TREATMENT 157

and leave the firm parts untouched. The soil is next
packed well round the sides of the boxes to prevent any
escape of steam. It is always advisable to remove the

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soil used for packing, before lifting off the box, as it is
rarely sterilized, and is a source of contamination. It

158 DISEASES OF GLASSHOUSE PLANTS

should be thrown into the next box for sterilization.
When the boxes are filled they are fitted with lids and
tarpaulins thrown over the top. The steam is supplied
from a portable steam boiler of 12 to 30 horse power
(Fig. 43), and maintained at 60 to 90 lb. pressure per
square inch. Steam is passed through the grids for
_ 30 mins., which under normal conditions is sufficient to
raise the temperature of the soil to 200° F. Any leaks
of steam round the sides of the boxes must be stopped by
packing with earth. The passing of steam is continued
for another 20 to 30 minutes, and the box left on for still
another 30 minutes. To facilitate operations six boxes
and four grids should be employed. With a suitable
head of steam two boxes may be steamed at a time,
while two are being prepared and two previously steamed
are cooling. The grids, however, may be pulled out as
soon as the steam has been turned off. The boxes, being
smaller at the top than the bottom, are easily lifted off
the soil—a process performed by two men with lifting
irons—and the grids being shaped like forks are easily
pulled out from underneath. A house 250 feet by 26 feet
can be steamed in 70 hours continuous working.

(b) ‘‘ Small Grid” Method.—An adaptation of the grid
system has proved useful in some places (Fig. 42). The
apparatus, which resembles a comb, is made of 1-inch
iron tube from 8 to 10 feet long to suit the size of the
plot to be treated, and to fit the spaces between the pipes
and walls, etc. The ‘‘ teeth’ are each 2 feet 6 inches
long, fixed at intervals of 10 inches to 12 inches along the
“back”? of the grid. Holes a quarter of an inch in
diameter are made slightly on the lower side of the
laterals or “‘ teeth,’ and are placed alternately on either
side at intervals of three inches, so that holes on the same
side are six inches apart. The end of each lateral is
sealed, and the last hole should be made exactly on the
under side of the pipe to allow any condensation water
to run out. A short connexion of l-inch iron tube

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160 DISEASES OF GLASSHOUSE PLANTS

bent like the handle of a plasterer’s trowel placed on the
opposite side to the laterals serves to connect the grid
to a length of 14-inch best flexible hose piping fitted to
the main 2-inch steam barrel leading to the boiler. <A
trench 30 inches wide, 6 feet long, and 16 inches deep is
made and the soil wheeled away to the other end of the
bed to be sterilized. The grid is placed in the
bottom of the trench and a second trench is dug similar
to the first, and immediately behind it. The soil from
this trench is thrown into the first and covers the grid.
When the trench has been filled, a tarpaulin is thrown
over the top and steam passed for 10 to 15 minutes. The
grid is then pulled out and back into the empty trench,
which is filled up and steamed as was the first. By
working a pair of these grids one man can be fully
employed.

The success of this method of steaming depends very
considerably upon keeping the pipes and grid free
from condensation water. Any quantity of water lodging
in the pipes will cause failure by checking the proper flow
of the steam. The holes in the laterals, being placed
low down near the soil, prevent any condensation water
from filling the pipe, and the end hole, being made at the
bottom of the tube, allows the water to drain away. When
the actual steaming is done some distance away from the
boiler, and consequently a considerable length of 2-inch
steam barrel has to be used, there is a tendency for con-
densation water to lodge in the pipes. This is overcome
by placing a steam trap at the end of the steam barrel
just in front of the T-piece connected to the hoses.
In the absence of a steam trap a valve slightly opened
can be fitted as shown at Fig. 42 (e). This allows the
condensation water to drain away, but does not affect
appreciably the steam pressure.

(c) ‘ Tray’? Method.—In this method the soil is well
dug one spit deep, so as to assist the penetration of the
steam, and it is important that the soil should be moder-

Fig. 43. Steam Boiler. Fig. 44. Tray method of steaming, showing
trays and pipes ready to place in position.

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Fie. 45. Trays down ready for steaming to begin. [Facing page 160

REFLECTIONS ON DISEASE TREATMENT 161

ately dry. If it is too wet the steam condenses before it
has penetrated very far into the soil, and its latent heat
is lost. Furthermore, it tends to pass quickly through
the soil in certain definite channels, and the correct
general diffusion of steam between the soil particles is
prevented. The result is imperfect sterilization and
money wasted.

In the “tray” system the steam is passed into
inverted shallow trays placed on the soil and from them
diffuses down into it (Fig. 44). The trays are designed
to combine lightness with efficiency, and generally are
made of match-boarding lined with zinc to make them
air tight. A better type of tray can be made of galvan-
ized sheet iron reinforced with ‘“‘ T”’ irons and “ angle”
irons. They vary from 6 to 9 inches in depth and are
made in various sizes suited to the house to be treated,
having regard to the position of pipes, purlins, stays, etc.
The trays are placed on the soil and the steam fed
from suitable pipes introduced at one side. These pipes
are laid in shallow trenches a few inches deep, and serve
to connect one tray with another or with the main steam
supply pipe. The number of trays treated with steam
at one time depends upon their size and the amount of
steam available. Frequently when more than two trays
are employed at a time it is necessary to pass the steam
for a long period. The pipe introducing the steam into
the tray extends to the centre of it, and it is an advantage
to place a brick close to the end to assist in dispersing the
steam as it issues from the pipes. Trays 8 feet by 4 feet
may be steamed in pairs joined by a T-piece, the left half
of the top of the T-piece passing under one tray, the right
half passing under the other, and the pipe being con-
nected to the main pipe. When the pipes and trays are
in position, the latter are pressed down into the soil, well
packed round the edges and pipe junctions with earth
(Fig. 45), and covered with tarpaulins. Steam is then
turned on, and 14 hours is usually necessary before the soil
at a depth of 6 inches is raised to a temperature of 200° F.

II

162 DISEASES OF GLASSHOUSE PLANTS

From observations taken by the author when using trays
(9 feet by 24 feet by 5 inches) in pairs, it was found that
by passing steam at 60 lb. pressure into moderately
heavy soil for 14 hours the temperature } hour after the
steam was cut off was 210° F. at 6 inches depth, 190° F.
at 9 inches depth, and 150° F. at 15 inches depth. The
general practice is to pass the steam for 2 hours, thus
allowing 30 minutes after the desired temperature is
reached, and then to leave the boxes in position for
+ hour after the steam has been shut off. The governing
factor to the man in control must be the actual soil
temperature obtained. This should be not less than
190° F. at a depth of 6 inches.

(d) Tank Method.—For this method a galvanized tank
of about 100 gallons capacity is employed (Fig. 46). A

False Wooden
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false bottom consisting of wooden boards supported on
bricks is placed about 6 inches from the bottom and a
piece of steam barrel fitted into it, so that steam maybe
introduced into the space between the true and the false
bottoms. When the tank is almost full of soil a lid is
fitted and a tarpaulin thrown over the top. Steam is
then passed for half an hour, after which time it is shut
off and the tank emptied. This method has the advan-
tage that every piece of soil is removed and sterilized
and there is little chance of any being missed.

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164 DISEASES OF GLASSHOUSE PLANTS

(e) Drain Pipe Method.—Where trays are unobtain-
able or too expensive to use, steam may be passed
through a buried system of pipes (Fig. 47). For this
purpose 2-inch agricultural tiles may be used. These are
laid at a depth of about 9 inches in rows 15 to 18 inches
apart, each row consisting of 12 tiles, the far end of each
row being stopped up. It is convenient to steam four
rows at a time, the steam being introduced by a hollow
four-branched tube of steam barrel, the branches of
which fit into the four rows to be steamed. It is advisable
to dig the soil first, and in covering the pipes the soil
must be compacted uniformly to enable the proper
diffusion of steam. The surface of the soil should be
covered with a tarpaulin or steam-proof sail-cloth, and
then the steam may be turned on. Four rows of 12 tiles
with 18 inches between the rows will steam 72 square feet
of soil surface. Steam should be passed for 1 hour in
order to heat the soil sufficiently to a depth of 18 inches.
It is advisable not to disturb the soil before it is quite
cold in order to obtain the maximum benefit from the
steam, and so an extra supply of tiles should be obtained
to allow the work to proceed continuously. Two men
can be constantly employed in laying and removing tiles,
making up beds, and attending to the steam boiler for
each set of tiles. It is a saving to employ several sets
of tiles simultaneously.

Surface Sterilization with Hot Water

The ground is first well dug and broken up; and in
this respect a mechanical cultivator is helpful. It is
sufficient to run the cultivator over the ground once, as
by repeating the process the soil is broken up to such a
fine state that it pans badly. Boiling water is applied
freely, and temperatures of 180° F. at 3 inches depth,
140° F. at 6 inches depth, and 120° F. at 9 inches depth
are usually reached.

A special hot water boiler can be obtained to deliver

REFLECTIONS ON DISEASE TREATMENT 165

1,500 gallons of boiling water per hour. It takes 130
hours to treat one acre of land, in the course of which
approximately 200,000 gallons of water are applied.

Deep Sterilization with Hot Water

In this process the normal winter dressing of caustie
lime is first applied to the soil. Drills are next made
across the house and as close together as possible ;
generally they are a foot apart and 6 inches deep. Three
drills are treated at a time, and these are filled with
boiling water three times. During the process extra
heat is provided by the rapid slaking of the lime, which
“‘ explodes ” all over the ground, and, with the repeated
filling with boiling water, produces a temperature of
150° F. at a depth of 9 inches. Three floodings are
sufficient to level the soil and the next three drills are
then filled up three times and so on.

By this method, 180,000 gallons of boiling water are
applied to each acre, or 37 gallons per square yard.
Using a boiler delivering 1,500 gallons of boiling water
per hour, and working 10 hours per day, the process takes
12 days.

The Boiler Tray Method

In this method soil is heated in a shallow tray placed
on the top of an ordinary heating boiler. ‘The tray, made
of galvanized iron, 8 feet long, 3 feet wide, and 1 foot
deep, is made to fit over the top pipes of an ordinary
horticultural tubular boiler. The tiles and brickwork of
the flues, etc., are made to fit round the tray in such a
manner that the boiler may be used for ordinary heating
purposes at the same time. Suitable brickwork is made
to prevent the weight of the tray and soil being borne
solely by the top pipes, as otherwise there is a risk of
causing the ring joints of the castings to leak. Moist
soil to a depth of 12 inches is placed in the trough, which
is then covered with wood and waterproof sheeting.

166 DISEASES OF GLASSHOUSE PLANTS

Each lot of soil is heated for 24 hours, the heat being
evenly distributed throughout the mass by means of the
moisture, and after some time a temperature of 180° F.
is obtained throughout. It is important that the soil
should not be too dry or the heat will be imperfectly
distributed and the bottom layers overheated.

In sterilization by the “tank” and “ boiler tray ”
methods it is easy to over-sterilize. For this reason
these methods should be performed in the summer, at
least three months before using the soil. The soil should
be mixed with clean stable manure and allowed to
recover before using.

The main principles governing the practice of steriliza-
tion by heat have been given, but growers may have
occasion to alter the small details to suit their own
houses and convenience. Thus, in the tray method of
steaming, the size of tray is optional and may be adjusted
to conform to the structure and size of the house. Galvyan-
ized iron is often useful for constructing small trays to
fit into narrow places between pipes and walls, etc.
Modifications in size and shape of grids and combs may |
also be devised to suit individual requirements.

Sterilization by Baking

When sterilizing small quantities of soil for propaga-
tion purposes it is often inconvenient to employ steam.
In such cases the soil may be sterilized by baking. In
this method damp soil is placed in a suitable oven (of
which there are a number on the market) and heated
until the temperature at the interior of the soil mass is
between 205° F, and 210° F. This should be maintained
from 60 to 120 minutes and the soil removed from the
sterilizer to a suitably covered position, where there is no
danger of reinfection.

Our experience has indicated that for satisfactory
work it is necessary to have a factory thermometer fitted
through the side of the oven, with the bulb in the soil

REFLECTIONS ON DISEASE TREATMENT 167

mass, otherwise there is danger of over-heating the soil,
especially if it is too dry. Investigations with regard to
the temperature details are now in progress at Cheshunt
Experimental Station. Even with the crude methods at
present in practice satisfactory results are generally
obtained. In any case, it is advisable to bake the soil
six weeks ahead of using it, and in the interval the heap
should be well worked and watered to bring the soil into
a suitable physical state for cultivation.

Sterilization by means of Chemical Compounds

The researches of Sir John Russell and his colleagues
at Rothamsted Experimental Station have elucidated
many problems of soil sterilization and led to the know-
ledge that soil treatment with chemical compounds pro-
duces partial soil sterilization effects. The intrinsic value
of any method of soil sterilization depends upon the ease
of application, thoroughness of sterilization, and the cost.
Chemical treatment of soils has proved to be slightly
cheaper and easier of application than that of heat treat-
ment by steaming or baking ; but while the compounds at
present on the market have proved successful for certain
purposes, there is need for a more efficient and cheaper
compound than is at present available to the practical man.

Investigations conducted at Rothamsted indicate
that such compounds will be available in the future.
Caustic lime and cresylic acid are the only compounds
used extensively in practice. Heavy dressings of caustic
lime of from four to ten tons per acre are employed in the
glasshouse industry with beneficial results.

Sterilization with Cresylic Acid

Pale straw-coloured “ carbolic”’ acid, composed of a
mixture of ortho-, meta-, and para-cresols of 97 to 99
per cent purity is used, one gallon being poured into a
barrel containing 39 gallons of water. On light soils the

168 DISEASES OF GLASSHOUSE PLANTS

40 gallons of diluted acid is applied to 9 square yards,
but on heavy soils the same quantity is applied to 12
square yards. The original method was to apply the
acid in two half-strength dressings with a fortnight’s
interval, and wash each into the soil by copious watering.
The present plan is to spread the diluted acid over the
soil, so as to saturate the top inch or so, and at once dig
the ground, so as to leave the saturated layer of soil a
foot below the surface. By so doing the vapours per-
meate the top foot of soil and sterilize it.

Probably the best way of applying the acid is to
absorb it in a quantity of dried soil containing gypsum,
which is then well mixed with the soil by digging.
Cresylic acid has proved useful in increasing soil fertility,
but while it reduces certain diseases to some extent it
does not eradicate them completely.

Sterilization with Formaldehyde

Formaldehyde has proved eminently satisfactory for
bringing about the destruction of some disease organisms
in the soil, but the present cost prohibits its extended
use on a large scale. For sterilizing a few tons of soil in-
tended for propagation the excellent results obtained
more than compensate for the expense. A 2 per cent
solution prepared by adding 1 gallon of 40 per cent
formaldehyde to 49 gallons of water will effectively
sterilize most soils. To sterilize each ton of moderately
heavy soil 14 gallons of 40 per cent formaldehyde or 75
gallons of the diluted solution is used. A suitable place
to perform the operation is first chosen, and this should
be fairly flat and if possible protected by an overhead
covering to keep off the rain. The ground should be
beaten flat and damped with formaldehyde. The soil to
be sterilized is laid out in a layer not more than 6 inches
deep and saturated with the diluted formaldehyde.
Another layer of soil is placed on top and saturated in
turn, until the whole of the soil has been treated. The

REFLECTIONS ON DISEASE TREATMENT 169

heap is then covered with tarpaulins or sacks sprayed
with formaldehyde, and left so covered for 48 hours,
after which time the covering is removed and the heap
opened out to dry. This is accelerated by constant
turning, but all instruments should be sterilized with
formaldehyde to prevent reinfection of the heap. When
the soil ceases to smell of formaldehyde it is ready for
use.

Benches, staging, boxes, and pots may also be
sterilized by thoroughly wetting with the solution,
stacking, and covering for 48 hours, and then allowing
to dry. Where stable manure is suspected of carry-
ing infection formaldehyde offers the best method of
sterilizing it.

_ At Rothamsted many chlorine and nitro-derivatives
of benzene and the cresols have been tested with excellent
results. Among these may be noted dichlorcresol,
orthonitrochlorbenzene, and chlordinitrobenzene, but
these have not been tested yet on a commercial scale,
and large quantities are not yet on the market. The
grower may look forward with confidence to the future,
when thoroughly efficient chemical sterilizers will be
available.

Sterilization by Drying

The fact that some soil organisms can resist desicca-
tion to a greater degree than others is responsible for the
fact that the drying of soils produced effects similar to
those of partial sterilization. Under normal conditions
of cultivation, however, it is possible that glasshouse soils
are never sufficiently dry to benefit from this process.

The Effect of Different Methods of Soil Sterilization
upon Plant Growth

Sterilization of the soil results in the liberation of
plant foods previously locked up in unavailable forms and
in an increased production of ammonia. Plant growth,

170 DISEASES OF GLASSHOUSE PLANTS

therefore, is stimulated in much the same way as by the
addition of nitrogenous fertilizers. The extent to which
the plant is stimulated depends upon the method of
sterilization employed.

Heat is the most active in this respect, and liberates
large quantities of plant foods, while chemical sterilizers,
like cresylic acid, liberate considerably less. Again, the
amount of food liberated and ammonia produced varies
with soil type, being small in light sandy soils and large
in heavy soils rich in organic matter. When soils have
been sterilized by heat it is not advisable to give a base
dressing containing nitrogen, but to wait until the plants
develop, when nitrogen may be given as a top-dressing
if necessary. Many a grower has regretted disregarding
this principle when his crop has developed a soft, sappy
growth which could only be corrected by excessive
dressings of potash and phosphates. On some soils,
therefore, sterilization encourages a soft, sappy growth
if accompanied by normal base treatment, and thus the
safest procedure is to give the normal quantity of potash
in the base manure and to omit all nitrogenous manures.
Phosphates may be added at the grower’s discretion.
This applies especially to steam sterilization on all but
the poorest soils, and to cresylic acid sterilization on
heavy, rich soils.

The presence of large quantities of ammonia in the
soil has an inhibiting effect upon plant growth, which
is especially noticeable in young seedlings. Generally,
seeds sown in sterilized soil exhibit a retarded germination
in comparison with those sown in virgin soil. After a
time, however, those grown in sterilized soil forge ahead
and become much better plants than the controls. The
amount of ammonia produced is dependent upon a num-
ber of factors, such as the richness of the soil, method of
sterilizing, and duration of the sterilizing processes. It
is greater in rich soil than in poor soil, in heated soil than
in chemically treated soils, and in soils heated for a long
time than in those heated for a short time. The growth

REFLECTIONS ON DISEASE TREATMENT 171

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172 DISEASES OF GLASSHOUSE PLANTS

inhibiting factors may be removed by thoroughly soaking
the soil with water and allowing it to drain, or by leaving
the soil to weather under cover for three or four weeks
before using. Of all methods employed for sterilizing
the propagating soil that of formaldehyde has proved to
produce the best plants. Occasionally tomato seedlings
raised in baked soil have turned a dark blue-green colour
and refuse to grow. This is generally due to the soil
having been over-sterilized, and a slight top-dressing of
virgin soil is usually sufficient to restore normal con-
ditions in a short time.

The table on p. 171 indicates the effect of different
methods of sterilization upon crop yield, and is drawn up
from the results of experiments conducted on a com-
mercial tomato nursery in the Lea Valley.

The Effect of Sterilization upon Plant Disease

One of the most important functions of the steriliza-
tion of soil is the eradication of the disease organisms
which inhabit it, and numerous investigations have shown
that diseases may be prevented by sterilization by
steaming, baking, or with formaldehyde and other
chemical compounds.

It is necessary to bear in mind, however, that steriliza-
tion to be completely satisfactory must be very
thoroughly done. If patches of untreated soil remain in
corners, under pipes, etc., infection rapidly spreads into
the adjacent sterilized parts, and after a year or so the
whole of the soil may be reinfected. It has been shown
that disease organisms spread more rapidly through
sterilized soil than through virgin soil—a fact which is
easily understood, as the enemies and competitors of the
parasitic fungi, which restrict their growth in normal
soils, have also been destroyed by sterilization.

An experiment performed by the author is instru ctive
in relation to the fact. Two troughs 8 feet long, 18 inches
wide and 2 feet deep were filled, one with steam sterilized

REFLECTIONS ON DISEASE TREATMENT 173

soil and the other with virgin soil, and six tomato plants
were planted at equal distances apart in each trough.
Five pounds of soil copiously infected with Verticillium
albo-airum was introduced at one end of each box, and
the temperature maintained as near as possible to 65° F.
The tomato plant nearest the point of inoculation in the
sterilized soil was the first to show signs of wilt, which
appeared in twenty days. In six weeks all plants in the
sterilized soil, but only two in the virgin soil, were
infected. This indicates clearly that the fungus in
question spreads more rapidly in sterilized soil than in
unsterilized soil, and provides a reason for the fact that
diseased soil may be more heavily infected two or three
years after sterilization than it was before treatment.
In sterilizing soil, therefore, every possible means to
avoid reinfection should be taken. It is a good plan to
sterilize with formaldehyde all brickwork under the
ground and a foot above it, and any soil that cannot be
treated by steam.

Water Sterilization

It has been shown previously (3) that the water supply
of modern nurseries may be so highly contaminated with
disease organisms as to form a continuous and important
source of infection to glasshouse plants. In considering
the elimination of this form of infection the grower is
faced with two alternatives: he may either discard the
contaminated well and construct a new one upon such
lines as to ensure its freedom from pollution, or he may
endeavour to cleanse the 6ld one. The former plan will
appeal to growers as being the most satisfactory in the
end. ‘The best wells are made by boring into the subsoil
to a depth of 100 feet or more, and should be well bricked
in for the top 10 to 20 feet, to prevent the entrance of
surface drainage, by which means contamination enters.
All deep artesian wells have proved practically free from
fungi and should be used wherever possible.

174 DISEASES OF GLASSHOUSE PLANTS

Complete cleansing of a polluted well is by no means
simple. The first thing is to clear away the surface scum
and as much of the growth round the sides as possible.
When the water is thick with decaying plant materials
it may be cleared by precipitation with alum and sodium
carbonate, added at the rate of 18 lb. potash alum and
5 lb. sodium carbonate for every 25,000 gallons of water.
Perchloride of iron at the rate of 9 lb. per 25,000 gallons
of water is also a useful clarifying agent. After precipi-
tating the organic matter in the water the well may be
pumped low and cleaned out as much as possible. When
the well has filled up again the water should be “ chlorin-
ated” to destroy disease organisms, and a convenient
method is to add 50 gallons of “Chloros”’ for each
100,000 gallons of water. After this treatment the well
should be left alone for a week before using. Complete
sterilization of the water can only be obtained by boiling,
and this may be accomplished by arranging the pipes
leading to the tank in such a manner that they are heated
by passing through the boiler. The treated water is
then stored in a tank adjacent to the boiler. While it
may not be so necessary to supply the more mature
plants with disease-free water it is advisable to sterilize
the water used in the stages of propagation when the -
plants are most susceptible to disease.

CHAPTER IX

GENERAL REFLECTIONS AND CONSIDERATIONS
ON DISEASE TREATMENT—Continued

Sprayins and Dusting

THE value of spraying has long been recognized by the
potato and fruit growers, the more progressive of whom
regard it as an essential cultural operation—almost as a
form of insurance. The glasshouse owner, however, has
not yet learned to regard it in the same light, and many
of those who have attempted to spray against fungus
diseases have been disappointed by the results, mainly
because they have not understood the rationale and
function of the process.

Spraying consists of the application of fungicidal and
- insecticidal fluids to the surface of a plant as a protection
against pests causing diseases.

Some fungi and most insects live on the surface of
the leaves and stems and may be destroyed directly by
contact when a suitable spray is applied. On the other
‘hand, the majority of parasitic fungi and bacteria spread
deeply into the plant tissues, and therefore cannot be
touched by spray fluids, which are external. In such
cases the application of a fungicidal spray destroys the -
fruiting parts of the fungus, which are produced on the
outside of the plant, but does not prevent the further
development of the deeply buried parts, which in course
of time are able to grow out into the air and produce new
fruiting bodies.

The true function of a fungicidal spray is protective,
not curative; and realization of this fact will lead the

| 175

176 DISEASES OF GLASSHOUSE PLANTS

grower to a better understanding of the effects of spraying
and perhaps prevent undue disappointment. This pro-
tection is obtained by covering the entire plant surface
with a thin, poisonous coating able to destroy any fungus
spores which may settle on the plant. Spraying efficiéney
is governed by a number of different factors, each of
which must be taken into account if success is to follow.

In the first instance it is necessary to determine
correctly the cause of the disease. This is important
because different organisms are more sensitive to one
poison than to another, and what is a certain cure for
one disease may not affect another in the slightest degree.
Thus spores of Botrytis are extremely sensitive to calcium
bisulphite, being rapidly killed by a two per cent solution,
which is quite innocuous to spores of Cercospora melonis.

Having, however, determined what organism is
causing the trouble, a suitable spray liquid may be
chosen; but it is necessary to remember that complete
protection of the plant tissues depends upon every part
of the surface being covered by the poisonous film.
Some plants are easily covered by watery solutions, but
others, such as those possessing waxy or hairy surfaces,
offer considerable resistance. This is overcome by
adding a “‘ wetting agent’ or “ spreader ”’ to the liquid,
which assists it to wet the entire surface. Further, any
new growth which develops after spraying has been
carried out is of course unprotected, and so at critical
periods the spraying operation must be repeated at
frequent intervals.

Because the work of the spray is to protect the plant
by destroying any fungus spore which happens to fall
on its surface it is necessary to spray only immediately
prior to and at the time when the spores are being shed
and blown about in the air. Spraying at other times,
when spores are not being produced and therefore do not
exist in the air, is merely wasted time, money, and
energy. ‘This being so, a knowledge of the life-history
and methods of growth of the disease organisms is

REFLECTIONS ON DISEASE TREATMENT 177

important, so that spraying may be conducted at the
right time. |

The method of applying the spray is a matter of
considerable importance, for no matter how destructive
to the disease organisms the liquid may be, or how
perfectly it may wet the surface, it is of no avail if not
directed in the proper direction or with the required
force from suitable machines.

It must also be remembered, however, that under
certain conditions the cure may be worse than the
disease, and consequently the effect of the spray upon
the health and growth of the plant must be taken into.
consideration in choosing a spray liquid. In this respect
physical conditions of the environment at the time of
spraying and for some days afterwards are important.
Thus a spray which is perfectly harmless to the plant
under cool, moist conditions may scorch it beyond remedy
when the air is hot and dry.

Fungicides

A fungicide is a chemical compound which is poisonous
to fungi and which, when correctly applied to the surface
of the living plant, serves to protect it from attack by
fungus parasites. A good fungicide should be extremely
poisonous to the pathogen against which it is directed,
but harmless to the plant to which it is applied; it
should completely wet the plant surface and adhere
strongly once it is applied; and finally it should be
cheap and easy to use.

The discovery of Bordeaux Mixture by the Frenchman
Millardet in 1883 may be said to mark the commencement
of fungicidal treatments. Since that time scientific
investigations have produced a large number of prepara-
tions of proved fungicidal value.

Bordeaux Mixture

The discovery of the fungicidal value of Bordeaux

178 DISEASES OF GLASSHOUSE PLANTS

Mixture was entirely the result of an accident. In order
to prevent loss by thieving, the vineyard owners near the
city of Bordeaux were in the habit of sprinkling the vines
near the roads with verdigris to make them look as if
they had been poisoned. Later a mixture of lime and
copper sulphate was used to replace the verdigris on
account of cheapness. Downy mildew, caused by
Peronospora viticola, was introduced accidentally from
America, and it soon became apparent that the vines
sprinkled with lime and copper sulphate were less affected
by the disease than those untreated. Bordeaux Mixture
is prepared by mixing a solution of copper sulphate and
lime in varying proportions in accordance with the
disease and kind of plant to be sprayed.

Standard Bordeaux Mixture is made of 6 lbs. copper
sulphate, 4 lbs. of lime and water to make 50 gallons.
The copper sulphate should be fully 98 per cent purity
and the lime should be freshly burnt stone lime of good
quality. The copper sulphate should be dissolved in
40 gallons of water in a barrel, and the lime slowly
slaked in another vessel. This is best done by adding
as much water as the lime will absorb, but no more. The
remainder of the water, making 10 gallons in all, should
be added gradually to the slaked lime, taking care to stir
thoroughly at the same time. The 10 gallons of lime and
water are then added to the 40 gallons of copper sulphate
solution, stirring all the time. No matter what quantity
of the mixture is being prepared it is necessary to strain
the materials through a sieve, the best type being made
brass wire having 18 to 20 meshes to the inch.
Correctly prepared Bordeaux Mixture possesses a brilliant
sky-blue colour, but the improperly prepared mixture
generally has a greenish colour, and is dangerous to use
as it will injure the foliage. The following precautions
must be observed in preparing the mixture:

(1) Only pure 98 per cent copper sulphate and freshly
burnt stone lime should be used. Air slaked lime is
useless.

o

REFLECTIONS ON DISEASE TREATMENT 179

(2) The copper sulphate solution must not be placed
in vessels of tin, iron, or zinc, as it corrodes them and
loses its strength.

(3) Concentrated solution of copper sulphate and lime
should never be mixed.

(4) The lime and water should be added to the copper
sulphate solution and not vice versa.

(5) Hot water should not be used in the preparation.

(6) The mixture can only be used when freshly
prepared, as it loses its fungicidal powers after standing
12 hours.

(7) Bordeaux Mixture must be strained before trans-
ferring to the spraying machine, and constantly shaken
or stirred during use. |

The action of the lime on the copper sulphate is to

produce compounds called basic copper sulphates, which

possess valuable fungicida] powers. If insufficient lime
is used to combine with all the copper sulphate (which
happens when the lime is not up to standard), a certain
amount of the latter will remain free in the solution and
will scorch the foliage when the solution is applied. It
is important, therefore, to make quite sure that enough
lime has been added and that no free copper sulphate
remains, as shown when the yellow prussiate of
potash test is applied. A 10 per cent solution of
yellow prussiate of potash, which may be secured from
any chemist, is used. If a drop of this solution be
allowed to fall on to the surface of the mixture after
thorough stirring the drop will turn a reddish-brown
colour should any unchanged copper sulphate be present.
If this happens more lime must be stirred in until the
brown colour fails to appear when a drop of the test
solution is added.

As a rule, the normal mixture is used upon plants
with strong foliage grown in the open, but plants under
glass are generally more sensitive, and a modification of
the mixture is used.

For very tender plants 3 lb. of copper sulphate, 6 Ib.

180 DISEASES OF GLASSHOUSE PLANTS

of lime, and 50 gallons of water are employed. This is
called the 3—6—50 formula.

For less delicate plants the 4—4—-50 formula, or 4 lb.
copper sulphate, 4 lb. lime, and 50 gallons of water,
may be used, while on occasions it may be necessary to
employ the 5—5—50 formula.

Burgundy Mixture

This is a modification of Bordeaux Mixture and was
devised for use where good freshly burnt lime is unobtain-
able. In this mixture washing soda is used instead of
lime. The normal mixture is prepared from 4 lb. copper
sulphate, 5 lb. washing soda, and 40 gallons of water.
In practice, the copper sulphate is dissolved in 5 gallons
of water, which is then made up to 35 gallons. The
washing soda is dissolved in 5 gallons of water, and when
solution is complete is added to the 35 gallons copper
sulphate solution, stirrmg thoroughly all the time.
When prepared correctly the mixture is bright blue in
colour, and the fine precipitate it contains remains in
suspension for a considerable time. Jf the colour is
greenish the mixture has not been correctly made,
and if the precipitate settles rapidly it will not adhere
to the foliage. It is important to procure pure
materials, and both copper sulphate and washing soda
should be fully 98 per cent purity. The mixture must
be used fresh and certainly not more than ten hours
after preparation.

On most plants correctly made Burgundy Mixture
has a greater tendency to cause scorch than correctly
made Bordeaux Mixture.

Just as Bordeaux Mixture containing an excess of
lime is less likely to scorch foliage than the normal
mixture, so the same effect is produced in Burgundy
Mixture by an increase in the proportion of washing
soda. In cases where normal Burgundy Mixture has
caused scorch, the 4—6—40 mixture, in which the washing

REFLECTIONS ON DISEASE TREATMENT 181

soda is increased from 5 lb. to 6 lb., should be used.
Both Bordeaux and Burgundy Mixtures have a high
fungicidal value, but when freshly burnt lime is easily
obtained the former is advised.

Ammoniacal Copper Carbonate

One objection to Bordeaux and Burgundy Mixtures
is that they stain the leaves and fruit, and when this is
an important factor, ammoniacal copper carbonate is
employed, for this, upon drying, deposits practically no
stain. This compound, however, possesses less value as
a fungicide, and plants susceptible to injury by Bordeaux
Mixture are more likely to be injured by ammoniacal
copper carbonate. It is prepared from 5 ozs. copper
carbonate, 2 to 3 pints strong ammonia (‘880), and 50
gallons of water. As copper carbonate is more soluble in
dilute ammonia than in the strong solution it is necessary
to dilute it. The copper carbonate should be rubbed
down to a thin paste with a little water, and then 1 pint
of the ammonia diluted to 1 gallon with water, poured
over it. The mixture should be shaken well and then
allowed to settle, when the clear blue, supernatant liquid
is poured off. A further quantity of diluted ammonia
is added to the residue and well shaken, pouring off the
clear liquid after standing. This process is repeated
until the whole of the carbonate is dissolved, but care
should be taken to use no more ammonia than is necessary
to do this. After adding the remainder of the water the
solution is ready for use. It should be used immediately,
for it rapidly loses ammonia and its fungicidal value
becomes reduced.

Sulphur Fungicides

Sulphur in various forms has long been used as a
fungicide. Apart from the pure “ flowers of sulphur,”
which are employed as a “ dust’ and will be considered

182 DISEASES OF GLASSHOUSE PLANTS

later, the best known compound is “ liver of sulphur,”
or potassium sulphide. This is dissolved in water at
the rate of 3 to 10 ounces per 10 gallons of water for
spraying purposes, but it is important to obtain a guaran-
teed pure sample, as some samples have been found to
be highly caustic and cause injury to delicate foliage. |
For general purposes a solution of 4 ounce to 1 gallon of
water will be found most suitable. The solution should
be used immediately it is prepared as it loses value upon
standing.

Lime sulphur prepared from freshly burnt stone lime,
flowers of sulphur, and water has valuable fungicidal
properties. The finished product is a deep amber-
coloured liquid, and while it can be made at home its
preparation requires a considerable amount of skill, and
it is advisable to procure the ready-made article from a
reliable commercial firm. ‘The manufactured article is
sold at different concentrations, which are indicated by
the specific gravities marked on the label. Generally
the specific gravity is 1:3, but it is necessary to know
this, so that suitable dilutions may be made for spraying
purposes. The solution deteriorates upon standing and
should be used immediately.

Of recent years ammonium polysulphide has come
into use as a fungicide as the result of investigations by
Professor Salmon and Dr. Eyre. It has proved extremely
valuable in controlling many diseases, and will probably
have a wide application in the future. Concentrated
ammonium polysulphide is of a dark red colour in bulk,
becoming yellow in dilution. It is extremely pungent
and smells strongly of ammonia. It should be stored in
a cool place in tightly stoppered vessels, and the vapours
should not be inhaled, as they are harmful. The diluted
solution, however, is harmless. The compound can be
obtained in two concentrations, namely, ‘“‘ A.P.S. 1918”
and “‘ A.P.S, 1919,” the latter being approximately twice
as strong as the former.

For spray purposes 1 gallon of “ A.P.S. 1918” or

REFLECTIONS ON DISEASE TREATMENT 183

4 gallon “ A.P.S. 1919” is diluted to 100 gallons by the
addition of water. Saponin at the rate of 2 ounces per
100 gallons of spray should be added, and two sprayings
are necessary at a week’s interval. Solutions of colloidal
sulphur are now on the market and should prove valuable
aids in the combating of plant disease, but at present
little can be said about them.

Spreaders

It is a matter of some difficulty to wet thoroughly
the foliage of some plants if only aqueous solutions
are being used. In order to overcome this difficulty
“* spreaders,” which increase the wetting power of the
spray solution, are added.

The first known spreading agent to be used was soft
soap, but this has now been replaced by other and more
suitable compounds. Of these saponin, calcium caseinate,
flour paste, and certain resin mixtures are the most
important. Saponin at the rate of 2 ounces per 100
gallons may be added to most fungicides with beneficial
results. In the case of Bordeaux and Burgundy Mixtures
it increases the wetting power and helps to keep the
precipitates in suspension for a longer time than would
otherwise be possible.

Certain plants, such as the cucumber and carnation,
are only imperfectly wetted, even with the addition of
saponin, but this may be overcome by using flour paste,
which imparts an efficient wetting power to the solution.
In the Lea Valley it was first used with “ liver of sulphur ”
and ‘lime sulphur” as a means of combating the
Colletotrichum leaf spot of the cucumber, for which the
following sprays are recommended :

Liver of Sulphur and Flour Paste—

5 Ib. flour.
4 1b. potassium sulphide (liver of sulphur).
100 gallons water.
Smaller quantities may be made up at a time by taking

184 DISEASES OF GLASSHOUSE PLANTS

proportional parts of these quantities. Thus for two
gallons of spray the quantities are as follows:

14 ounces flour.

14 ounces potassium sulphide.

2 gallons water.

Two gallons of spray are prepared in the following
way: Fourteen pints of water are placed in a bucket
and 14 ounces of liver of sulphur added, which will
completely dissolve while the flour paste is being prepared.
A very little water is added to 14 ounces of ordinary
wheat flour and the mixture carefully rubbed into a
smooth paste. Water to 2 pints is then added till the
mixture is as thin as milk and free from lumps. This is
next boiled, with constant stirring, until it froths
up, when it is added to the solution of liver of
sulphur and mixed thoroughly. The spray is then.
ready for use.

Lime Sulphur and Flour Paste—

5 lb. flour.
2 pints lime sulphur (specific gravity = 1°3).
100 gallons water.

For 23 gallons of spray the following amounts are

necessary :
2 ounces flour.
1 fluid ounce lime sulphur (specific gravity = 1:3).
23 gallons water.

Two and a half gallons of spray are prepared in the
following manner: Two ounces of flour are mixed and
boiled in 3 pints of water in the manner described above,
and added to 17 pints of water in a bucket. One fluid
ounce of lime sulphur, the specific gravity of which is 1°3,
is then added to the liquid in the bucket and thoroughly
stirred.

The spray compounds described above constitute the
most important fungicides in general use. They have
been widely used and their value has been proved
repeatedly. Other spray compounds have been devised
for special purposes, but as they have not proved of

REFLECTIONS ON DISEASE TREATMENT 185

special value under glasshouse conditions there is no
need to discuss them here.

The Process of Spraying

Having determined the spray compound best suited
to control the disease in question care should be exercised
in its preparation. Only the purest ingredients should
be obtained, and the method of preparation should be
carefully studied and followed to the smallest detail.
Any doubts or difficulties which arise should be discussed
with an expert who is familiar with the details. Hurried
carelessness in preparing the spray compound invariably
leads to disappointing results.

. Spraying should follow immediately the fungicide is
ready. The ultimate aim of the process is the covering
of the entire plant surface with a uniformly thin film of
the fungicide, and in attempting to spray this should
be kept constantly in mind. Sufficient liquid must be
applied to do this, but an excess must be avoided,
otherwise a deposit of irregular blotches will result. To
obtain the best results the fungicides should be applied
to the plants as a very fine mist, and many excellent
spraying machines have been designed for this purpose ;
these are fitted with nozzles especially designed for the
purpose. The amount of pressure used is important, for
while high pressures assist in the production of a fine mist
spray, low pressures with the same nozzle produce a
much coarser spray.

It is thus apparent that an efficient machine must be
employed and used correctly if the process is to be »
successful, All spray fluids must be strained through a
wire sieve before placing in the machine, otherwise
clogging of the pipes and nozzles will result, with loss of
time and patience. Most machines are fitted with
suitable strainers. It is important to clean thoroughly
the machine immediately after using, otherwise its
efficiency will be impaired for the next operation.

186 DISEASES OF GLASSHOUSE PLANTS

In small nurseries a small portable machine holding
about 12 gallons of spray will be found most convenient.
These may be taken into the houses, and a short length
of hose will enable one of the operators to work along
the rows of plants while another works the machine. On
larger nurseries a motor driven machine placed outside
the house is convenient. The spray is delivered through
two or more long hoses taken into the house. To be
efficient the spray must be carefully applied to each
plant, and should be directed towards the lower surfaces
of the leaves before spraying the upper surfaces. If the —
upper surface is sprayed first the leaves tend to drop
and it becomes increasingly difficult to spray the lower
surface. At the best, spraying plants in glasshouses is
a difficult and somewhat unsatisfactory process, because
of the difficulty of reaching every part of the plants
when they are close together and the foliage is dense.
It is of considerable assistance if the foliage can be
suitably thinned out before spraying, especially in the
case of cucumbers and tomatoes.

It must be remembered that spraying does no more
than destroy the fungal parts on the outside of the plant
and prevents further infection through the poisoned
surface. It does not destroy those parts of the fungus
within the tissues which later grow out and produce
new spores. Where plants such as the cucumber are
growing rapidly, new leaf areas unprotected by the
fungicide are continually being produced, and may be
infected by any spore which may settle there. This
state of affairs may only be prevented by spraying at
exceedingly short intervals. As this is undesirable and
may be impracticable, special methods must be adopted.
These consist of spraying and next day removing all
infected leaves. About a week later the spraying and
removal of diseased leaves should be repeated. This is
generally sufficient to check a normal attack, but in
severe cases it may be necessary to repeat the process
again.

REFLECTIONS ON DISEASE TREATMENT 187

The Effect of the Spray on the Plant

One of the properties connected with a satisfactory
fungicide is that it shall be of a composition and strength
which will be non-injurious to the plant to which it is
applied. This, however, cannot always be determined
precisely, for under different meteorological conditions
the injurious action of a fungicide may vary considerably.
Bordeaux Mixture may be perfectly harmless to apple
foliage under one set of conditions and ruinous to it under
different conditions. Generally, the atmospheric con-
ditions of glasshouse cultivation are unfavourable to
fungicidal treatment, and special precautions are neces-
sary to prevent undue damage to the plants. Too strong
a fungicide, or one applied under unfavourable conditions,
produces damage to the foliage, which may vary from a
slight scorching of the tender shoots to the entire
destruction of the leaves and the defoliation of the plant.

In order to reduce to a minimum the possible harmful
effects under glass due observation must be paid to the
following rules:

(1) In the absence of expert advice or previous
experience no new fungicide should be applied to the
main body of plants in a house before a preliminary test
has been made upon a small number.

(2) Spraying should be done in the cool of the evening,
and never before the sun has ceased to shine directly on
the houses or whilst the temperature is above 70° F.

(3) After spraying with copper compounds the foliage
should be kept as dry as possible, as surface moisture
increases the amount of scorching. :

(4) Before spraying cucumber plants with liver of
sulphur, lime sulphur, or ammonium polysulphide, care
should be taken to see that the houses are sufficiently
shaded, otherwise scorching will result. The morning
after spraying with these compounds the ventilators
should be opened to allow the escape of the hydrogen
sulphide given off from the fungicidal compounds.

188 DISEASES OF GLASSHOUSE PLANTS

(5) Cucumbers, tomatoes, and other plants with
heavy masses of foliage should be trimmed before—
spraying to allow the fungicide to be applied thoroughly,
but the spraying must follow immediately after com-
pletion of the trimming to prevent infection from the ©
spores which have been distributed over the leaves by the
disturbances of the foliage.

Soil Fungicides

While spraying the aerial portions of plants for
purposes of disease control has long been in practice,
but little attention has been given in the past to the
treatment of diseased soils in which plants are growing.
The general view has been that any compound capable
of destroying the disease organism would injure the ~
plant at the same time. In consequence, any treatment
directed towards the destruction of disease in the soil
has been applied at a time when no plants were being
grown. Recently, while investigating “ damping off”
of the tomato, a soil drench was devised which, while
causing no injury to the living plant, possesses fungicidal
powers. This compound, which for convenience has
been named “ Cheshunt Compound,” has already been
described on page 61. The treatment was devised to
control “‘ damping off,’ but has given satisfactory results
against other fungal diseases originating in the soil.

Dusting

Of recent years the practice of dusting plants with
dry powdered fungicides and insecticides has been
introduced to replace spraying. The process, which is
largely due to the initiative of Professor H. H. Whetzel,
of Cornell University, is still in its infancy, but the
excellent results obtained have earned for it considerable
praise by both scientific and practical investigators.

To be entirely satisfactory the dust should take the

REFLECTIONS ON DISEASE TREATMENT 189

form of an impalpable powder, for in this state it may be
easily and thoroughly applied. In this country dusting
with flowers of sulphur has been practised for many
years, and recently the so-called “ green sulphur” has
gained many supporters among glasshouse cultivators
because of its ease of application.

Progress along the lines of dusting seems inevitable,
and should lead to the preparation of a dust possessing
high fungicidal powers combined with a sufficiently fine
state to render application very easy.

In America, dry preparations containing lime, sulphur,
and various copper salts have been tested extensively
against liquid fungicides and have compared favourably
with them. So much is this the case that special mechani-
cally driven machines able to emit dense clouds of dust
are employed for dusting on a large scale. In this
country, however, dusting has barely passed the experi-
mental stage, but there is little doubt that its importance
as a factor in disease control will soon be recognized.

Cleansing, Glasshouses

Since it has been shown that certain fungi may live
from season to season in decaying wood, paper, etc., it
is obvious that very careful cleansing methods must be
adopted during the winter in any nursery where disease
has occurred. Fumigation with sulphur has not proved
entirely satisfactory in every case, and it is advisable
after vigorous attacks of disease, or in old houses, to
adopt some method of cleansing the houses with an
emulsion of cresylic acid and soft soap. Such an emulsion
is prepared in the following manner: Pale straw-coloured
cresylic acid 97 to 99 per cent purity and pure potash
soft soap are placed in a bucket at the rate of 1 gallon
of the former and 8 lb. of the latter. The bucket
is then heated over a brisk fire until all the soap is
completely dissolved, the process taking about ten
minutes to complete. The strong emulsion is used at

1909 DISEASES OF GLASSHOUSE PLANTS

the rate of about 1 part in 50 parts of water, i.e., 1 pint
in 6 gallons.

It will mix properly without agitation, it being
sufficient to place it in the tank of the sprayer and run
in the correct quantity of water. The diluted emulsion
is best applied by means of a strong power sprayer, and
should be carefully directed into every part of the
woodwork, soil, etc. Special attention should be Be.
to any rotten part of the structure.

The ventilators should be left open while the spraying
is in progress, so that they may be thoroughly treated,
but must be closed down when the spraying is finished,
in order to retain the strong vapours. The houses should
remain closed for four days after treatment, and then
opened to allow the vapours to escape. Fourteen days
afterwards the house may be planted if necessary. The
operators should wear goggles while spraying, as the
liquid causes the eyes to smart, and rubber gloves should
be worn to protect the hands. In high houses it is
advisable to fit bamboo lances to the hoses, so that the
nozzle may be held close to the woodwork and the spray
forced into the cracks and crevices. One hundred feet
of house, 13 feet wide and 8 feet to the ridge, requires
about 100 gallons of spray, which takes about four hours
toapply. Asa final precaution every cavity in the wood-
work should be filled up with putty and painted over.

Breeding

It is a matter of common observation that among
large colonies of plants exposed to infection from disease
organisms some are completely destroyed, others only
slightly diseased, while an occasional plant remains
unaffected and continues to function in a normal manner.
Such phenomena are attributed to varying degrees of
susceptibility and disease resistance possessed by the
individual plants. Disease resistance in a plant may be
defined, therefore, as the ability to develop normally

REFLECTIONS ON DISEASE TREATMENT 191

when exposed to conditions of infection under which
normal plants of the same variety are unable to function,
Many reasons have been brought forward to account for
disease resistance, but with these the practical man, in
the present state of our knowledge, is unconcerned. It is
sufficient for him to know that such phenomena do exist,
and to realize the great possibilities of disease control
which they open before him. Resistance is variable in
extent—some individuals being slightly resistant, others
completely resistant. Plants belonging to this latter
class are commonly said to be immune. Plants which
are resistant to one disease are not necessarily resistant
to another. This is well known to farmers, who find
that certain varieties of potatoes immune to wart disease
are yet susceptible to blight caused by Phytophthora
infestans. While this is disappointing it should not
prevent us from taking full advantage of the benefits to
be derived from the existence of resistant varieties, even
though these are limited in the scope of their resistance.

All growers of plants, and especially those of glass-
house plants, have long since experienced the difficulties
connected with disease control, and many have voiced
an opinion that the control may be worse than the
disease, for it requires infinite care and patience before
it is effected. Moreover, many diseases are only imper-
fectly controlled by any known means, and the production
of resistant varieties in such cases would solve the
problem satisfactorily. The use of resistant varieties is
becoming more and more important, and in the future
the production of such varieties must become one of the
most important problems of the pathologist and grower.

The chief means by which disease resistant varieties
can be obtained is by selection or by hybridization.

Selection

By this method plants are grown under conditions
which expose them to a considerable amount of infection

192 DISEASES OF GLASSHOUSE PLANTS

from the disease in question. Where the disease is
controlled through the roots they are grown in highly
infected soil, while if the disease attacks the aerial portions
of the plants they may be grown in normal soil and
artificially inoculated by spraying with a suspension of
spores of the disease. Among these plants, the majority
of which become diseased, a few remain healthy and are
selected for seed. From this seed plants are again
grown under similar conditions and healthy plants again
selected. Out of the original number only one or two
healthy plants may be obtained, but after many genera-
tions of selection the entire batch may be found to be
healthy and a strain immune to the particular disease
may be obtained. A necessary detail of the process is
the exposure of each generation of selected plants to
copious infection of the disease to resist which the plants
are being cultivated.

The selection method has been successfully employed
in America, where varieties of cotton, cabbage, and
tomato have been selected which are highly resistant to
wilt disease due to various species of Fusarium.

Recently Egerton (19) has elaborated a system of
selecting in the seed-bed varieties which are resistant to
attacks of Fusarium lycopersici. Selection has proved
to be an easy way of obtaining resistant varieties, but
‘in this country there is need for much investigation in
this line, especially in relation to glasshouse plants.
Practically the only resistant variety of any glasshouse
plants produced in England is Butcher’s Disease Resister
cucumber, which was selected during an epidemic of the
leaf spot disease due to Cercospora melonis, and which
so successfully resisted attacks by the fungus.

Hybridization

This process consists in raising a disease resistant
variety by means of cross-breeding, but is more laborious
and less certain than that of selection. Usually it is

A

REFLECTIONS ON DISEASE TREATMENT 193

resorted to when selection is impossible because the first
slightly resistant plant is unobtainable.

Wilt resistance has been shown by Orton (35) to be of
a heritable character by his creation of a wilt resistant
edible water melon. For this purpose an inedible form
of melon, Citrullus vulgaris, known as citron or stock
melon, which was resistant to wilt, was crossed with the
Eden variety of melon. The first generation resulting
from this cross proved to be extremely vigorous and
productive. From the second generation ten fruits
from 3,000 or 4,000 plants were selected for resistance
and quality, and the seeds from them were planted in
infected soil. Continued selection in this manner resulted
in a resistant variety of good quality fruit.

_ At Cheshunt Experimental Station a similar investiga-
tion is being carried out in an endeavour to produce a
tomato resistant to Verticillium wilt, and of good quality
and productiveness.

For breeding investigations large areas are necessary,
but any expenditure is amply rewarded if a resistant
variety is ultimately obtained.

CONCLUSIONS

THE ultimate end of investigations upon plant diseases
must be an effective control, and in conclusion it is fitting
that the general principles governing disease control
should be recited briefly.

General principles must come under three main
headings : plant hygiene; spraying, dusting, and
sterilizing ; resistant varieties.

Plant Hygiene
(a) The Elimination of Centres of Infection.—This
process has been exploited extensively in the control of
human diseases, and the resultant success may readily
be seen when one compares the death-rate due to disease
during the last war with that during previous wars.
13

194 DISEASES OF GLASSHOUSE PLANTS

Similar benefits result when hygienic methods are applied
to glasshouse work, and it is in the interests of the
grower and the whole country that cleanliness should
prevail in every nursery and market garden. Contami-
nated soil, manure, water, seed, imported plants, and
weeds are important sources of infection, as are also
contaminated buildings, market baskets, workers, visitors,
and insects. It is important to realize this, for in every
case a little knowledge before the event is better than a
good deal more afterwards. Often growers are reluctant
to spend time and money in destroying weeds outside
the houses, but it is always worth the expenditure, and ~
where centres of infection are known to exist no pains
should be spared in their elimination.
(b) Cultural Methods.—The wise grower has learned
by experience, often dearly bought, that plants which
are grown strong and vigorous, without any tendency
to soft, sappy growth, are often resistant to many
diseases. So obvious is this fact to the observant
grower and pathologist that no study of a particular
disease can be complete unless a study of the effect of
different cultural conditions upon it has been made.
The rapid forcing of glasshouse produce so that early
markets and high prices may be captured often ends in
producing plants susceptible to disease, while a healthy
growth, developing uniformly, often produces plants
upon which disease can make no headway. Cultural
details are vastly important, and there is an immediate
need for investigation, so that when a particular disease
appears the best conditions for assisting the plant in its
struggle for health and life are known and can be applied.

Spraying, Dusting, and Sterilization

In cases where the disease has got the upper hand
and cultural means are ineffective or unknown, the above
devices must be resorted to, but they require careful
application, and frequently are difficult to carry out.

REFLECTIONS ON DISEASE TREATMENT 195

All available information should be obtained and every
detail followed minutely.

Resistant Varieties

The employment of such varieties as are highly
resistant to disease is the simplest way of guarding
against it, and where disease is rampant these varieties
should be grown when possible. Unfortunately the
number of resistant varieties of cultivated plants is very
limited, but it is in the interests of the country that no
money should be spared in obtaining a great many more.
The usual criticism against employing resistant varieties
is that such are lacking in quality and productiveness.
This fact must be borne in mind by every hybridist, so
that resistant varieties of high commercial value may
be produced. ;

Finally, a word to pathologists and students of
disease problems, the outcome of much experience in
commercial nurseries. First learn to grow your plants
so that they may pass the most critical examination for
health, quality, and productiveness by practical com-
mercial growers, and then experiment upon the healthy
plant. It is useless to base detailed knowledge of any
disease or the method of its control on experiments
conducted on half-starved, physiologically weak plants
grown in tiny pots.

APPENDIX I

TOMATO DISEASES COMMONLY FOUND IN
ENGLAND

Name

Causal Organism

‘“‘ Damping off ” of seedlings (a) Phytophthora cryptogea

* Foot rot ” or ‘“‘ Blackleg ”
of young plants

Fusarium root rot
Colletotrichum root rot

Verticillium wilt or “sleepy
disease ”’
** Stripe ”

** Mildew ”’ or “‘ leaf mould”
Botrytis stem rot
*“ Buckeye ” rot of the fruit

Fusarium fruit rot
Botrytis fruit rot
Rhizopus fruit rot
Penicillium fruit rot

Pethybridge and Laf-
ferty ;

(b) Phytophthora parasitica
Dastur ;

(c) Rhizoctonia solani Kuhn

(a) Phytophthora cryptogea
Pethybridge and Laf-
ferty ; -_

(b) Phytophthora parasitica
Dastur

Fusarium spp.

Colletotrichum tabificum
Pethybridge

Verticillium albo-atrum
Reinke and Berthold

Bacillus lathyri Manns and
Taubenhaus

Cladosporium fulvum Cke.

Botrytis sp.

Phytophthora parasitica
Dastur

Fusarium spp.

Botrytis sp.

Rhizopus nigricans

Penicillium sp.

197

198 DISEASES OF GLASSHOUSE PLANTS

Name Causal Organism

Bacterial soft rot of the Bacillus carotovorus Jones
fruit

Blossom end rot Physiological causes

Mosaic Gause unknown

Potato disease Phytophthora infestans

(Mont.) De Bary

TOMATO DISEASES FOUND OCCASIONALLY IN

ENGLAND
Fruit rots resembling (a) Phytophthora cryptogea
** Buckeye ” rot Pethybridge and Laf-
ferty
(6) Rhizoctonia solans Kuhn
Sclerotinia stem rot Sclerotinia sclerotuorum
Massee
Fusarium wilt Fusarium lycopersict Sace.
Macrosporium blight Macrosporium solant
Stem canker Diplodina lycopersics
(Cooke), Hollos emend.
Brooks and Searle
Fruits rots (a) Phoma sp.

(6) Gleosporium sp.
(c) Colletotrichum sp.
(d) Diplodina lycopersict

SELECTED BIBLIOGRAPHY
(1) Alcock, N. L., Royal Botanic Gardens, Kew. Bull.

XVIII.

(2) Allard, H. A., 1915. Jour. Agr. Res., 3, 295-299.
Ditto., 1915. Ditto., 5, 251-255.
Ditto., 1916. Ditto., 6, 649-674.
Ditto., 1918. Ditto., 13, 619-637.

(3) Allard, H. A., 1916. Jour. Agr. Res., 6, 649-674.
(4) Atkinson, G. F., 1892. Alabama Exp. Sta. Bull,

41, 19.

(5) Bewley, W. F., 1920. Ann. App. Biol. VI, 2 and 3,
156-172.

(6) Bewley, W. F., 1921. Journ. Min. of Agr., XXVITI,
7, 653-654.

(7) Bewley, W. F., 1921. Cheshunt Exp. and Res. Sta.,
~ 7th Ann. Rep., p. 39.
(8) Bewley, W. F., and Buddin, W., 1921. aia App.
Biol., VIII, 1, 10-19.
(9) Bewley, W. F., 1922. Journ. Min. of Agr., X XIX,
5 and 6.
(10) Bewley, W. F., 1922. Ann. App. Biol., IX, 2,
~ 116-134.
(11) Beijerinck, M. W., 1899. Abstr. in Centralb. Bakt.
Abt. 2, 5, 27-33.
(12) Boncquet, P. A., 1917. Phytopath., 7, 269-289. .
(13) Brooks, F. T., and Searle, G. O., 1921. Trans.
British Mycol. Soc., VII, 3, 173-197.
(14) Chapman, G. H., 1913. Mass. Agr. Exp. Sta. Rep.
25, 94-104. .
(15) Chapman, G. H., 1916. Science N.S. 44, 537-538.
(16) Dastur, J. F., 1913. Mem. Dep. Agr. India, Bot.
Series, V, 4, 177.

199

200 DISEASES OF GLASSHOUSE PLANTS

(17) Dickson, B. T., 1922. Studies Concerning Mosaic
Diseases. Macdonald College, Technical Bulle-
tin No. 2.

(18) Doidge, E. M., 1921. Ann. App. Biol. VII,
4, 407.

(19) Egerton, C. W., 1918. Phytopath. 8, 5-14.

(20) Egerton, C. W.,and Moreland, C. C.,1920. Louisiana
Agr. Exp. Sta. Bull., 174.

(21) Flammarion, Camille, 1898. Exp. Sta. Record, 10,
103-114.

(22) Freiberg, es W., 1917. Ann. Miss. Bot. Gardens, 4,

(23) Halsted, = D., 1892. New Jersey Agr. Exp. Sta.
Rep., 18, 297,

(24) Hunger, F. W. T., 1905. Zeit. Pflanzenkr., 15,
257-311.

(25) Iwanowski, D., 1903. Zeit. Pflanzenkr., 13, 2-41.

(26) Johnson, J., 1921. Phytopath. II, 11, 446.

(27) Kunkel, L. O., 1921. Bull. Exp. Sta. Hawaiian
Sugar Planters’ Assoc., Vol. 3, Part 1, 44-58.

(28) Levin, E., 1916. Michigan Agr. Exp. Sta. Bull., 25.

(29) Lodewijks, J. A.,1910. Rec. Trav. Bot. Neerlandais
7, 107-129.

(30) Masse, G., Diseases of cultivated plants, p. 266.

(31) Masse, G., 1896. Jour. Roy. Hort. Soc., XIX.

(32) Matz, J., 1919. Jour. Dept. Agr. Porto Rico, 3,
65-82.

(33) Mayer, A., 1886. Landw. Versuchs. Sta., 31,
450-467.

(34) Mercer, W. B., 1915. Jour. of Roy. Hort. Soc.,
XLI, 2, 227-229.

(35) Orton, W. A., 1907. Repts. Am. Breed. Assoc., 3.

(36) Paine, S. G., and Bewley, W. F., 1919. Ann. App.
Biol. VIL 2 and 3, 183-202.

(37) Pethybridge, G. H., and Lafferty, H. A., 1919. Sci.
Pro. Roy. Dublin Soc., XV (N.S8.), 35, 487.

(38) Pritchard, F. J., and Porte, W.S., 1921. Jour. Agr.
Res., XXI, 3, 179.

SELECTED BIBLIOGRAPHY 201

(39) Quanjer, H. M., 1922. Report of Internat. Potato

Conference of 1921, Roy. Hort. Soc., London.

(40) Ramsbottom, J. K.,1918. Jour. of Roy. Hort. Soc.,
XLII, 1, 65-78.

(41) Rand, F. V.,and Enlows, E. M. A.,1916. U.S. Dept.
of Agr. Jour. Agr. Res., 6, 417-434.

(42) Rosenbaum, J., 1920. Phytopath., 10, 9, 415.

(43) Sherbakoff, C. D., 1917. Phytopath., VII, 2, 119.

(44) Smith, E. F. Bacteria in relation to plant diseases,
Vol. 3, 174-219.

(45) Smith, E. F. Bacteria in relation to plant diseases,
Vol. 3, 161-165.

(46) Smith, E. F. Centralb. f. Bakt. Abt. 2, Bd. 1, 9-10,
364-375; also Bacteria in relation to plant
diseases, Vol. 2, 209-299.

(47) Smith, E. F. and Bryan, M. K., 1915. Jour. of Agr.
Res., V, 11, 465-476.

(48) Stone, G. ¥., 1913. Massachusetts Agr. Expt. Sta.
Bull., 144, 3-39.

(49) Woods, A. F., 1899. Centralb. Bakt. Abt. 2,
5, 745-754,

(50) Woods, A. F., 1902. U.S. Dept. of Agr. Bureau
Plant Indus. Bull., 18, 1-24.

(51) Vogel, I. H., 1919. Phytopath. 9, 9, 403.

INDEX

A tomato eanker, 135.
Actinonema rose, 111.
Aeroplane type of glasshouse, 22.
Air, changes of, 22

a *

circulation, 22.
movement of, 22.
space, 18.
Aleurodes vaporariorum, 149.
Alger, 49.

Allard, 147, 148. ©

Alternaria brassice, var. Nigrescens, 99.

Amosba-like bodies, 147.

Ammonium earbonate, 61.
polysulphide, 182.

A.P.S. 1919, 182; 1918, 182.

Ammoniacal copper carbonate, 181.

Anchorage, 45.

Angle of root, 18.

Antirrhinum, 84.

Aplanobacter michiganense, 135.

Arum: soft rot of, 138.

Ascospores, 56, 89.

Aspect of houses, 18.

Assimilatory processes, 18.

Associations of glasshouse growers, 24.

Aster, 59.

Bacillus aroidee, 138.
Bacillus carotovorus, 48, 124, 137, 138.
Bacillus lathyri, 46, 132.
hosts of, 132.
Bacillus tracheiphilus, 123.
hosts of, 123.
Bacteria: 121.
ammonia producing, 155.
rapid multiplication of, 121.
resting spores of, 121.
the effect of high temperatures
on, 36.
Bacterium vesicatorium, 136.
**Bad penny,” 113.
Balance of plant foods, 46.
Bars, 18.
Baskets and sacking as sources of
infection, 31.
Beets, 69.
Beijerinck, 146.
Berkefeld filter, 148.
Bittersweet, 150.
Black nightshade, 150.
* Blaok rot,’’ 113.

Blosk, advantages of dividing parti-
tions, 14.

Blossom end rot of the tomato, 15, 41,
42.

effect of ammonium salts on, 42
effect of lime on, 42.
effect of nitrate of soda on, 42.
effect of stable manure on, 42.
Bordeaux Mixture, 100, 104, 107, 113,
119, 178.
preparation of, 178.

Botrytis, 28, 176.

Botrytis foot rot of the tomato, 64.
control of, 65. ;
organism causing, 64.
sources of infection, 65.
symptoms of, 64.

Botrytis on petunias :
effect of shade on, 34.

Botrytis stem rot, 90.

Botrytis rot of bulbs, 71.

Boucquet, 147.

Breeding, 190.

Broadbalk wheat plots, 47.

Brooks and Searle, 89.

‘‘ Buckeye ” rot of tomato fruits, 25,

29, 59, 114.
effects of careless watering on, 25.
in relation to process of watering,25.

Bulb rots, 71.

Bulbs, Botrytis rot of, 71.

Fusarium and Penicillium rots of,
72.
Sclerotium disease of, 71.

Burgundy Mixture, 119.
preparation of, 180.

Cabbages, 69.
Calcium bisulphite, 65, 91, 176.
Calcium caseinate, 183.
Capsicum, 84.
Carbon dioxide, 18.
Carnation :
** Dieback ” of, 107.
leaf mould of, 106.
Macrosporium leaf spot of, 106.
powdery mildew of, 107.
root rot, 71.
rust of; 104.
Septoria leaf spot of, 106.
stigmenose of, 52.

202

Castor bean, 59.
Cercospora apii, 34.

Cercospora disease of the cucumber, 92.

Cercospora melonis, 92, 176, 192.
Chamberland filter, 148.
Chapman, 147, 151.
Chlamydospores, 54, 59.
Cheshunt Compound, 61, 69, 70, 83, 90,
114, 188.
Chlordinitrobenzene, 169.
Chlorophyll, 18.
Chlorosis, 52.
relation of lime to, 49.
in waterlogged soils, 40.
Chrysanthemum :
leaf blight of, 108.
leaf spot of, 109.
powdery mildew of, 109.
rust of, 107.
Citrullus vulgaris, 193.
Cineraria, 59.
Cladosporium cucumerinum,
126.
Cladosporium fulvum, 14, 100.
effect of slope of houses on, 14.
Coleus, 21.
Colletotrichum leaf spot of cucumber, 96,
97.
cleansing houses after, 96.
conditions favourable to, 98.
sources of infection, 95.
spraying against, 96.
Colletotrichum oligochetum, 62, 95, 117.
carried by straw manure, 30.
hibernation of, 95.
Colletotrichum tabificum, 70.
Complete sterilization, 155.
Congested areas, 17.
Conidia, 59.
Coniothyrium fuckelii, 91.
Coniothyrium rosarium, 91.
Copper sulphate, 61, 90.
Corticium vagum, 58.
Cotton, 84.
Cow manure, 50.
Cresols, 169.
Crown canker of the rose, 66.
Cucumber, 84.
angular leaf spot of, 126.
Alternaria leaf spot of, 99.
bacterial wilt disease of, 122.
Butcher’s disease resister, 92, 145,
192.
Cercospora leaf spot of, 92.
Cladosporium leaf spot of, 99.
Colletotrichum leaf spot or anthrac-
nose of, 94.
control of angular leaf spot of, 127.
* Foot rot ”’ of, 124.
downy mildew of, 99.
fusarium wilt of, 86.

98, 117,

INDEX

203

Cueumber—conid.
mosaic disease of, 142.
powdery mildew of, 99.
Verticillium wilt of, 82.
Cucumber fruits, gammosis of, 117.
Cucumber mildew, 34.
Cuoumber seedlings, “damping off,”
of, 62.
Cucumber wilt, 23.
Cucumbers, spring physiological wilt
of, 33.
Cylindrocladium scoparium, 6T.
Cylindrosporium chrysanthemi, 108.

Daffodil :
** yellow stripe ”’ disease of, 53.
** Damping off,’ 17, 29.
** Damping off ’’ of cucumber seedlings,
62.
** Damping off” of tomato seedlings:
control of, 60.
effect of lime on, 48.
effect of temperature on, 62.
effect of thick sowing on, 56.
effect of shade on, 34.
organisms causing, 58.
relation of contaminated water
supply to, 60.
relation to process of watering, 26.
sources of infection, 60.
symptoms, 56.
Daphnes, 69.
De Bary, 34.
Deep sterilization with hot water, 165.
Depressions in ground, 14.
Diabrotica vittata, 123.
D. duodecimpunctata, 123.
Dichlorocresol, 169.
Dickson, 149, 151.
Diplocladium, 73.
Diplodina lycopersici, 89, 117.
Disease :
effect of atmosphere humidity on, 39.
effect of light on, 33.
effect of manurial treatment on, 47.
effect of temperature on, 35.
Disposal of diseased tissue, 28.
Doidge Miss, 135.
Doolittle, 149.
Drainage, 14.
contaminated by disease, 16.
effect on tomato crop, 15.
Dropping of flower buds, 51.
Dropsy, 42, 43.
of the geranium, 42.
of the tomato, 42.
Dusting, 175, 188.

Edgerton, 37, 192.
Effect of shade on early blight of the
celery, 34.

204

Eggplant, 84.
Elm, 84.
seedlings of, 59.
Epichle typhina, 47.
Erysiphe polygoni, 99.
Etiolation, 18.
Experimental and Research Station,
Cheshunt, 32.
BHyre, 182.
Ferns, 24.
Flagella, 121.
Flammarion, 20, 21.
Flour paste, 183.
Flowers of sulphur, 181.
Formaldehyde, 60.
Freiberg, 147.
Fruit diseases, 88.
Fuel bill, 24.
Fungi:
effect of high temperatures on, 36.
effect of low temperatures on, 36.
in nursery water supplies, 29.
Fungicides, 177.
Fungus spores :
condition for germination of, 25.
Fusarium lycopersici, 73, 84, 85, 192.
spores of, 85.
temperature relations of, 86.
Fusarium root rot of the tomato, 68.
Fusarium sp., 117.
Fusarium vasinfectum var. niveum, 87.
Fusarium wilt, 73.
Fusarium wilt of cucumber and melon,
86.
Fusarium wilt of the tomato, 84.
control of, 86.
effect of lime on, 48.
effect of temperature on, 37.
temperature relations of, 86.

Gas injury, 52, 53.
Gilia tricolor, 59.
Glass :
blue, indigo and violet rays, 18.
big overlaps, 19.
importance of using white glass of
consistent purity, 21.
irregular surface flaws and bubbles,
20.
light transmitting properties of
different kinds, 20.
quality of, 19.
relation between root development
and colour of, 21.
relation of colour of flowers to
colour of, 21.
relation of dry weight of plants to
colour of, 20.
relation of plant growth to colour
of, 20.

DISEASES OF GLASSHOUSE PLANTS

Glass—contd.
relation of temperature of house to
colour of, 20.
small panes, 19.
size of, 18.
white, red, green and blue, 20.
Glasshouses :
effect on crop management of large,
well lighted, 22.
heating of, 23.
humidity in those with high gutters,
25.
Glasshouse construction, 17.
in relation to air capacity and the
ventilation factor, 21.
in relation to the light factor, 18.
Gleosporium, 117.
Grape mildew, 34.
Growth :
at night, 18.
effect of light on, 18.
maximum, 23.
Gummosis of hyacinth, 53.
Gypsum, 168.

Hardpan :
effect on growth of plants of, 15.
Head space, 22.
Heating :
cost of, 24.
Heterosporium echinulatum, 106.
Horse nettle, 66.
Humidity of glasshouse atmospheres,
24, 38.
Humidity :
effect on foliage, flowers, setting of
fruit and disease, 24.
effect of low, 24.
epidemics associated with high, 39.
limiting factor in many diseases, 24.
relation to spore production, 24.
relation to temperature, 25.
relative requirements of different
crops, 38
Hunger, 147.
Hyacinth :
gummosis of, 53.
Hybridization, 192.
Hydrangeas, 69.

Infection centres, 28.

Impervious stratum, 15.

Imported plants as disease carriers, 30.
Iwanowski, 146, 147.

Johnson, 38.
Jones, L. R., 37.

Kitasato filter, 148.
Leaf diseases, 88.

INDEX

Lettuce, 21.
Levin, 103.
Light, 33.
efiect on resistance to disease, 33.
orange rays, 18.
plants grown’in weak, 33.
red rays, 18.
in relation to plant growth, 18.
Light intensity :
afternoon light, 19.
effect of roof angle on, 20.
morning light, 19.
Lilies, 20.
Lime, 48.
Lime and sulphur mixture, 125.
Lime sulphur, 113, 182.
Lime sulphur and flour paste, 97.
preparation of, 184.
Liver of sulphur, 90, 156, 182.
Liver of sulphur and flour paste, 109,
111, 184.
Livingstone atmometer cup, 148.
Lodewijks, 151.
Low lying plains, 16.
Lysol, 133.

Macrosporium dianthi, 106.
foot rot of tomato, 63.
symptoms of, 65.
Macrosporium solani, 119.
M. tomato, 120.
Malnutrition, 50.
Mangold leaf spot, 47.
Manx Marvel, 81.
Massee, 73.
Mayer, 147.
Melons, 69, 84.
** foot rot ’’ of, 124.
fusarium wilt of, 86.
resistant to wilt disease, 193.
verticillium wilt of, 82.
Melon “‘ canker,” 124.
control of, 125.
Methods of building glasshouses :
evolution in, 19.
Millardet, 177.
Moisture :
film of, 25
Mosaic disease, 38, 140.
abnormality due to, 141.
carrier plants, 150.
control of, 151.
cross inoculations, 149.
distortion due to, 141.
effect of environmental conditions
on, 151.
host range of, 141.
mottling, 141.
pathological anatomy of diseased
plants, 145.
properties of the virus of, 148.

205

Mosaic disease—conid.
symptoms of, 141.
the determination and elimination

of agents transmitting, 152.
the determination and elimination
of infection centres of, 151.

the infectious nature of, 146.
transmission of, 148.
varieties immune to, 153.

Mosaic disease of the cucumber :
seed transmission of, 149.
symptoms of, 144.

Mosaic disease of the tomato:
symptoms of, 142.

Mulching, 26, 114.
conservation of soil water by, 26.
effect on crop yield, 27.
effect on disease, 27.
plant food supplied by, 26.
straw, 27.

Musbroom beds, 60.

Mycelium, 54.

Mycospherella citrullina, 89.

Neighbouring highlands, 16.
disease introduced from, 16.
effect on drainage water passing
through site, 16.
height of water table depending on,
16.
Nursery workers, 31.

Gdema, 42.
of the geranium, 42.
of the tomato, 42.
Oidium chrysanthemi, 109.
Orientation of glasshouses :
effect on health and yield of the
crop, 19.
Ortho-nitrochlorbenzene, 169.
Orton, 193.

Paraffin vapours, 53.
Parasites, 54.
Partial sterilization, 154.
Peronospora sparsa, 110.
Peronospora trifoliorum, 113.
Peronospora viiicola, 178.
Perithecia, 55.
Petrol vapours, 53.
Petunia, 59.
botrytis disease of, 34.
Phoma, 117.
Phoma beice, 47.
Phosphates :
in relation to root development, 48.
Physiological disorders, 25.
wilting, 33.
Phytophthora cryptogea, 17, 29, 48, 64,
, 114.
optimum temperature for, 62.

206

Phytophthora foot rot of the tomato, 63.
Phytophthora infestans, 118, 191.
Phytophthora parasitica, 29, 48, 58, 64,
67, 114.
optimum temperature for, 62.
Phytophthora terrestria, 59, 114.
Pig manure, 50.
Plant hygiene, 193, 194.
Point-rot of the tomato, 41
Posts, 18.
Potash :
in relation to disease resistance, 48.
Potato, 59, 66, 69, 84.
Propagating soil, 16.
Protozoa, 49.
Prichard and Porte, 66.
Pruning, 28.
result of careless, 28.
result of jagged wounds, 28.
knives as carriers of disease, 31.
Pseudomonas lachrymans, 126.
Pseudomonas solanacearum, 134.
hosts of, 134.
Pseudoperonospora cubensis, 100.
Puccinia chrysanthemi, 108.
Purlins, 18.
Pycnidia, 55, 90, 91, 103, 106, 109.
Pythium de Baryanum, 62.

Quanjer, 143.

Rands and Endlows, 123.
Resin mixtures, 183.
Resistant varieties, 195.
Rhizopus nigricans, 116.
Rhizoctonia solani, 58, 71.
hyphe of, 58.
optimum temperature for, 62.
Root rots, 67. :
Roots :
in cold damp beds, 23.
Rose :
bronzing of, 50, 51.
downy mildew of, 110.
graft disease, 91.
leaf blotch of, 111.
powdery mildew of, 111.
stem canker of, 91.
Russell, E. J., 167.
Rusts, 48.

Salmon, 182.
Saponin, 183.
Saprophytes, 54.
Sclerotia, 69, 71, 88.
Sclerotium rolfsw, 68.
Sclerotium root rots of the tomato, 69.
Sclerotium tuliparum, 71.
Sclerotinia sclerotiorum, 88.
hosts of, 88.
Sclerotinia stem rot, 88.

DISEASES OF GLASSHOUSE PLANTS

Scorching, 20.
Seedboxes and pots, 60.
Selection, 191.
Sensitive plants, 20.
Septoria chrysanthemella, 109.
Septoria dianthi, 106.
Septoria lycopersici, 103.
Setz, 70.
Shaded positions, 34.
Site, 14.
Situation, 13.
“‘ Sleepy disease,”’ 73.
Slope :
effect on temperature of glasshouse
atmospheres, 14.
relation to efficient drainage, 14.
Smith, E. F., 123, 134.
Soft rot of the Calla lily :
effect of lime on, 48.
Soil :
biological factors, 18.
chemical factors, 18.
physical factors, 18.
waterlogged, 25.
Soil fungicides, 188.
Soil particles, 44.
Soil and air temperature :
effect of marked difference between,
36
Soil micro-organisms, 49.
Soil moisture, 39.
effect on plants of an excess, 39,
effect of a deficiency of, 41.
Soil population, 49.
down grade changes, 49.
disease producing organisms, 49.
up grade changes, 49.
in relation to fertility, 49.
Soil reaction, 45.
Soil sterilization, 60, 154.
boiler tray method, 165.
effect on ammonia production, 155.
effect on bacterial numbers, 155.
effect on plant growth of different
methods of, 169.
Soil temperature, 36.
Soils :
** Sick,” 154.
sterilization of, 154.
Sources of infection :
baskets and sacking, 31.
imported plants, 30.
nursery workers, 31.
pruning knives, 31.
straw manure, 30.
water supply, 29.
Spectrum, 18.
Spherotheca pannosa, 111.
Sporangia, 60.
Sporangiophore, 60.
Spraying, 175.

|

INDEX

Spraying—contd.
effect on the plant, 187.
the process of, 185.
Spraying, dusting and sterilization, 194.
Spreaders, 176, 183.
Steam sterilization, 90, 156.
box and grid method, 156.
drain-pipe method, 164.
small grid method, 158.
tank method, 162.
tray method, 160.
Stem diseases, 88.
Sterilization :
by baking, 166.
by drying, 169.
by chemical compounds, 167.
effect on plant disease, 172.
of propagating soil by formalde-
hyde, 168.
with cresylic acid, 167.
Sterilization of water, 173.
Sterilized soils:
ammonia production in, 155.
Stigmonose, 52.
Stone, 19, 20.
Straw manure :
as a carrier of disease, 30.
necessity of obtaining
material, 30.
Strawberry mildew, 34.
** Streak ” disease, 132.
** Stripe ” disease of the tomato, 46.
control of, 133.
effect of ‘‘ damping ”’ on, 130.
effect of forced and slow growth

clean

on, 130.

effect of manurial treatment on,
129

effect of nitrogenous fertilizers
on, 47.

effect of potash on, 47.
organism causing, 132.
symptoms of, 131.
Striped cucumber beetle, 123.
Sulphur fungicides, 181.
“Summer ”’ spores, 54.
Sunburn of tomato fruit, 35.
Surface drainage, 16, 60.
Surface moisture, 26.
Surface sterilization with hot water, 164.
Sweet pea, 84.
downy mildew of, 113.
Rhizoctonia root rot of, 71.
Verticillium wilt of, 83.
Sycamore, 84.

Talc, 148.

Temperature :
difference between soil and air, 23.
effect on disease, 23.
relation to humidity, 25.

207

Temperature—contd.
maximum, 38.
minimum, 38. ~
optimum, 23, 37.
soil, 23.
in relation to different stages of
growth of the same plant, 23.
in relation to disease resistance, 35.
in relation to plant growth, 35.
in relation to the plant and patho-
gen complex, 37.
of soil covered by straw mulch, 27.
The soil, 43.
aeration of, 45.
artificial aeration of, 45.
degree of compactness of, 45.
physical conditions of, 44.
ramming of, 45.
the chemical conditions of, 45.
the food holding capacity of, 44.
the texture of, 44.
the water holding capacity of, 44.
in relation to disease, 44.
“Tip burn ”’:
of lettuce, 34.
method of combating, 35.
of the tomato, 34.
Tissues: maturing of, 23.
Tomato, 24, 84.
bacterial wilt of, 134.
Botrytis stem rot of, 90.
Grand Rapid’s disease of, 135,
hollow stem of, 51.
leaf spot disease of, 103.
** Nailhead ” spot of, 120.
Macrosporium disease of, 119.
mosaic disease of, 142.
potato blight of, 118.
sleepy disease of, 73.
stem canker of, 90.
‘stripe ’’ disease of, 127.
Verticillium wilt of, 73.
Tomato fruits :
Botrytis rot of, 115.
brown rot of, 137.
“ buckeye ” rot of, 114.
Fusarium rot of, 117.
soft rot of, 136.
sunburn, 35.
Penicillium rot of, 117.
Rhizoctonia rot of, 115.
Rhizopus rot of, 116.
Tomato mildew, 14, 100.
conditions favourable to, 101.
control of, 102.
Tomato seedlings:
“damping off ” of, 56.
“Tops,” $4:
Training, 28.
Transpiration, 23.
Tulip blindness, 53.

208

Twelve spotted cucumber beetle, 123.

Uromyces caryophyllinus, 104.

United States Department of Agri-
culture, 37.

University of Wisconsin, 37.

Ventilation, 21.
Ventilators :
number, size and position of, 22.
Verticillium albo-atrum, 34, 73, 75, 173.
hosts of, 83.
Verticillium collar rot of the tomato,
63, 66.
Verticillium lycopersici, 66.
Verticillium wilt, 73, 84, 193.
Verticillium wilt of cucumber and
melon, 82.
Verticillium wilt of sweet pea, 83.
Verticillium wilt of tomato :
beneficial effect of shade on, 34.
control of, 81.
effect of lime on, 48.
effect of manurial treatment on, 48.
effect of shade on, 80.
effect of temperature on, 37.
effect of type of growth on, 76.
symptoms of, 74.
temperature relations of, 76.
Vine, 24.
Virus diseases, 140.
Vitality, reduced, 23.
Vogel, 91.

Wallflower, 59.

DISEASES OF GLASSHOUSE PLANTS

Wart disease of the potato, 191.
Water, 38.
advantages of pure supply, 30.
analysis of nursery supplies, 29.
careless splashing of, 25.
Watering, 25.
effect of insufficient, 25.
effect on physical condition of
soil, 25.
excessive, 25.
overhead, 26.
relation to infection, 25.
surface, 26.
sub-irrigation, 26. ;
use of perforated hoses, 26.
Waterlogged soil, 16.
‘¢ Water rot,’”’ 113.
Water supply, 29.
from brooks and ponds, 30.
from deep artesian wells, 29.
in relation to disease, 29.
from surface wells, 29.
Wells, methods of cleansing, 174.
Wetting agent, 176.
Whetzel, 188.
White fly, 149.
Wilt, caused by waterlogging, 39.
Wilt diseases, caused by fungi, 73.
Woburn manurial trials, 47.
Woods, 147.
Wounding by woodlice or wireworm, 68.

Yellows, 41.

Zoospores, 59.

Printed by Jarrold & Sons, Lid., Norwich

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