oS

Sectional Volume No. 1.

taistry of Agriculture and Fisheries.

UC-NRLF

B 3 fiT3 531

COLLECTED LEAFLETS

FUNGUS PESTS

FEUIT TEEES.

LONDON :
PRINTED FOR HIS MAJESTY'S STATIONERY 01

To he obtained at tin • o1

MINISTRY OF AGRIG1I LTI.'HE AND K1SHKKILX
10, Whitehall Place, London, S.W.I.

: : . VF!>KQ,\ :. SGN\) Lin . .. , ': ,,,! . V (•/./«, ; ,.,.. ..... '- ,

1921.
Price 8d. net, Post Free.

AGRIC.

UBRARY

8061 '13 WlVd
'A 'M '

Sectional Volume No. 1

Ministry of Agriculture and Fisheries.

COLLECTED LEAFLETS

ON

FUNGUS PESTS AND ALLIED DISEASES

OF

FRUIT TREES.

LONDON :
PRINTED FOR HIS MAJESTY'S STATIONEEY OFFICE.

To be obtained at the Office of the
MINISTRY OF AGRICULTURE AND FISHERIES,

10. Whitehall Place. London, S.W.I.
Agents for Advertisements : C. VERNON & SONS, Ltd., 33, Holborn Viaduct, London. EJC.1.

Price 8d. net, Post Free.

HGRIC.

UBRAR.Y

Vll

LIST OP CONTENTS.

;

• PAGE

£3 Introduction

LEAFLET No. TITLE.

56. Apple Canker (Nectria gallir/ena.) ... 9

131. 'Apple and Pear Scab (Venturia inoequalis,

and V. pirina.) ... ... ... 12

204. Apple Mildew (Podosphaera leucntricha,

Salm.) ... 18

86. Brown Rot of Apples (Monilia fructigena,

Pers.) 21

312. Blossom- Wilt of Apples (Monilia cinerea,

forma mali. ) ... ... ... ... 24

245. Crown Gall (Bacterium tumefaciens.) ... 27

302. Silver Leaf in Fruit Trees (Stereum pur-

purewri)... ... ... ... ... 31

367. Wither-Tip and Brown Rot of Plums

(Monilia cinerea, forma pruni.) ... 37

87. The Die-Back Disease of Fruit Trees

(Cytospora leucostoma.) ... ... 40

120. Peach Leaf -Curl (Exoascus deformans.) ... 42

195. American Gooseberry Mildew (fiphaerotheca

mors-uvae, Berk. ) ... ... ... 45

52. European Gooseberry Mildew (Microspliaera

grossulariae, LeV.) ... ... ... 49

115. Coral Spot (Nectria cinnabarina.) ... 51

234 The Die-Back Disease of Gooseberries

(Botrytis cinerea.}... ... ... 53

133. Powdery Mildew of the Vine (Uncinula

necator, Burr.) ... ... ... ... 56

377. "Reversion" or Nettlehead of Black

Currants ... ... ... ... 58

Conclusion ... ... .. ... ... ... 601

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INTRODUCTION,

Unlike insect pests, which are for the most part more or less
clearly understood, there is often a certain mystery in the public
mind connected with fungi. Their strange form, sudden appear-
ance and devastating habits are not readily explainable. To the
practical fruit grower painful familiarity with their presence
soon robs fungus parasites of any mysterious charm, and, by
force of circumstances, he is compelled to study them, if it be
only their mode of attacking plants and the best methods of
keeping them at bay. Whether, however, the origin and!
habits are understood or not, it may not be inadvisable to point
out at the commencement of this volume the main outlines of
the life of any ordinary fungus.

The Nature of Fungi. — The fungi consist of a group of
organisms belonging to the vegetable kingdom. In common
with bacteria, fungi differ from all other members of that
kingdom in that they possess no green colouring matter
(or chlorophyll) and are therefore unable to obtain their carbon
food supply from the air as do ordinary flowering plants. This
accounts for their peculiar habitats, fungi being found on
materials derived from dead plant or animal tissues or on the
living organisms themselves. In size they vary greatly; some
are large, but the majority are minute. Those which cause
serious epidemics amongst crops are usually amongst the
smaller species. Each fungus consists essentially of two parts :
(1) a system of fine threads, technically called, the mycelium,
and (2) spores, which correspond to seeds in ordinary plants.

Mycelium. — It is well that growers should be clear as to the
term mycelium, especially as no English equivalent is avail-
able. In a large earth-growing species such as the mushroom
it consists of the white threads or " spawn," and a similar
kind of mycelium can be found in any of the toadstools or
puffballs. The mycelium absorbs nourishment from the sub-
stance in which it grows — soil, wood or humus — and in the
case of a parasitic species it extracts this food at the expense
of the host-plant and causes more or less definite injury. In
the smaller fungi the mycelium, though very minute, is similar
in form and has the same function as in the larger species typi-
fied by the mushroom. In mildews and certain moulds it
exists on the surface of the host and may be seen with the
aid of a lens as a loose, cobwebby growth. In others it is
internal, or located within the host, and only shows itself after
the damage it occasions is completed.

Spores. — It is equally important to know something of the
nature and significance of the spores. They serve to propagate

(39112) P.1130/R.201. 3,000. 11/21. M. & S. Gp. 95.

the species and, although produced in enormous numbers, are
always very minute. In the larger fungi they are borne on
an elaborate cap-shaped structure, e.g., the cap or edible portion
of a mushroom. Anyone can demonstrate the vast quantity
of spores which are produced by a mushroom by cutting off
the stalk and laying the cap, gills downward, for a few hours
on a plate or piece of white paper and covering with a tumbler.
A copious fall of spores will be found and an outline of the gills
will be seen on the paper. If the cap is not covered only a few
spores will be deposited owing to their being blown away as
they fall by draughts.

Precisely the same spore-discharge takes place daily in
orchards and gardens. Silently and unobserved, fungi both
large and small, are, if the weather be favourable, discharging
myriads of dust-like spores. Most of these are wafted as a
totally invisible dust over neighbouring plants or neighbouring
fields; some may be blown away in bulk, others washed down
by rain. The big toadstool may discharge its hundred millions,
the Coral Spot fungus its one million, but in all fungi, when
the reproductive phase is reached, an exceedingly copious crop
of spores is developed.

When it is remembered that this spore-discharge takes place
all over the country and that, borne by the breeze, spores are
rained on every garden, field and orchard, the sudden appear-
ance and rapid spread of a mildew like American Gooseberry
Mildew or a tree disease like Silver Leaf is no longer myste-
rious. The wonder is that fungus epidemics are not more
prevalent and that, relatively speaking, so very few spores
fulfil the function for which they were produced.

Saprophytes and Parasites. — As is the case with seeds, spores
germinate under suitable conditions of warmth and moisture.
Each spore gives rise to fresh mycelium and thus produces a
new plant. Whether this infant fungus will grow and mature
depends on the nature of the substratum Fungi differ
enormously in their requirements. Some are so adaptable that
they can flourish almost anywhere where moisture and- vege-
table matter of any kind are present. Others have a more
restricted range, requiring either leaf mould, dead wood or
rotten fruit as the case may be. Yet others show a decided
tendency to feed on living organisms and thus pass from the
realm of harmless saprophytes (i.e., organisms that live on
dead organic matter) to that of parasites.

Types of Parasitic Species. — As far as the present volume is
concerned, the interest lies wholly with the parasitic species.
These show varying degrees of parasitism and, for this and
other reasons, there is considerable diversity in the amount of
damage they cause. There are the feebly parasitic fungi which
occur abundantly on dead shoots, but under certain conditions
spread into the living tree and cause damage. Amongst species
of this class attacking fruit trees Coral Spot is a good example.
Others, although they commence on a dead branch or wounded

5

area, become more readily and seriously parasitic and will,
unless checked, invariably proceed rapidly in the living tissues
and occasion much damage. This form of parasitism, in
various degrees of intensity, is seen in the following diseases of
fruit-trees: — Brown-Rots, Die-Back of Plums, Apple Canker
and Silver Leaf. Lastly, there are the exclusively parasitic
forms of which it may be said, in a general way, that their power
to bring about the infection of sound, uninjured tissue is con-
fined to the leaf -attacking species, which are found principally
in three natural groups, the rusts, the mildews and the false
mildews. Alighting on a leaf, the moisture provided by a
light dew allows the spores to germinate and the germ tube
of the spore rapidly enters the leaf, either through the stomat.a-
(or breathing pores) or by piercing its way through the epider-
mis or outer skin of the leaf. The true rusts (such as Plum
Bust), the false mildews (few of which attack fruit) and the
Apple and Pear Scab fungi effect an entrance in this way.
The true mildews, such as American Gooseberry Mildew and
Apple Mildew, are highly parasitic but differ from other fungi
in being superficial and extract all their nourishment by means
of minute suckers which penetrate the epidermal cells. The
damage that mildews cause, however, is very severe.

Mode of Attack and Measures of Control. — The method
of attack, subsequent development and above all the means of
hibernation of fungus parasites are, from the practical stand-
point, of very great importance and should be mastered by
every fruit grower. In the leaflets a short outline is given in
each case, but a very brief summary is added below, the dis-
eases being grouped according to the nature of the damage
caused. An indication of the type of treatment required is
also stated.

(a) Fungi attacking the Leaves. — Leaf fungi, though com-
prising many forms, are usually combatted by spraying or
dusting. Spraying may aim at the prevention of an attack or,
more rarely, at the destruction of the mycelium and spores. If
the leaves are covered with a film of fungicide, the slender
germ tube which the spore produces on germination is killed
and infection does not- take place. The action of the spray
here is preventive. To mildews, the mycelium of which is
superficial, a spray or powder may be applied with the object
of the actual killing of the mycelium. This is often largely
successful, but, even with mildews a preventive spray is more
effective and should always be employed if possible.

The method of hibernation varies considerably. Where this
is known it is often possible almost to exterminate the fungus
by the removal of the winter stage. In mildews the normal
method of hibernation is by means of resting spores
contained in spore-cases produced on the old mildew patches
either on the leaves or shoots. The affected leaves and shoots

in this case should be collected and destroyed. The operation
of tipping for American Gooseberry Mildew has this object in
view. In some mildews, e.g., Apple Mildew, few or no resting
spores are formed and the mycelium hibernates in the buds
and develops with them in spring. In this case affected buds
should be cut out.

In most other leaf-fungi winter or resting spores form in
the old leaves, hence the invariable advice of the mycologist to
burn affected foliage. It is now known, however, that leaf-
fungi not infrequently also attack the young wood and pass the
winter on twigs on the tree. Apple and Pear Scab are the
best known and most striking examples of this form of hiber-
nation, and the great/ importance of removing, scab-infected
wood is now recognised by all up-to-date growers.

(b) Fungi attacking the Fruit. — The most destructive fungi
of this group are those causing the Brown Eots of pear, apple,
plum, cherry and other stone fruits. In these diseases the
fungus remains in the attacked fruit and, drying them up,
forms the well-known mummies which hang on the trees
during winter. The mycelium hibernates in the mummies,
becomes active the following season and liberates spores. In
addition to spore-infection the inoculation of new fruit is also
brought about if the latter is in direct contact with a mummy
as is often the case when fruit is borne on spurs. An impor-
tant point to remember, however, is that Brown Hot fungi
also attack flowers and shoots, forming Blossom-Wilt, Wither-
tip and Canker, and that when the shoots are infected the
fungus hibernates in them also. In each of the Brown Rot
diseases, therefore, attention should be concentrated on remov-
ing not only mummies, but all wood that has been killed by
Brown Eot the previous season.

Apple and Pear Scab are only second in importance to the
Brown Eots. The scab fungus hibernates on the wood and
passes from this to the leaves. From the leaves it spreads
to the young fruit. A somewhat similar method is found with
the less prevalent diseases Bitter Eot and Black Eot of apples.

(c) Fungi attacking the Wood. — This group is connected
with the last by the Brown Rot and Scab fungi which at times
seriously injure the wood of both plums and apples. The
fungi particularly in view under the present heading are, how-
ever, those which attack the wood only, and these invariably
gain entrance through a wounded surface or an area of dead
tissue such as a twig or broken branch. The most important
fungi of this group are those causing Canker, Silver Leaf, Die-
Back of plum, although there are many others.

In all these diseases the first point to ascertain is the manner
in which the fungus gains entrance to the tree. Any wound
may probably serve, but in ordinary orchard conditions1 there
are generally particular injuries which act as weak points in
the armour of the tree. As an instance, Apple Canker may

be taken. It has long been known that pruning and other
wounds are liable to admit the canker fungus, but quite re-
cently it has been shown that the ordinary leaf scars offer a
particularly easy mode of entrance and that in certain seasons
at any rate the majority of infections arise in this way. The
discovery in itself is not consoling, but it points out an un-
suspected weak spot and the efforts of scientific workers must
now be concentrated on devising a new method of control. In
the case of Silver Leaf it is known that the spores, if placed
on a snag or large wound, rea,dily bring about infection, but
it is not impossible that further research will indicate additional
or unperceived doors of entry.

A. group of fungi which, since they invade the wood of the
plant and require dead or injured tissue to gain admittance,
may be included in this section, are the root fungi. The most
important of these is the Honey Fungus which sometimes
attacks apples. Damage in fruit trees by root fungi in England
is, however, insignificant, and need not be further considered
here.

The principal measures for combating the attacks of wood-
destroying fungi are three : (1) The removal of the sources
of infection, viz., dead or infected wood which, by a constant
liberation of spores, is a continual menace. This includes
diseased or dead wood on the tree or dead wood lying about in
the orchard. (2) The prevention of injury and the protection
of 'wounds. Damage through neglect should be reduced to a
minimum, pruning being carefully carried out and broken
branches sawn off with a clean cut. The most important item
under this heading is the protection of all injured surfaces with
a coat of tar or good paint which, in the case of larger wounds,
should be renewed after three months. (3) The prompt use
of the knife. Affected shoots or diseased areas should be cut
out a,s soon as infection is apparent. By this method large
branches or even whole trees may often be saved. The excised
area should not be larger than necessary, but must extend to
healthy wood. The wound must, of course, be protected at
once by tar.

The foregoing are merely general recommendations. To be
successful it is obvious that the particular habits of each fungus
should be understood thoroughly. No apology, therefore, need
be made for inserting paragraphs on the life-History of the
parasite in leaflets intended for the practical grower.

Methods of Cultivation and the Use of Resistant
Varieties. — In urging the importance of the knowledge of
the life-history of fungi and recommending such measures oi
control as spraying and dusting, the supreme importance of
good all-round cultivation has not been overlooked. This lies at
the very foundation of successful fruit-growing. Scientific

8

method should be employed in all cultural operations of prun-
ing, choice of soils, fertilisers, stocks and the like, just as it
should be in the destruction of pests. All these cultural factors
immensely affect the incidence of disease, and every mycologist
would admit that sound cultivation does more than anything
else to reduce outbreaks. Approved methods of culture for fruit
trees will be found in Sectional Volume No. 4 (Fruit Cultiva-
tion), to be published shortly.

One of the chief weapons of this nature at the disposal of
the grower is the use of disease-resistant varieties. The breeding
and raising of new varieties is a slower process with fruit trees
than with many other crops, but with the large number of
varieties already in existence much may be done by judicious
selection. In substituting resistant varieties for those pre-
viously grown, it is first of all necessary to ascertain whether
they are suited to the soil and climate of the district. This
being assured their employment in the orchard will not only
save endless labour but may mean the difference between
success and failure. In most of the leaflets notes are given
on the relative susceptibility of varieties. It is sometimes
found, however, that when good growth and resistance to the
disease in question have been obtained, the variety suddenly
falls a victim to another pest. Early and prompt attention is
then essential and if given will often prevent the new fungus
from becoming firmly established.

In spite of the very best culture and the most suitable
varieties certain diseases invariably appear, and it is here that
the most accurate knowlege and the most recent results of
scientific inquiry will be of most value. No disease has yet
been exhaustively investigated, and it is only by further re-
search that the many problems can be solved and effective
measures of control discovered.

A. D. COTTON,

(Mycologist, Ministry of Agriculture and Fisheries).

London, S.W.I.

October, 1921.

APPLE CANKER.

(Nectria galligena.)

Apple canker is one of the most destructive diseases with
which fruit growers have to contend. It is a serious menace
to fruit trees on the continent of Europe, whilst in England
it has ruined many a plantation and placed certain kinds of
apples very largely outside the pale of commercial cultiva-
tion. Though the term " canker " is often applied to various
open wounds where the bark is swollen and rugged, the true
apple canker, as understood in Britain, is a specific disease,
and is caused by the minute fungus Nectria galligena.* The
same fungus also causes canker in the pear.

Description. — The general appearance of apple canker will
be recognised from the figures accompanying this leaflet.
Fig. 1 represents an early stage of the disease, Fig. 2 one
which is far advanced. The wounds may occur anywhere
on the branches, but are found most frequently at a node or
at the junction of a small branch with a larger one.

The commencement of the canker-wound and the various
stages which follow may readily be observed if a diseased
tree is studied with a little care. The young stages will be
seen as small depressed areas, which gradually break away
from the surrounding part of the shoot and are somewhat
darker in hue. Such areas very frequently originate at a leaf
or scar around a dead twig (Fig. 1), and are caused by the
activity of the fungus which has gained an entrance to the
tissues. This small damaged area increases in size and
usually becomes more or less elongated in outline. The
tissue in the central portion dies and gradually decays, so
that the damaged area assumes the form of an open wound
surrounded in later stages by rugged bark. Such wounds,
termed " canker " by gardeners, are found in many plants
and may be produced by a variety of causes. In the apple,
canker is due to the fungus Nectria galligena, although, as
explained below, the injury caused by Woolly Aphis is often
very similar in appearance, and in fact the two pests are not
infrequently found together and aid each other in the damage
they cause. One of the most marked features of apple canker
is the presence of more or less regular concentric rings around
the wound, and it may frequently be recognised by these even
in the absence of the fruits of the fungus.

As the disease progresses the wound enlarges and a mass
of swollen tissue, due to the repeated formation of callus by
the branch, arises around it. The subsequent behaviour
varies under different circumstances and with different

* Kecent investigations show that Nectria galligena and not
N. ditissima as formerly supposed, is the correct name for this fungns.

10

varieties of apple. In many cases the wound is confined to
one side of the branch and the branch continues to live for
many years. In other cases the parasite " rings," that is com-
pletely encircles, the branch or shoot, as shown in Fig. 2,
and causes its death. Provided the branch is not ringed the
sap continues to flow, and such branches remain alive and
may even grow and yield heavy crops of small fruit.

The fruiting bodies of the fungus, by which alone the
disease can be identified with absolute certainty, are
minute and occur especially in autumn and spring.
Those formed in spring and early summer are perhaps
the best known. They occur on various parts of
the wound and consist of round, rather tough bodies
(perithecia), deep crimson in colour and about the size of
a poppy seed. Large numbers of two-celled spores are
liberated from these fruit-bodies and are conveyed by rain,
wind, insects, &c., to other trees, and thus spread the disease.
In late summer and autumn especially the fungus develops
another form of fruit, which shows as white, very minute
spots. The fruiting bodies bear masses of sickle-shaped spores
(conidia) and these also serve to spread the disease.

Causes Leading to Attack. — The canker fungus gains
entrance to a tree by means of wounds or small injuries, and
apart from such wounds the fungus spores are not able to
infect the branches. The wounds may be large, such as
those caused by breakages, careless pruning, Ac., but minute
injuries, especially those occurring naturally in the region
of the buds, provide a very efficient means of entry.
Recent work at Long Ashton shows that the leaf-scars left
by the fall of the leaves in the autumn afford an excellent
opportunity for the fungus to enter the tree, and, during
the winter and following spring, examples of this type of
infection 6f the previous year's shoots can be found in almost
every orchard. Other wounds which may frequently lead to
canker are frost cracks and the breaking of young shoots due
to careless picking or pruning. The " heading back " of
badly grown trees exposes large cut surfaces to infection, and
main branches of trees treated, in this manner are often found
to be severely attacked by canker.

Another, and very important, means of entry is through
injuries caused by Woolly Aphis. The soft swollen tissue
produced by the Aphis is very apt to become cracked' during
the autumn, exposing the woody tissues of the stem. The
fungus quickly infects such splits, and infections of this type
are quite commonly found on the young wood during the
winter. One other method by which the canker fungus
attacks the tree must be mentioned. The scab fungus towards
the autumn attacks the young shoots formed during the
summer, resulting in the formation of numerous scars on the

APPLE CANKER.
FIG. 1. — Young branch of Apple with a small canker present on the

right-hand side. The commencement of the wound is at the base

of short side shoot.
FIG. 2. — Older branch. The fungus here has not only destroyed the

tissues on one side of the branch, but has completely encircled

and killed it.

10-1

APPLE AND PEAR SCAB.

FIG. 1 (above).— Affected apple showing small scabs.

FIG 2 (below).— Affected pears. The fruits are badly scabbed and as a
result are cracked and deformed. The black blotches on the leaf
indicate infection of the leaves from which the fungus spreads to
the fruit.

11

stem. (See Leaflet No. 131 — Apple and Pear Scab.) These
scars frequently allow the entrance of the canker fungus,
which causes much more serious damage than the superficial
injury of the scab fungus.

Influence of the Soil. — Although the presence of wounds
is essential for successful infection by the fungus, there are
other factors intimately concerned with the prevalence of
canker. Chief among these is the question of location and
soil. A variety which cankers in one district may remain,
comparatively free in another. Canker is always worse on
a low site and a clay subsoil, so much so that it is often
impossible in such positions to grow certain varieties of apples
successfully (e.g., Lord Suffield, and Cox's Orange Pippin).
The effect of damp, heavy soil is to cause rank growth, which
probably permits the fungus not only to infect more readily
but also to develop more rapidly in the tissues. Although a
hill slope and more open soil are desirable much good may
be done, where a Bad subsoil is present, by careful drainage
and by the encouragement of surface rooting. With improved
conditions affected trees will sometimes grow out of canker,
and by careful pruning, clean, healthy trees may be produced.

Susceptibility of Varieties. — Another factor of great
importance is the nature of the variety, some sorts being
exceedingly liable to the disease, whilst others are more
resistant. Varieties such as Lord Suffield and Cox's Orange
Pippin are so prone to attack that they are liable to canker even
on good soil, whilst Bramley's Seedling is resistant and will
often remain free even when grown on heavy land. The
following varieties are amongst those which under most
conditions are subject to attack: —

Cox's Orange Pippin. Peasgood's Nonsuch.

Dumelow's Seedling (Welling- Pott's Seedling,

ton). Kibston Pippin.

Ecklinville Seedling. Stirling Castle.

Lady Henniker. Warner's King.

Lord Derby. White Transparent.

Lord Suffield. Worcester Pearmain.

Amongst the varieties usually less susceptible may be
mentioned : —

Annie Elizabeth. Bramley's Seedling.

Beauty of Kent. Lane's Prince Albert.

Blenheim Orange. Mr. Gladstone.

Bismarck. Newton Wonder.

Measures of Control. — 1. Cutting-out of Diseased Wood. —
Old and badly -diseased trees should be cut down and burned. An
exception to this rule may be made if, as occasionally happens,
the trees are bearing well. In the case of trees not so badly
diseased, cankered shoots and badly-cankered boughs should
be cut out. If a sharp look-out be kept for the commence-
ment of the canker, affected branches may sometimes be
removed without much loss of symmetry to the tree. During
winter-pruning all infected shoots should be carefully removed.

12

In the case of large boughs the canker-patch can sometimes
be cut out with a sharp knife or chisel. If care be taken
to cut down well into sound wood, and to protect the exposed
surface, the wound may completely heal over, and the bough
may thus be saved.

2. Protection of Wounds. — All cut surfaces should be
covered by a protective substance, such as coal-tar, Stockholm
tar, paint, painters' knotting, styptic or grafting wax. If
nothing better is procurable the surface may be luted with clay.
Eubbing over the surface with earth, a frequent custom, should
not be practised.

3. Top Grafting. — In certain cases, especially where the
variety is very susceptible, the tree may be cut back and top-
grafted with a more resistant variety, such as Bramley's
Seedling.

4. Prevention of Woolly Aphis. — Woolly Aphis, which not
only itself injures the tree, but frequently also introduces
canker, should be kept down. Instructions as to this pest will
be found in Leaflet No. 34. f

5. Drainage. — Apples are more liable to canker in heavy
subsoils and in damp situations. Where a wet, heavy soil is
present an effort should be made to drain it. In the case of
young trees growing down into an unsuitable soil, the trees
should be root pruned, and surface-rooting encouraged. On
exceptionally heavy and stiff land trees may sometimes be
planted on the surface and earthed up.

6. Selection of Varieties. — When planting a new garden or
orchard, varieties resistant to canker, and those which have
been found to do well in the locality, should be chosen.

7. Sanitation. — Spraying is practically useless, but in order
to reduce liability to infection all general sanitary measures
should be adopted, and diseased shoots and prunings should be
burned.

APPLE AND PEAR SCAB.

(Venturia inaqualis, and V. pirina.)

Scab is probably the most general and most widely dis-
tributed fungus disease which attacks apples and pears. The
well-known black blotches or scabs on the fruit caused by this
fungus are familiar to everyone (Fig. 1 and 2).*

In this country the disease causes an enormous amount of
damage in apple orchards and plantations, the annual losses
amounting to many thousands of pounds. During certain

* The Ministry are indebted to Mr. E. S. Salmon for permission to
use Figures 2, 3 and 4. For more detailed description of the scab
fungus see Mr. Salmon's article in the Jour. Board Agric. (Vol. XV,
1908-9, pp. 182-195).

| This leaflet can be obtained from the Ministry and also is included
in Sectional Volume No. 2, price lOd.

13

seasons entire crops are much depreciated in value and even
rendered unsaleable. Scab is a disease, however, which can
be well controlled if growers will carry out the necessary
treatment for spraying.

Although the fungi causing apple and pear scab respec-
tively are different species they are very closely allied. The
general appearance and method of treatment are the same in
each case so that separate descriptions of the two diseases are
unnecessary.

Description. — On the Leaves. — The first active stages of
the fungus-attack appear on the leaves during the early spring.
The general appearance at this stage varies according to the
variety; in every case the part of the leaf which actually
bears the fungus becomes a dark olive-green or sooty colour.
Sometimes the fungus confines its attack almost entirely to
the veins of the leaf, while in other cases it forms distinct
and sharply marked patches scattered over the surfaces
(Fig. 3). The spores, which are produced on these dark
patches in enormous numbers, are distributed by air currents,
and in this way spread the infection to other leaves, young
fruit and also new wood. It is most important to remember
that the early infection of the fruit takes place almost entirely
from the leaves, hence unless these are kept healthy a clean
crop cannot be expected.

On the Fruit. — Figure 1 depicts the disease as it occurs
on the fruit. The spores alighting on the fruit germinate
and penetrate the skin, beneath which they form a mass of
fungus tissue or " mycelium." THis does not, however,
grow deeply into the flesh of the fruit, but spreads immediately
beneath the surface. In course of time the covering skin of
the fruit is broken, with the result that thousands of dark
coloured spores escape from the open wound or scab. At
the margin of the scab the silvery fragments of broken skin
may often be seen. The extent and nature of the injury
depend largely on the date of infection and the variety of
apple or pear. When attacked quite young the fruits are
often much distorted and may even be injured to such an
extent that they fall off. In other cases, especially with pears,
more or less severe cracking takes place (Fig. 2). If the attack
occurs late in the season the fungus does not make so much
headway, small scabs or spots only being formed.

The spores produced on the fruit, as in the case of those
formed on the leaves and wood, may infect leaves, young
twigs, or other fruit. From this it will be noted that the
fruits of apples1 and pears are subject to infection right through
the growing season. Susceptibility to fresh infection ceases
when the growth is completed and the fruits are picked and
stored.

U

On the Wood. — For many years the mariner in which the
scab fungus lived through the winter was unknown. The
matter has, however, been carefully investigated at Wye
College and it has been clearly established that the fungus
passes the winter in a dormant condition on the young wood
of many varieties of apples and pears. These diseased shoots
provide for the initial infections the following season. Affected
wood is readily distinguished by its blistered appearance
(Fig. 4) due to the formation of numerous small cushion-
like growths of mycelium, just beneath the bark. These
cushions remain dormant during the winter. As soon as
the spring arrives they recommence growth and finally
liberate thousands of spores which infect the young leaves
and thus start afresh the life-cycle of the fungus. The in-
fected wood varies considerably in appearance with the variety
of apple or pear, the blisters being large or small, scattered
as in Cox's Orange Pippin (Fig. 4), or crowded together, as
is often the case with Lord Sumeld. So far as is possible
this scab-infected wood should be cut out when pruning
the trees in winter.

The fungus is probably capable of existing in the wood of
most varieties of apples and pears, but it is known to occur
plentifully on the following : —

Apples. — Cox's Orange Pippin, Cox's Pomona, Ecklinville
Seedling, James Grieve, Lord Sumeld, Warner's King, Welling-
ton, White Transparent, Worcester Peannain, Yellow Ingestre.

Pears. — Beurr6 Bose, Beurr6 Diel, Clapp's Favourite,
Doyenn6 Boussoch, Doyenn6 du Cornice, Jargonelle, Le
Lectier, Marie Louise, Pitmaston Duchess, St. Germain.

The young wood of Cox's Orange Pippin and Lord Sumeld
atnonst apples is often severely crippled by scab, but on
other varieties the occurrence of this stage of the disease is
to be dreaded more from the spring-infections which it causes
than from any actual damage to the wood.

Nature of Damage. — In addition to the loss due to cracked
and deformed fruit and to the disfigurement of less infected
specimens, apple scab causes a considerable amount of damage
in a way less familiar perhaps to many growers. Scabbed
fruits keep badly. They shrivel and in many cases, owing
to the growth of moulds and other fungi entering through the
scab wounds, they rapidly decay. Once a fruit becomes rotten
any sound specimens in contact with it become similarly
affected. In this way considerable loss in storage may be
incurred.

Susceptibility of Varieties. — The degree of intensity with
which the fungus attacks the fruit varies considerably.
Difference in variety, soil, locality, and seasons are all
determining factors. Whilst all varieties are probably more

15

or less subject to scab, the following may be mentioned as being
particularly liable to attack wherever grown : —

Apples — Pears —

Bismarck. Beurre Clairgeau.

Cox 's Orange Pippin. Beurre D'Amanlis.

Cox's Pomona. Doyenne du Cornice.

Duchess's Favourite. Duchesse d'Angouleme.

Ecklinville. Glou Morceau.

Keswick Codlin. Pitmaston Duchess.

King of the Pippins. Souvenir de Congress.

Lord Grosvenor. Vicar of Winkfield.

Lord Suffield. Williams Bon Chretien.
Quarrenden.
Warner's King.
Wellington.
Yellow Ingestre.

The following varieties of apples are more or less resistant : —
Beauty of Bath, Bramley's Seedling, Early Victoria, Grenadier,
Lane's Prince Albert, and Lord Derby.

Measures of Control. — These consist of cutting out in-
fected wood and spraying.

1. Infected Wood. — As it is from diseased shoots that the
scab starts afresh each year, it is worth making a determined
effort to attack the fungus in this position and also to prevent
it from infecting the wood in future seasons. When the
wood is badly diseased, as much as possible without injuring
the trees should be cut out. This should be completed by
the end of March. The source of infection will in this way
be reduced to a minimum, although a certain amount is almost
certain to remain. To prevent, as far as possible, the new
wood being affected it is necessary to spray during spring
and summer (see details below). If this is properly done it
should be possible, not only to secure a clean crop that season,
but to preserve to a large extent the new wood and thus
eliminate the source of infection for the following season.
It is particularly important to spray Cox's Orange Pippin
and Lord Suffield (which develop scab badly on the wood),
as the disease spreads from these to other varieties which
would not otherwise be attacked.

2. Spraying Mixtures. — There are two mixtures which
may be used for spring and summer control, namely Bordeaux
Mixture and Lime-sulphur* (Burgundy Mixture has not been
found suitable). Lime-sulphur is the more easy to prepare,
but Bordeaux Mixture is undoubtedly the most effective,
though in the case of a few varieties of apples it is apt to cause

* If caterpillars are likely to prove troublesome, lead arsenate paste
may be added to either of these mixtures at the rate of 1 Ib. to 20-25
gal. of the mixture. Further details as to the use of this substance,
which it should be remembered are highly poisonous, will be found in
Leaflet No. 4 ( Winter Moths], obtainable from the Ministry and also
included in Sectional Volume No. 2, price lOd.

16

scorching. These varieties include: Beauty of Bath, Cox's
Orange Pippin, Duchess's Favourite, Gladstone, James Grieve,
Lady Sudeiy. On all of these a lime-sulphur spray only
should be used. Bordeaux Mixture is safe to use on all
varieties of pears and can be relied upon to prevent scab.

Application of Mixtures. — The dates at which the mixtures
should be applied are of considerable importance. In very
bad cases of apple scab two or even three applications are
necessary, the first immediately before flowering, the second
as soon as the petals fall, and the third about three weeks
later. As a rule, the first spraying may be omitted.
Where labour and time necessitate one spraying only, the
third should be omitted in preference to the second. On
pears the first spraying should be given directly the fruit
is set and a second spraying three weeks or a month after-
wards. It is particularly important with pears to spray the
under surfaces of the leaves as far as possible.

The spraying should be carried out on a still day, and the
mixture applied through a fine nozzle with a strong pressure
and the leaves uniformly wetted with a very fine mist.
Spraying should cease before the leaves begin to drip.

Preparation of Bordeaux Mixture. — The best formula for
apple spraying in this country is: Copper sulphate, 4 lb.,
best quicklime (in lump form), 4 lb., and water, 50 gallons.
The copper sulphate* should be dissolved in a small wooden
vessel at the rate of 1 gal. of water per 1 lb. of sulphate (iron,
or tin vessels must not be used). The limef should be slaked
to a fine paste with a little water in another vessel, and water
added gradually to make a milk, and finally diluted in a
large barrel to the requisite amount (46 gal.). The 4 gal.
oT copper sulphate may now be poured slowly into the diluted
milk of lime and the mixture stirred thoroughly during the
process. The two solutions may be kept separately for a
long time, but after mixing the solution should be used as
soon as possible — at all events within 24 hours.

Stock Solutions. — When used on a large scale it may be con-
venient to make up stock solutions of each ingredient which
may be diluted down and mixed as required. For this purpose
50 lb. of copper sulphate may be dissolved in 50 gal. of water,
and 50 lb. of lime, slaked and diluted to 50 gal. of nvlk of
lime. Each gallon will then represent 1 lb. of copper sulphate
and 1 lb. of lime. When required for use, the barrels should
be thoroughly stirred and the requisite number of gallons

* Growers are strongly advised to buy their own copper sulphate and
lime, and prepare their own mixture It is of great importance also
that only pure copper sulphate be used and a guarantee of 98 ppr cent,
purity should be obtained.

t It should be remembered that fresh quicklime is essential for
making Bordeaux Mixture and that the air-slaked lime so often found
in builders' yards is useless for the purpose.

1

17

taken out and diluted according to the above formula. For a
50-gallon barrel, for instance, 4 gal. of lime-milk should be
removed and diluted with 42 gal. of water and when thoroughly
stirred and strained the 4 gal. of copper solution may be
slowly added.

Safety test for Bordeaux Mixture. — If the copper sulphate
and quicklime have been accurately weighed out and properly
mixed there is no need to test the Bordeaux Mixture before
use. Should there, however, be any doubt on this point a
simple and reliable test is the use of a bright blade of a knife
which must not be greasy. The blade should be immersed
in the liquid for at least a minute; if at the end of this time
the blade remains unchanged in appearance the mixture may
safely be used, but if it assumes the yellow colour of copper
more lime must be added.

" Excess Lime " Bordeaux. — On account of the scorching
of the foliage and russeting of the fruit which is liable to
occur on some varieties with the " equal lime " Bordeaux
(4-4-50), an " excess lime " Bordeaux has been advocated
in Canada and the United States. " Excess lime " Bordeaux
is now being tested at the East Mailing Eesearch Station
and" preliminary trials with a mixture made at the rate of
4 IB. copper sulphate and 12£ Ib. lime to 50 gallons of water
gave promising results.* The damage in the case of James
Grieve, Worcester and Allington, though not entirely elimi-
nated, was much reduced and the high fungicidal effect was
maintained.

Lime-sulphur. — This fungicide should always be used for
delicate varieties such as those already noted and it may
also be resorted to in cases where it is not convenient to
prepare Bordeaux Mixture. It is not so efficient as Bordeaux
but has the advantage over the latter in that the concentrated
wasH may be purchased ready-made in large or small drums
so that it is only necessary to dilute it to the proper strength.
Numerous brands of lime-sulphur of a guaranteed strength
of 1*3 sp. gr. are on the market and these are to be preferred
to those of which the strength is not guaranteed.

For spraying against scab " summer-strength " lime-
sulphur, namely 1 gal. of concentrated solution mixed with
29 gal. of water, should be used. The lime-sulphur should
be poured slowly into the water, stirred well, and used at
once. Spraying machines with copper parts should not be used
for lime-sulphur spraying. It should be noted that Cox's
Orange Pippin and James Grieve are particularly sensitive,
and for these varieties, and also for Wellington and Newton
Wonder, half " summer-strength " is necessary, namely 1 gal.
to 59 gal. of water.

* See N. H. Grubb. Tests of Fungicides on Apple Trees. Journal
of Pomology (Vol. II, pp. 93-104). February, 1921.

18

As is the case in the United States and Canada, a reduction
of crop occurs, specially with certain varieties, when lime-
sulphur is used, a certain number of fruits falling in June.
This matter is also being investigated at East Mailing.

APPLE MILDEW.

(Podosphaera leucotricha, Salm.)

Apple Mildew is an extremely widespread disease and it
occurs in almost all countries wiiere the apple is cultivated.
The mildew has been repeatedly studied, especially in
Europe and America, and, with the exception of a few
special cases, it is now possible to keep it under control if
the proper measures be adopted. In England the disease is
of common occurrence and may cause a considerable amount
of damage, especially in the case of certain varieties. During
recent years it appears to have been increasing in severity,
and reports have been received from several apple growing
centres in which, owing to serious and persistent outbreaks,
extensive damage has been caused. Pears also are attacked
by the same mildew, but in this country not severely.

Cause of the Disease. — Apple mildew is caused by the

fungus Podosphaera leucotricha. It is closely related to the
other true mildews, such as those of the Hop, Eose, and Goose-
berry, and especially to that so commonly found on Hawthorn
hedges, but is distinct from all, and is confined in its attacks
to the genus Pyrus. The fungus consists of the mycelium,
very slender threads (corresponding to the spawn of a
mushroom) which creep over the surface of the host plant,
and masses of conidia or minute spores, which appear as a
white mealy powder on the surface of the mildewed area,
and which, like seeds, serve to propagate the fungus. The
mycelium of the fungus obtains its nourishment by means
of minute suckers inserted into the tissues of the leaf.

Description of Affected Plants. — Affected leaves show the
curled and white appearance characteristic of most mildews.
The mealy substance or mass of spores is at times very
abundant and hence with favourable weather the disease
spreads rapidly. The mildew commences each year with the
unfolding of the buds in spring. Both flower bud? and leaf
buds may be affected (Fig. 1). In some cases the flowers are
so injured that the whole truss fails to set and the buds die,
and if many are thus attacked a considerable loss of crop may
result. Leaf buds which are affected . may in like manner
be killed outright, but in the majority of cases they survive
and continue to grow, each new leaf which develops being
generally more or less affected with the mildew as also is the
" wood " itself. The leaves or other part of jhe tree are

19

infected by means of spores blown from the rosette of first
formed diseased leaves or flowers, and from them additional
spores are produced which provide for further infection.

The appearance of diseased trees later in the year varies
with the season and the locality, but more especially with the
variety of apple. In favourable seasons and in localities which
do not suffer badly, the mildew, except for crippling a few
shoots, does not cause extensive damage, and those varieties
which in other districts are usually badly injured may almost
entirely escape. In other localities, especially in parts of
Cambridgeshire and Worcestershire, the mildew is always
troublesome, and in the case of susceptible varieties (see list
below) may so injure the leaves that by July a considerable
amount of defoliation may take place. Such trees present a
very characteristic appearance, the young shoots being devoid
of leaves, except near the growing tips, and, if the attack has
been very severe, altogether leafless (Fig. 2). The affected
shoots are much weakened and most of them die back very
considerably during the winter. If the number attacked in this
way be not too great the mildew can, with judicious pruning,
be controlled and the trees saved, but if such measures are not
resorted to whole trees will in course of time become so
seriously injured as to be almost worthless.

Though the flower buds are often affected the fungus in
later stages is confined as a rule to the shoots and leaves,
but in a few varieties, notably in Lane's Prince Albert, it
may attack and injure the young fruit. Bad cracking of the
fruit of both apples and pears by the mildew has been reported
from the Continent.

Method of Overwintering. — As is well known, the mildew
fungi possess two kinds of spores, conidia or summei
spores, and resting or winter spores. The former are pro-
duced very abundantly in spring and summer, their function
being to distribute and reproduce the fungus during the
-growing season, the latter, which occur in minute spore
cases termed perithecia, are formed in autumn and normally
remain dormant till spring, when they germinate and
infect the new foliage. The apple mildew is remarkable
in possessing an additional method of hibernation. Though
its perithecia are now known to be produced more
frequently than was formerly thought, they are, except
on certain varieties, always scarce and often entirely
absent. When they occur they are produced on»the " wood,'1
usually at the base of the current year's growth. It
is very doubtful, however, if the resting spores contained
in these perithecia play any part in the propagation of the
disease, as it is quite certain that the first outbreaks of the
mildew in spring are to be found, not scattered! here nnd
-there on a few leaves as in cases resulting from, spore-

20

infection, but all over certain newly expanded buds. Careful
examination makes it evident that the buds were infected
before they had opened, and this implies that the mycelium
of the mildew entered the young soft bud during the pre-
vious autumn and remained in the bud in a dormant state
during the whole winter. This method of overwintering by
means of hibernating mycelium is rare amongst mildews,
but occurs also in the mildew of hawthorn, coppiced oak and
in that of the garden Euonymus.

The infected buds as they open soon produce a crop of
spores, consequently the leaves and flowers early assume the
mildewed aspect and appear as white mealy rosettes (Fig. 1).
These mildewed rosettes, which arise exclusively from the
hibernating mycelium, are termed " primary " infections, and,
as it is from them that all later infection directly or indirectly
arises, it is clear that in seeking to eradicate the disease it is
of the utmost importance that they should be removed.

The buds infected with hibernating mycelium are most
often fruit buds, either terminal or spurs, but wood buds,
especially terminals, are also invaded. The infected buds
can hardly be distinguished from the normal, but all shoots
showing the whitened appearance of the " wood " (Fig. 2),
recognisable even in winter, are liable to contain infected
buds and should in consequence be pruned away. If this
were done thoroughly the trees would be largely freed from
mildew, as little secondary infection (unless introduced from
neighbouring gardens or other trees) could arise.

When the leaves only have been attacked and the " wood "
and buds have escaped infection, the mildew disappears with
the normal shedding of the leaf in autumn.

Susceptibility of Varieties. — The varieties most suscep-
tible to apple mildew are Lane's Prince Albert, Bismarck, and
Cox's Orange Pippin. In the Wisbech and Evesham districts
large plantations of the first two varieties may be seen com-
pletely ruined by mildew attacks. Amongst other varieties
which frequently suffer are Bramley's Seedling, Grenadier,
Irish Peach, Lord Grosvenor, Lord Suffield, and Mr. Gladstone.
Very frequently young stocks 2 or 3 years old, both in this
country and in America, are badly attacked by mildew, and in
some cases killed outright.

Measures of Control. — 1. Winter Pruning. — Since the new
outbreaks commence each year from infected buds these latter
should, as far as possible, be removed. When pruning the
trees all shoots which were mildewed during the past summer,
and recognisable in winter by the white appearance of the
young " wood " should be cut away. The operation should
be carried out early, as if delayed beyond the new year the
distinction between the mildewed and the non-mildewed shoots
may be lost.

•s

I

f^^

3-g

ii

< s

?,0— 1

FIG. 2. — BROWN EOT OF APPLES. Two mummied apples as found on the
trees in winter. The mummies should be removed as they are
permeated with the fungus which will develop spores and thus bring
about the infection of the new apples in summer.

I

Fin. 3.— BROWN ROT OF APPLES. Fruiting spur, from a tree of
Lord Derby, bearing pustules of the same fungus.

21

2. Cutting out in Spring. — When the buds expand in the
spring the trees should again be gone over and all affected
shoots, whether flower trusses or leaf -buds, carefully cut off
and burned. If this operation and the previous one were
carried out thoroughly, there would be little or no trouble
with mildew unless the trees were infected from an outside
source.

3. Spraying. — If as a result of the removal of all primary
infections the trees are clean, but are known to be liable to
serious infections from neighbouring gardens and other mil-
dewed trees, it is advisable to spray with lime sulphur shortly
after the blossoms set and again later if necessary (1 gallon
lime sulphur to 30 gallons water, except for the delicate
varieties, Cox's Orange Pippin, James Grieve, Newton "Wonder
and Wellington, for which 1 in 60 must be used).* This
spraying, though of no value for keeping primary infection in
check, will do much to prevent secondary infections, and will
in this way help to reduce the number of bud infections which
initiate the new outbreaks the following season.

BROWN ROT OF APPLES.

(Monilia fructigena, Pers.)

Brown Eot of Apples carries off hundreds of tons of ripe
or half ripe fruit every season. The soft brown patches of decay
commence as mere spots and gradually increase in size until
the whole apple is affected, while, small pustular swellings
appear beneath the skin and soon burst through as yellowish,
powdery, cushion-like outgrowths, usually in concentric circles
(see Fig. 1 b and c). The diseased apples ultimately shrink
in size and the skin becomes wrinkled. Such fruits, when
hanging loosely, are easily detached, and many fall to the
ground during a high wind; the rot continues to develop on
these windfalls, and more pustules are produced, which act as
sources of further infection. When, however, a diseased
apple is in contact with other apples, or with a branch,
the pustules produced at the point of contact become adherent
and may so attach the apple to the tree that some little force
is required to detach it.

Method of Attack by the Parasite. — The disease is caused
by the fungus Monilia fructigena,, Pers. The powdery pustules
which appear on the affected apples are the reproductive
bodies of the fungus growing in the flesh of the fruit, and
each consists of numerous chains of spores or reproductive

* Lime-sulphur wash is now made by many firms, and is readily
obtainable, but the manufacturers should be asked to guarantee that the
concentrated wash is of 1*3 specific gravity.

22

bodies. The spores fall apart readily and are scattered by the
wind or carried by insects to other apples. If they gain access
to the flesh of an apple through a, cut or rupture of the skin
they germinate, producing fungal threads (mycelium) which
develop within the tissues of the apple and cause the charac-
teristic " brown rot." The rot extends very rapidly and within
a week or ten days the' fruit will be destroyed.

When a diseased apple is in contact with others on the
tree, the latter may become infected by contagion; frequently
a bunch of apples is found which shows fruit with the rot in
various stages of development. Fig. 1 is a photograph of such
a cluster of apples obtained in late summer from a tree of
Warner's King. The disease had commenced in the small
withered apple (a); below this and in contact with it is another
apple (b) which had evidently been affected for some time as it
is much shrunken and wrinkled. The rot had extended from
this apple to the one on the left (c), which, at the time the
photograph was taken, was permeated with the fungus and bore
numerous powdery pustules; but it was only very slightly
shrunken and its surface showed but little wrinkling. On the
right is an apple which is still quite sound.

Mummied Apples and Methods of Overwintering. — Those
diseased apples which become attached to the tree usually
remain in that position throughout the winter, becoming dry
and shrivelled, and they constitute the so-called " mummied
fruits (Fig. 2). Many of the spores on the pustules of these
" mummies " are washed away by rain in winter or dispersed
by the wind; others remain on the pustules but usually lose
their power of germination. As summer approaches, however,
the ' ' mummies ' ' produce a new crop of spores and cause
infection of the young fruit. A ' ' mummy ' ' frequently infects
the growing apples through direct contact, but in any case
apples in the neighbourhood of a " mummy " are liable to
spore-infection and such newly-infected fruit will soon produce
myriads of spores which serve to spread the disease. The
spores are very minute in size and are easily dispersed by
wind. Insects, too, crawling over the fruit, may not only carry
the spores from one apple to another, but, in the case of biting
insects, such as wasps, produce wounds which enable the spores
to reach the exposed flesh of the apple, where they develop
mycelium and reproduce the rot.

Spur Canker. — On some soft-wooded varieties of apples
(e.g., Lord Derby and James Grieve) the disease may extend
along the stalk of the affected fruit into the fruiting spur; or
even as far as the branch itself, producing in the latter a
canker round the base of the spur. In this capacity of form-
ing cankers the fungus resembles the " Blossom- Wilt " fungus
(see Leaflet No. 312). 5The two diseases are, however, quite

23

distinct. In the case of " Blossom- Wilt " infection occurs
through the open flower while, in the disease here described,
infection takes place, so far as is known, only through the
fruit. Fig 3 shows the pustules of the Brown Eot fungus
occurring on a fruiting spur.

The Disease on Stored Apples. — The Brown Eot fungus
attacks apples not only while still growing on the tree, but
also after they are picked and stored. The fungus spreads
readily by means of spores and by direct contact in the store
room; it may cause serious losses. Through careless selection
and packing extensive decay may also occur in apples boxed
for sending to a distance.

Under certain conditions stored apples affected by Monilia,
fructigen'a turn black, the skin remaining smooth or nearly
so for some time and bearing few or no pustules. Although
there is often no evidence on the exterior of such apples that
a fungus is present, the flesh is permeated by fungal threads,
and particles of the flesh, placed on suitable culture media,
give rise to the growth and pustules typical of Monilia
Jructigena.

Measures of Control. — 1. Removal of Mummies in Winter.
— The best preventive measure is the removal and destruction
of all mummies during winter. The mummies should be
collected and either burned or deeply buried.

2. Removal of Affected Fruit in Summer. — In gardens and
small orchards it is possible to go over the trees at intervals
during the season and remove all fruit showing the slightest
signs of Brown Rot. In large orchards such a course
is not always practicable, but every effort should be made to
remove diseased fruit as early as possible. It migKt be removed
at the time of picking, the diseased apples being gathered and
either destroyed at once or dropped on the ground, where they
are less dangerous than if left hanging on the tree.

3. Cutting out of Cankers and Infected Spurs. — All infected
spurs, together with cankers on the stem, should be cut out.
This operation is best performed in summer when the dead
or dying spurs are conspicuous, but it may be carried out in
winter provided it is completed before the fungus resumes
its growth in spring.

4. Picking nnd Handling. — Brown Rot will set in wherever
there is a wound. The greatest care therefore should be taken
not to injure or bruise the fruit, especially during the picking
and packing.

5. Precautions as to Storage. — When fruit is to be stored
all fruit showing signs of Brown Rot should be discarded, for
the disease will not only continue to develop in the affected
individuals, but will spread to others. These precautions
should be regarded even more scrupulously when apples are
boxed for transmission to a distance.

'24

BLOSSOM-WILT OF APPLES.

(Monilia cinera, forma mail.)

Blossom-Wilt is a comparatively new disease. It is a
trouble which affects not only the blossoms but the spurs and
the shoots. The disease was first described from Kent, where
it is very common, but it is now known to occur in many other
parts of the country and to be responsible for serious losses of
crop. In some orchards it has reached epidemic proportions
and in many cases the apple crop may be reduced to one
quarter or even less. Lord Derby is the variety most often
attacked, but Cox's Orange Pippin, James Grieve and many
others also suffer.

The disease is caused by the fungus Monilia cinerea, a very
near ally of Monilia fructigena, the cause of Brown Eot of
apple fruit (see Leaflet No. 86). The two fungi may be distin-
guished by the colour of the spore-pustules, those of the
former being a pale grey and of the latter being buff. Other
and sharper distinctions are discernible by laboratory methods.
M. cinerea is best known on plums and other stone fruit, and
the form which attacks apple blossom is known as forma mali.

The "Blossom-Wilt" Stage. — The first evident symptom
of the disease, which, in its early stages, is easily overlooked,
is the " blossom-wilt " condition (see Fig. 1). About a fort-
night after the flowers begin to open, the leaves on some of
the spurs will be seen to be beginning to wilt; within a day
or two these drooping leaves become brown and withered,
usually with incurved margins. The flowers of such trusses
will be found to have wilted and to be brown and dead. In
susceptible varieties many trusses on a tree may be affected.
The destruction of the blossoms is, however, only a minor part
of the damage caused by the fungus, as, after killing the
flowers, it usually proceeds to grow through the flower stalk
into the spur.

The life-history of the parasite has been worked out by
Dr. H. Wormald, of Wye College.* He showed that the
infection of the flower-truss takes place through spores
alighting on the stigmas of the blossoms. These spores are
derived from the spore-pustules which develop on the spurs
and wood which was attacked the previous season. It was
ascertained that the death of the flower follows very soon
after the infection of the stigma, and within a fortnight the
fungus has so far advanced as to a*ffect the young leaves and
cause them to droop as noted above. If the weather is dry
the fungus keeps within the tissues of the flowers and leaves
and does not show itself at all from without, but in moist
weather it develops externally on the flowers and flower-stalks

* For a fuller description see article by H. Wormald in Jour. Board
Agric.., Vol. XXIV., Aug. 1917, pp. 504-513.

5o«>

n
•H

24-1

25

in the form of small, rounded, grey pustules similar to those
found on the spurs.

The Attack on the Wood. — On the wood the fungus
may (1) attack and kill the spurs, (2) grow along the spur into
the shoot and form a spur canker, and (3) occasionally extend
considerably in the branch and kill extensive areas. An
example of an affected spur with a canker at its base is seen
in Fig. 2, whilst Fig 3 shows the killing of large areas of the
branch. The amount of canker which develops on the shoot
varies. In some cases the shoot may be only half girdled
(see Fig. 2), but in other instances the girdling may be com-
plete, in which case the portion of the branch above the canker
dies. The mycelium in the canker does not appear to spread
after the end of June, as the cankers do not increase in size
after that date.

In Figs. 2 and 3 the grey spore-pustules of the fungus are
seen on the surface of the bark, having been pushed out by
the mycelium from within. Though the pustules develop
on the flowers soon after infection, they do not make their
appearance on the newly-killed spurs and shoots until the
winter. About December immature pustules may be found
•bursting through the bark, which continue to increase in
number during the winter months. In spring they become
powdery in appearance, owing to the ripening of the spores,
and by the time the trees are in bloom they are liberating
spores in myriads. These are blown on to the opening flowers
and produce Blossom- Wilt.

Method of Spread. — As already explained, the disease
is spread by the spores of the fungus being blown on the open
flowers. It was found at Wye that the wilted trusses are far
more numerous in the vicinity of cankers and dead spurs
bearing spore-pustules, those trusses situated immediately
below such spurs or cankers being particularly liable to become
attacked with Blossom-Wilt. Thus, though infection through
spores carried by the wind from one tree to another or from
one plantation to another does to a certain extent take place,
the great majority of new attacks on any given tree arise from
spores produced by pustules on the same tree. The great
importance, therefore, of removing all wood which bears
pustules is obvious.

Damp weather favours spore-production; and such weather
also retards the " setting " of the flowers and so keeps them
in a condition in which they are liable to become infected by
spores for a longer period.

Varietal Susceptibility and Damage Caused, — The

varieties Lord Derby, Cox's Orange Pippin and James Grieve
are extremely susceptible, but, in addition, the disease occurs
on the following: — Ecklinville Seedling, Duchess of Oldenburg/
Worcester Pearmain, Allington Pippin, Early Julyan, Lane's

Prince Albert, Lord Grosvenor, Prince Bismarck, Chelmsford
Wonder, Newton Wonder, Domino, Beauty of Bath, Warner's
King, Keswick Codlin, Bramley's Seedling, Duchess'
Favourite, Rival, Fearn's Pippin, Dartmouth Crab, Ribstone
Pippin, and Han well Souring.

[n the highly-susceptible varieties, the Blossom- Wilt stage
is very conspicuous, and it is not uncommon to find 50-75 per
cent, of the flower spurs killed by the disease in a single season.
In Lord Derby the wood is very extensively attacked and
stretches of wood 1-2 feet in length may be found completely
killed, the mycelium spreading from the spur-canker along the
shoot and forming hundreds of spore-pustules (see Fig. 3).

Of varieties resistant to Blossom-Wilt, Bramley's Seedling
is the only one which shows this character to a very marked
degree.

Measures of Control. — 1. Spraying. — So far little success
has been obtained on a field scale by spraying. Owing to the
delicate nature of the floral organs, a strong spray cannot be
applied to protect the stigmas from the spores which produce
Blossom- Wilt, and owing to the difficulty of wetting them,
it is almost impossible to kill the pustules on the wood.

The most promising results have been obtained by a winter
wash containing caustic soda and soft soap, which if properly
applied will sterilize the pustules for two or three weeks.
The wash should contain 1 per cent, caustic soda and
1 per cent, soft soap, that is, 1 Ib. caustic soda and 1 Ib. soft
soap in 10 gallons of water. A slightly stronger solution would
be preferable, but, unless the mixture can be kept warm, it
becomes glairy or curded. The wash should be applied im-
mediately previous to the swelling of the buds.

-2. Cutting Out. — The most successful line of treatment at
present is the cutting out and burning of all infected spurs
and cankers.

The periods at which this can be done should be carefully
noted. Wherever possible, cutting out should be carried out
during the summer when the brown withered leaves of infected
spurs contrast with the living ones and constitute a sure guide
to places where the knife is required. If, through lack of
labour, pruning is left over until winter, the affected parts are
not so readily seen and the cutting away is therefore more
difficult. Cutting out can take place any time in winter, but
it is essential that it should be finished before the buds open.
During the winter, however, even with the greatest care, a
few diseased spurs are liable to be overlooked. Trees should
therefore be gone over a second time.

Another point to note when removing the dead spurs and
cankers is that it is necessary to cut back to the sound living
wood. It is not sufficient to break off the withered trusses
since some of the mycelium is invariably left behind and this
continues to develop and to produce pustules.

,

(Photo: Dr. H. Wormald.)

FIG- 1. — CROWN GALL as the base^of the stem on young seedling
apple stock.

3—1

27— 1

27

CROWN GALL.

(Bacterium tuinefaciens.)

The swellings or galls termed Crown Gall occur on a great,
variety of herbaceous and woody plants. They are abundant,
on various kinds of nursery stocks, particularly apple stocks,
but, amongst fruit, pear, quince, plum, cherry, apricot, peach,
raspberry, loganberry and blackberry also suffer, and in
America bad cases have been noted on gooseberries. Plants,
of many other kinds are attacked, chrysanthemum, hollyhock,
beet, mangel, and amongst stocks other than fruit trees,
Itanetti roses axe particularly susceptible. The disease
is now found in most parts of the British Isles, nurseries
having" distributed affected stocks not only all over England,
but to many other parts of the world. Although not so serious-
in its effects as are most diseases the health of plants attacked
by Crown Gall usually suffers sooner or later, the effects
being seen in dwarfing of growth or direct injury to roots
or branches, some plants being more seriously affected in this
way than others. Apart from actual injury, the disease
causes the, plants to appear unsightly and renders the stock
unsaleable. For this reason it is important to take what steps
are possible to control the disease.

It should be noted that galled plants will not be certified
for ej-pott, hence special attention to preventing attacks
should be given by all growers of fruit trees, roses and other
plants for export to America or other countries.

Cause of the Disease. — Crown Gall has been known for

very many years and has been the subject of much speculation
and investigation, but only comparatively lately has its true
nature been discovered. The disease in all its forms and on
a large number of plants has been the subject of the most
intensive study for over fifteen years by Dr. Erwin F. Smith
and his colleagues in the Department of Agriculture at
Washington. They have proved beyond question that the
disease is infectious and is caused by a bacterium which they
have named Bacterium tumefaciens. The galls can be produced
at will by inoculating appropriate tissues of healthy plants
with cultures of the bacterium. The bacteria are excessively
difficult to see in the tissues of the galls and, hence, for long-
escaped recognition. The American work has, however, quite
recently been confirmed in England in the case of the Paris
Daisy (Chrysanthemum frutescens); the same bacterium has
been isolated and inoculations with the organism have produced
new Crown Galls. * Further work on the Crown Gall is being
pursued in this country at more than one research institute.
On the practical side the disease is being investigated at the

* Ann. BoL, Vol. XXXV. pr. 137-8.

28

Jiast Mailing Research Station, and the Ministry is indebted
to Mr. N. H. Grubb of that station for the notes given below
on English fruit stocks.

Description of the Disease. — In its typical form Crown
Gall manifests itself as roughly spherical swellings which may
vary in size from that of a pea to that of a cricket ball. The
galls are at first white and soft, but gradually become hard and
dark in colour. On nursery stock they often decay and fall
away at the end of the season. Hedgecock has distinguished
the more woody perennial form usually found 011 older trees
as " hard crown-gall." These are slower growing, but continue
to enlarge for many years and become very hard. On old
plum trees very large galls, 8 or even 10 inches in diameter,
may sometimes be found.

The position of the swelling also varies considerably. As
the name implies the galls are often found at the crown or
collar of the attacked plants, but they may occur on the roots
or on the stems and branches. The latter is the usual form in
raspberries. Where root-grafting is practised they are particu-
larly frequent at the junction of the stock and scion.

In the case of apples it is found at East Mailing that in
stocks raised from layers or stools, by far the largest propor-
tions of galls occur on the base of the stem at the point where
it was separated from the parent stool or layer. Infections
higher up the stem are sometimes found and others on lateral
roots, but galls at the ground level, or " crown *' of the stock
are rare. The galls on these stocks usually take the form of
rough warty swellings of soft tissue which vary in development
from a slight irregularity of the " callus " to large, more or
less spherical galls up to three or four inches in diameter.

A form very different in appearance is frequent in America
and is known as " Haify Root " (see Fig. 3). In this case
dense tufts of fibrous roots are produced on various parts of
the root system. These have been found to be caused by the
same bacterium as that causing Crown Gall, the bacteria being
found in the flattened tumours on the main root from which
the tuft of fibres arises. Various types of Hairy Root have
been recognised, but in this country, though suspicions cases
Have been observed, no instances have been seen which can
with certainty ^6e referred to that disease.

Method of Attack. — It has been proved by direct inocula-
tion and other experiments that the bacteria gain entrance
through wounds and in all probability they cannot enter an
uninjured surface. This has been confirmed by trials in nur-
series where plants intentionally wounded gave rise to four
times as many galls as those not so treated. It is well known
also that seedlings are less often attacked. In the case of
cuttings and root grafts the period during which the plants
are vulnerable is jnst prior to and during t-be formation of the
callus.

29

Once inside the tissues the bacteria multiply, but instead
of destroying the cells they stimulate them to divide and en-
large, much in the same way as the Wart Disease fungus does
in the case of the potato tuber, and a gall or tumour is
produced. Crown Gall is, however, peculiar amongst plant
parasites, in that, in certain plants at any rate, the affected
areas form strands (termed tumour-strands) in the healthy
tissue along which the bacteria travel, and from which other
galls arise, sometimes at a considerable distance from the
original gall. This fact explains why it is that new galls arise
in other spots if the primary gall is removed.

Extent of Injury. — The amount of damage caused by
Crown Gall varies greatly in different crops. In some cases
it is almost negligible. Stone-fruits suffer more than apples or
pears. Soft fruits such as raspberries have been so injured in
Sweden a to bear practically no crop and similar cases have
been recorded in England. On the vine Crown Gall
is regarded as a serious disease in France and Italy. Manetti
roses, which are singularly subject to attack (see Fig 2), are
said to be much impaired for the uses for which they are
.required in America.

Few precise data have yet been collected in this country on
the nature and extent of the injury which is caused. In apples,
recent work tends to show that its effects are comparatively
slight and not so serious as was at one time thought. The
amount of injury varies in different varieties, not only as
regards the stock, but with the variety of apple grafted on it. In
America affected trees are liable to be retarded in development
and dwarfed in growth. Early death sometimes takes place.
The root system is often interfered with, as a result of which
the trees suffer. On the other hand, in some cases the stocks
appear to be able to throw off the disease entirely. In England
it has been observed that during tEe first few years after plant-
ing galled trees often grow almost, if not quite, as well as
trees free from galls. The amount of injury which takes place
depends largely on the position of the gall on the tree, galls
at the crown, for instance, being more serious than those on
lateral roots. Those galls, moreover, which have a narrow
neck are less harmful than those which are attached by a
broad base.

In addition to direct damage Crown Gall may be dangerous
in that it affords a means of entry for various 'parasitic fungi.
Further, the distribution of diseased stock among new and
clean orchards may lead to serious injury to other plants such
as raspberries and loganberries, which may subsequently be
planted. It is quite plain, therefore, that the disease cannot
be disregarded. Galled plants cannot be exported and will
not be accepted readily by buyers at home.

30

With regard to the susceptibility of apple stocks it has
been noted at- East Mailing that the different sorts of Paradise
differ markedly. The most susceptible are certain types which
root rather poorly when layered and which consequently have
to be torn or broken from the stool, instead of being cut with
the knife, e.g., the Doucin. While evidently this is due partly
to the ragged wound made by tearing the stock from the stool,
there is, apart from this, evidence of variation in susceptibility.
No kind of apple stock has yet been found to be immune, and
traces of the disease have been seen on several kinds of " Free "
stocks, which have been propagated by layers, as well as on
all the common forms of " Paradise."

Distribution in Britain. — The Crown Gall bacterium is
widely distributed in gardens and orchards and especially in
the soils of nurseries. This is evident from records kept by
the Ministry of Agriculture and receives confirmation from
the frequent occurrence of galls on stocks, canes and trees
received from all parts of Western Europe at the East Mailing
Research Station over a period of nine years. Judging by the
large percentage of attacked plants it would appear that the
disease must be exceedingly prevalent in certain localities.
From Erwin F. Smith's work it is clear that the bacteria*
causing the galls show little specialisation into distinct races
according to their hosts, but can, if opportunity offers, attack
a large range of plants. The importance, therefore, of pre-
venting the distribution of diseased stock and the infection of
clean areas is obvious.

Measures of Control — 1. Clean Soil. — In the cas'e of
Manetti roses which are particularly liable to infection it is
highly desirable when propagating these plants to avoid soil
badly infested with the Crown Gall bacterium. If possible new
or entirely clean land should be selected.

2. Prevention of Wounds. — It has been shown that the bac-
teria invade the plant through wounds, and that in all probability
they cannot enter an uninjured surface. The greatest care,
therefore, should be exercised in not injuring the stocks more
than necessary. This applies not only to their . propagation,
but to subsequent cultivation; all unnecessary shifting,
heeling-in, &c., should be avoided.

3. Budding and Grafting. — Where Crown Gall is liable to
occur, special care should be exercised in the operations of graft-
ing and budding, since very frequently the junction or point of
union is subsequently planted below ground, thus offering a
wound for immediate infection. Scions should be placed on
stocks of appropriate size and the graft protected by careful
wrapping.

4. Treatment of Attacked Stocks and Trees. — It should be
clearly understood that the removal of a gall from a plant does

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30-1

FIG. 4. — SILVER LEAF IN FRUIT TREES. Cross eection
of branch showing diseased wood, dark in colour,
and sound wood. Twice natural size.

YIG. 5. — SILVER LEAF IN FRUIT TREES.
Blenheim Orange Apple tree that has
died after being regrafted. The scions
also have died and the stock bears
fructifications of Stereum purpureum.

(Reproduced from Journal of Agricultural Science, Vol. V, Part 3, by
kind permission of Mr. F. T. Brooks and Camb. Univ. Press.)

31—1

31

not necessarily free the plant from the disease. As explained
above, the bacteria, in certain plants in any case, spread in the
tissues by means of the tumour strands and, if the gall is
merely cut off, the disease may break out again later. On
the other hand, if a deeper wound is made with the object of
cutting out the whole infected area, the wound may be more
serious than the gall itself and may injure or even kill the tree.
If the galls are on the lateral roots and only a few are attacked
the affected roots should be cut out, but if badly galled the
trees should be burned.

SILVER LEAF IN FRUIT TREES

(Stereum purpureum.)

During recent years fruit growers have suffered very severe
losses from the disease known as Silver Leaf. The trouble is
particularly common on trees belonging to the Order Ros&cece,
for example, plum, apple, almond, apricot, cherry, peach,
nectarine, and Portugal laurel, but trees and shrubs belonging
to altogether different orders are also affected. Amongst these
may be mentioned currant, gooseberry, horse chestnut, labur-
num, and tree lupin. It is, however, to the plum that Silver
Leaf has caused most serious loss, the Victoria variety
especially having been attacked and killed in large numbers, so
much so that unless drastic measures are taken to prevent the
spread of the disease this valuable variety of plum is threatened
with extermination. It is also a frequent source of death to
flowering almonds in London suburban gardens.

Description. — As the name implies, the foliage of trees
suffering from Silver Leaf develops a silvery or leaden sheen
which usually commences in a single branch. After a time the
affected branch begins to die back, and as branch after branch
becomes attacked large portions of the tree die, with
the result that in time the whole tree is killed. The
length of time between the first silvering and the death of the
tree varies with the species or variety, the extent of the
attack, and other factors. If affected branches are removed
as soon as the first signs of attack are manifest, trees may
often be saved, and in a few cases affected trees recover
without any treatment whatever. As a general rule, however,
unless promptly attended to, the tree inevitably succumbs.
After the death of a branch, but not necessarily after the whole
tree has died, a purple fungus named Stereum purpureum
will usually be found pushing its way through the dead bark
and forming flat incrustations (see Fig. 2) or bracket-shaped
bodies (see Fig. 3) on the surface of the dead bough and
branch. These structures are the reproductive portions of the
fungus, and produce innumerable spores. The fungus itself
had been present in the wood of the tree for one or more

32

seasons and had been responsible, as explained below, for the
silvering of the foliage and the death of the branches, but
it was only after the latter had died that it proceeded to develop
externally and to produce its fructifications.

Cause of the Disease. — Although further research is
needed as to the nature of Silver Leaf, and many
problems connected with it await elucidation, authorities
are agreed that Stereum purpureum is capable of producing
Silver Leaf, and that this fungus is primarily responsible for
the disease in plum orchards all over the country. It is not
maintained that Silver Leaf is invariably brought about by
this fungus, but exceptions are comparatively rare and from the
fruit-grower's point of view they are negligible.

The scientific knowledge which is possessed as to Silver
Leaf is due exclusively to the work of British botanists, and
for the last ten years Mr. F. T. Brooks of Cambridge has
made the disease a special study. This writer pointed out-
that the wood of a plum branch possessing silvered foliage
practically always shows dark brown or blackish markings
when cut across (see Fig. 4), although in the early stages of
attack this discoloration is usually considerably below the
silvered leaves. Microscopic examination revealed the fact
that the discoloured portion of the wood contained the fine
threads of fungus mycelium and that the brown colour was due
to the presence of a brown gum. Brooks further showed
that if portions of the mycelium of Stereum purpureum (derived
either from naturally-grown specimens or from the fungus
grown in pure culture) were inoculated into a healthy plum
under conditions which preclude the possibility of infection
from other sources, precisely similar mycelium was produced,
together with the discoloration of the wood and the formation
of gum. The foliage also developed the characteristic silvery
appearance. This experiment has been performed over and
over again on various kinds of plum and at different seasons
of the year with the same result. Inoculations with the spores
of the fungus also produced the same effect. No silvering of
the foliage, however, occurred if cuts were made but no
Stereum purpureum inserted, or if some other fungus (such
as Stereum hirsulum) was employed instead. It is important
to note that if the silvered foliage is examined no mycelium is
found in the tissues, nor as a rule is there any found in the upper
part of the affected branch and twigs, at all events in the
early stages of attack. The fungus is confined to the dis-
coloured portion of the wood which occurs lower down, though
its effects are seen in the silvering of the foliage. The silver-
ing itself is found to be primarily due to the accumulation of
air below the epidermal cells, much in the same way that,
owing to the presence of air, a white streak often shows in
a block of ice when it is cracked. The epidermal cells have a
tendency to break away from the cells immediately below, and

33

the presence of air interferes with the normal reflection of
light from the surface of the leaf.

From corespondence in the horticultural press it is evident
that many persons have not appreciated the above facts and
are still concerned with theories and speculations as to the
cause of Silver Leaf, or occupied with side-issues. There is
occasionally a silvering of plants, due to an unknown cause,
which simulates Silver Leaf, and the loss of colour in leaves
suffering from a bad attack of red spider or aphis may also
sometimes be confused with the true disease. It should be
clearly understood, however, that Stereum purpureum is the
responsible parasite in the fruit plantations of the country, and
that no other theory which has been put forward to account
for Silver Leaf in association with the dying back of fruit
trees has been proved.

Description of the Fungus. — It is important that growers
should learn to recognise the fungus causing the disease, as it
may occur on the dead wood of other trees besides plum, and
should not be allowed to persist in gardens. By the Silver
Leaf Order, moreover, all wood harbouring it must be burned.
(See last paragraph of this leaflet.)

The fructifications of Stereum purpureum are purple-mauve
when fresh, often with a white or pale woolly margin, but
they change colour with age. In consistency they are leathery.
They are very variable in form and appear either as flat in-
crustations up to several inches long covering the under surface
of the branches or on the sides of the trunk (see Fig. 2), or as
bracket- shaped projections of £-in. to 1-in. in width, and
arranged in tiers one above the other (see Fig. 3). In this case
the upper surface is hairy, and the under surface smooth. The
purple colour, however, is the characteristic feature and no
other fungus of this colour occurs on plum.

The spores are produced in abundance on the smooth under
surface of the fructifications. Although the latter shrivel up
in dry weather they are capable of reviving with rain and dis-
.charging a fresh crop of spores. In this way the spore-discharge
from a given fruit b'ody may last over a long period. The
fructifications appear at any time of year when the weather is
moist and mild, but they are produced in the greatest abun-
dance after the heavy rains of autumn.

Method of Spreading — The fungus is propagated by the
spores whicE are freely distributed by wind. They germinate
readily in moist weather, and the fungus gains admission to
the trees through wounded surfaces such as cracked branches,
injured trunks, fissures in the bark, and any other unprotected
wounds. On germination, a mycelium is produced which first
of all attacks the dead and injured tissues, but subsequently
invades the water-conducting cells and attacks the living part
of the tree. It has been shown that infection by spores cannot
take place through the sound and uninjured bark.

34

Where the roots of two trees overlap the mycelium from a
diseased root may attack a sound root if in actual contact
with it, and thus spread the disease. The mycelium, however,
does not spread through the soil itself, but in or along portions
of the woody roots. According to Brooks, however, it is not
by means of the roots that the disease is generally spread, and
in support of this it will be noted that affected trees, for in-
stance in a plantation of Victorias, usually appear scattered
about in an irregular manner, and not radiating out from
definite common centres.

Silver Leaf is especially prevalent in trees which have been
cut down and regrafted, and in the case of apples it is as a
rule only top-grafted trees which suffer to any large extent.
The varieties badly attacked when top-grafted are mostly those
which are known to callus poorly, such as Lord Sumeid, Lord
Grosvenor, Manx Codlin, Ecklinville and Pott's Seedling. The
explanation, doubtless, is that the unprotected cut stump offers
exceptional facilities for spore-infection. This infection may
not take place immediately but if it does occur and mycelium
develops in the stock it usually spreads to the graft and brings
about its rapid death. Fig 5 shows a case where the fungus
obtained an entry soon after grafting and where the scions
made little growth.

In connection with spore-infection it should be remembered
that the spores which give rise to new infections do not neces-
sarily originate from a tree of the same kind. Stereum
purpureum, for instance, is often found on laburnum and
poplars in gardens, and Brooks has shown that spores from
these host-plants may bring about Silver Leaf in plums.

Susceptibility of Varieties. — All varieties of plum are
liable to Silver Leaf. At one end of the list stands 'Victorian,
by far the most susceptible of all, with Czar (very subject
to the disease in certain localities) as second, and at the other
end River's Early Prolific and the Yellow Pershore, which are
highly resistant. The explanation of the frequency of attack
in Victoria has been sought in its heavy cropping powers which
not only weaken the trees, but bring about breaking of the
branches, thus affording points of entry for the fungus. This
is, however, probably only partly responsible, the true explana-
tion doubtless lying in those subtle factors which govern sus-
ceptibility and immunity in all kinds of plants and animals.

The question of the effect of stocks has recently attracted
attention and in some quarters it has been stated that|
Victorias grafted on highly resistant varieties partake of the
resistance of the stock. Many trees have consequently been
worked recently on the Yellow Pershore Plum, especially in
the Evesham district, but whether a permanent beneficial effect
has been produced or not is at present uncertain, since the use

35

of this stock on an extensive scale has only been practised for
the last 10 or 12 years. In the case of Victorias it is usually in
trees 20-25 years old that Silver Leaf causes such wholesale
damage, hence it is too early to form an opinion based on any
extensive trial. The, Yellow Pershore plum itself is un-
doubtedly occasionally attacked by Silver Leaf, and, according
to reliable growers, Victorias worked on it have, after a time,
shown signs of the disease. The whole question of stocks is
now under investigation both from a scientific and practical
standpoint.

Measures of Control. — Although no cure for Silver Leaf
is at present known, the following recommendations, if carried
out thoroughly, will materially help in checking the spread of
the disease : —

1. Removal of Trees and Cutting out Branches. — Some
growers advocate the removal and destruction of all trees show-
ing silvered foliage ; but, judging from experience in Cambridge-
shire on a fairly extensive scale, it appears that this drastic
treatment is not necessary. If affected trees are systematically
and energetically dealt with as soon as the disease appears, it
is possible very considerably to control its spread. To effect
this the following two operations must be rigorously
enforced : —

(a) All dead trees must be grubbed up and destroyed, and

also all trees which have begun to die back.

(b) All silvered branches, even though they show no signs

of dying back must be cut out. It will be remem-
bered that the minute threads of the fungus are
usually found in the tissues of the wood consider-
ably further down the branch than the level at
which the silvered leaves appear. Silvered
branches, therefore, mnst be cut back to a point
where no dark stain in the wood can be found.
Unless this be attended to the operation will not be
successful and the disease will spread to other
branches. It should further be remembered that,
as callus-formation takes place much more readily
if the branches are cut back flush with the main
branch or stem, it is important, if practicable, to
cut back in such a manner.

2. Protection of Wounds. — This is exceedingly important.
All wounds made by the removal of branches should be pared
over with a knife, and covered at once with Stockholm tar. As
tar often loses its effect after three months it is necessary, in the
case of the larger branches at any rate to renew the tar dressing
at frequent intervals. Wounded surfaces made by the breaking
off of branches through wind or other causes should be pared
and tarred in the same way, and in the case of the highly

36

susceptible Victoria, branches which have cracked through
heavy crops of fruit should be cut off. Trees should never
be injured more than is necessary, and in every case the wound
should be immediately protected with tar.

3. General Measures. — Accumulations of woody debris
must not be allowed. Sawn-off branches and trees that have
been grubbed up should be removed from the plantation
immediately and be used for firewood. Small branches should
be burnt on the spot. To cut down dead trees without sub-
sequently removing them is useless, and to keep a wood-pile
in or near a fruit garden is a practice that cannot be too
strongly condemned. Tree stumps, e.g., poplar, occurring on
the borders of fruit plantations should either be removed or
covered with soil, as they are liable to give rise to fructifica-
tions of Stereum purpureum. (See Silver Leaf Order below.)

Drainage also should be attended to. Silver Leaf is believed
to make more rapid headway on heavy soils and in damp
situations. Any improvement, therefore, in the drainage of an
orchard will help the trees to resist the disease.

The application of lime, moreover, must not be neglected.
Where the soil is sour through lack of lime, the general health
of the trees suffers, and, as a consequence, they more readily
fall a prey to disease.

4. Resistant Varieties. — Where Silver Leaf has been very
severe it is advisable to plant plums other than Victoria or
Czar. This applies especially to cases where the ground is
surrounded by orchards or plantations in which the disease is
still rampant and where the new trees will be in constant
danger of spore-infection. Of varieties to be recommended,
Pond's Seedling, Monarch, Purple Egg Plum and Damsons are
generally fairly resistant, whilst Pershore Yellow Plum,
Eiver's Early Prolific, Blaisdon Red and Damascene are
extremely seldom attacked.

Silver Leaf Order of 1919. — Silver Leaf has now been
scheduled under the Destructive Insects and Pests Acts of
1877 and 1907. The new Order requires occupiers of any
premises on which plum trees are growing to cut off and destroy
by fire on the premises all the dead wood on each plum tree
before the 1st of April of every year. Where the dead wood
on the trunk extends to the ground the whole tree, including
the root, must be burned. An occupier of premises on whicn
trees are growing may also be required to cut off and destroy
in like fashion the dead wood of any kind of tree whatsoever on
which Silver Leaf fungus is visible. Any Inspector of the
Ministry of Agriculture or the Local Authority may enter
premises on which he has reason to suspect the presence of
trees or bushes to which this Order applies.

Fid. 1. — WlTHKR-TlP AND BROWN ROT OF PLUMS. Mum-

mied plums. The pustules are seen on the centre plum,
and are commencing to become active and produce spores.

WlTHER-Tl? AND BROWN ROT OF PLUMS.

FIG. 2. — Blossom-Wilt. The FIG. 3.— The same 9 days after

lower Flower, which was inocu- inoculation. The calyx lobes of

lated 5 days previously, shows the lower flower have collapsed

the style commencing to turn and the stamens have withered

black, completely.

36-1

IG. 4. — WiTHER-TiP as seen in winter and spring, showing the
dead leaves which often remain attached, and the spore-
pustules (especially en the shoot to the left).

37—1

37

WITHES-TIP AND BROWN ROT OF PLUMS AND
CHERRIES.

(Manilla cinerea, forma pruni.)

The decay of ripe plums known as Brown Rot is a disease
familiar to all growers. It is a serious trouble wherever plums
are cultivated, and the losses occasioned, both on the
trees and in market consignments, are often very heavy.
Quite as serious, and at times more serious, is the attack by the
Brown Rot fungus on the flowers and shoots. This form of
attack, which is known as " Blossom-Wilt and Wither-Tip "
respectively, has only recently been recognised and carefully
studied. The details of the various Brown Rot fungi and then-
method of attack have been followed with extreme care and
minuteness by Dr. H. Wormald, of Wye College, to whom
practically the whole of our knowledge of Brown Rots, as they
occur on fruit trees in this country, is due, and on whose work
this leaflet is based.

The fungus concerned in the various attacks on plums is
Monilia cinerea, forma pruni. It is a form of this same species
(M. cinerea, forma mali) which causes the Blossom- Wilt of
apple. The two forms, however, are quite distinct. The plum
form is apparently unable to cause Blossom- WTi It of apples and
the apple form has not been found on plums. When the
number of apple orchards interplanted with plums is con-
sidered, this fact should be a matter of considerable satisfaction
to growers.

Brown Rot of cherries is caused by the same fungus as
Brown Rot of plums, and in this case, also, it attacks the
blossom and the wood as well as the fruit. The description
given in this leaflet, and also the control measures recom-
mended, would apply equally to cherries.

Blossom-Wilt. — The Brown Rot fungus produces a
Blossom-Wilt of plums similar to that found on apples (see
Leaflet 312). In the spring, spores derived from the mummied
plums (Fig. 1) on the trees, and from spore-pustules on infected
wood, are blown on to the flowers. The spores readily infect
the stigma, and the mycelium of the fungus passes down the
style and causes the death of the flowers (Figs. 2 and 3). The
mycelium subsequently passes through the flower-stalk into
the wood and attacks the shoots. Trees which have suffered
from Blossom- Wilt may be recognised by the brown, withered
leaves which usually remain attached instead of falling off.

The damage caused by Blossom-Wilt is in certain seasons
very serious. The factors chiefly contributing to an epidemic
attack are a low temperature, and a very moist atmosphere

All the photographe used in this leaflet are by Dr. Wormald, and
Figs. 2, 3 and 4 are reproduced from the Annals of Applied Biology,
Vol. V, by kind permission of Dr. Wormald and the Cambridge Un:ver-
sity Prese. •

38

during the flowering period. The former retards the develop-
ment oi the flowers, and causes them to remain susceptible
to infection for a longer period than usual, and the latter
favours the production (by means of the pustules on mummied
fruit and dead wood), of an abundance of spores.

Wituer-Tip. — In Wither-Tip the fungus attacks the young
green shoots and causes them to die-back. In this way many
of the leading shoots are killed (Fig. 4), the buds at the base are
then stimulated' to precocious development, and instead of
fruit buds, a number of weak, ill-ripened shoots are produced.

The Wither- Tip form arises from spores formed on the
pustules on mummies, on wood attacked the previous season,
and probably also from flowers killed by Blossom- Wilt. It
usually commences about 'the end of April or the beginning of
May, when some of the young shoots may be seen to wither.
The wilt does not always begin at the tip of the shoot; it may
commence some distance down, the fungus in this case entering
at a node or through a leaf. Shoots attacked by Wither-Tip
hang downwards; at first they are flaccid, but later the tissues
harden and dry, and the dead shoots may be recognised by the
characteristic curve. The dead brown leaves, being killed pre-
maturely, do not usually fall from the tree, but hang on during
winter and even until spring (Fig. 4). When it is a spur that
is invaded the fungus sometimes forms a canker similar to that
formed in apples.

Spore-pustules develop during the following winter and
spring, on all the shoots and spurs which have been killed
(Fig. 4, see specimen to left, lower part), and these give off
spores which provide for the fresh infection in spring. The
pustules are small and grey in colour, they commence to show
in December, and increase in number as spring advances.
Large quantities o>f spores are liberated in March, April and
May. The fungus in dead twigs may retain its vitality for
more than one season and liberate a crop of spores during the.
second winter after attack. The importance, therefore, of
removing such shoots is obvious.

As is the case with Blossom- Wilt, the severity of Wither-Tip
is greater in some seasons than in others. Wet, cold weather
in spring favours its development, but since epidemics of
Wither-Tip have been found to correspond with severe attacks
of aphis, it is possible there may Be a connection between them.
The aphides, by puncturing the leaves, would injure the
tissues, and render them particularly susceptible to invasion
by the fungus mycelium.

Brown Rot on Fruit. — The fruit is most often attacked
when approaching maturity, the slightest wound or bruise
allowing the spores to bring about infection. Once it has gained
an entrance the mycelium rapidly destroys the fruit, and finally
reduces it to the hard, wrinkled structure known as a
" mummy." The mummies may fall to the ground, -but usually

39

they remain on the tree until spring, stuck together in groups
of two or three by means of the fungus mycelium (Fig. 1).

Although it is on ripe or nearly ripe fruit that Brown Rot is
best known, it may also attack young fruits. These may be
infected by direct contact with mummies or with other diseased
fruit, but apart from this it is clear that wounds or abrasions
of the surface are necessary for infection. Soon after the
fruits are infected, small grey spore-pustules appear, often in
concentric rings.* The spores liberated from these pustules
infect other fruits. When the fruit is destroyed, and the dry
mummy stage is reached, the pustules for the most part cease
forming spores and remain dormant till spring, when they
regain their activity, liberate myriads of spores, and thus bring
about new attacks on the flowers, shoots, and fruits.

Varietal Susceptibility. — With regard to Blossom- Wilt
and Wither-Tip, as a general rule, Victoria and Czar Plums
suffer most. In 1920 the attack on these varieties in East
Anglia was particularly severe, and it was estimated that 80 per
cent, of the flowers and shoots were killed in certain districts.
Later in the season the trees, made new growth, but this was
very weak, and on Victoria especially much of it was sub-
sequently killed. Occasionally Monarch Plums suffer more
extensively than either of the above. Pond's Seedling and
River's Early are not usually so severely attacked.

The fruit-rot form is common on all varieties, although if
trees are extensively affected with Blossom-Wilt and Wither-
Tip the likelihood of the fruit becoming attacked by Brown Rot
is greater.

Measures of Control. — At present there are three methods
of treatment, as indicated below: —

1. Removal of affected fruits and mummies. — It cannot be
understood too clearly that new infections are mainly brought
about by the mummies hanging on the trees, and by diseased
twigs. Diseased and decayed plums should therefore be picked
and dropped to the ground at the same time as the crop is
gathered. Any mummies that are left on the tree should be
removed in winter before the buds swell. They should be col-
lected and burned or deeply buried.

2. Cutting out of diseased shoots. — As far as possible all
dead twigs bearing pustules should be cut out. If the attack is
not extensive it will be possible to do this without excessive
labour or serious injury to the trees. When large areas are
concerned the labour is of course prohibitive, and the trees may
be injured through excessive cutting out. At the same time, it
should be remembered that the affected wood is quite dead, and
therefore useless.

* It may be noted that Apple Brown Hot (M. fructigena) which is
distinguished by its buff instead of grey pustules, is sometimes found
on ripe plums causing a rot. It appears, however, to be confined to
such fruit and not to attack flowers or shoots.

40

3. Spraying. — In spraying the aim is to destroy the spore-
pustules on the wood by means of a caustic winter wash, but
the treatment is only in an experimental stage. The chief
difficulty is to wet the pustules thoroughly, as on account of
their powdery nature the spraying fluid runs off. This can
partly be overcome by the addition of soft soap. The mixture
which has proved most promising is as follows : —

1 lb.- caustic soda.
1 Ib. soft soap.
10 gal. water.

The best time for applying a winter wash depends to some
extent upon the season. It should be applied as late as possible,
but not too late to run the risk of damaging the opening buds.
Generally speaking, the correct time is about the end of
February or the beginning of March, but in a cold, late season
the spraying may be delayed until the end of March.

THE DIE-BACK DISEASE OF FRUIT TREES.

(Cytospora leucostoma or other species.)

Many varieties of fruit trees in almost every part of the
world are subject to a disease characterised chiefly by a dying-
back of the branches (see Fig. 1). This disease is caused by
a fungus named Cytospora. Plums, peaches, apricots, apples
and cherries (both wild and cultivated) have all been found
to be attacked by various species of this fungus. The dying-
back, which may extend to the large branches and even cause
the death of entire trees, is common to them all, but as the
symptoms vary somewhat, a brief description of the disease as
it occurs on the different kinds of trees is given below.

Plums. — Young, vigorous growing trees suffer more often
than older ones, the varieties " Victoria," " Czar," " Purple
Egg Plum " and " Prince of Wales " being frequently
attacked. The first sign of disease is a withering of the leaves,
often of a single branch. At the same time areas of bark which
may be on the main shoot or on a side branch, collapse and
turn brown, and the shoot or branch attacked commences to
die back. The dying-back does not as a rule confine itself to
the younger branches, but extends downwards and gradually
involves the largest branches and the trunk, and may then
endanger the life of the tree.

After the branch or main stem has been dead some time,
the fungus produces Ifcs fructifications which can be recog-
nised externally as pimples or minute pustular swellings in
the bark (see Fig. 2). The fruit-bodies themselves, which
are found within these pustules, soon rupture the bark
horizontally and are seen on the surface as more or less
elliptical, flattened bodies, white in colour and perforated by

FIG. 1. — THE DIE-BACK DISEASE OF FRUIT TEEES. Cherry tree suffering

from a Die-Back Disease caused by the fungus Cytospora.
(From a Photograph by Dr. H. Wormald and reproduced by kind per-
mission of South Eastern Agricultural College, Wye, Kent.)

40—1

FIG. 2. — THE DIE-BACK DISEASE OF FRUIT TREES. Portion of diseased
wood of Plum Tree showing pustular swellings caused by the fungus
Cytospora. The spores are extruded in damp weather from these
pustules in tihe form of minute, coiled threads of a reddish colour
(nat. size).

41-1

one or more pores which appear as black dots. Minute spores,
which readily spread the disease, are produced from the pus-
tules in great abundance. In damp weather they are extruded
through the spores as minute, coiled, tendril-like threads of a
white, pink or reddish colour. This is due to the fact that the
spores are held together by a mucilaginous substance. When
wetted by rain this substance is dissolved and the individual
spores are separated from one another and blown or washed,
away. The spores may be extruded from the pustules over a
period of several weeks but in dry weather spore-liberation is
practically at a standstill.*

The conditions under which infection takes place have
been carefully examined in the case of the cherry disease,
and it has been found that* the fungus can only gain entrance
to the tree through a wound. If the wound contains no dead
wood the injury caused, is very slight, but if dead wood is
present, the fungus lives upon it first and afterwards invades
the living parts killing them as it advances. This is probably-
true for other fruit trees. Late frosts or other adverse condi-
tions not infrequently cause the death of small branches, or of
portions of large ones, whilst careless pruning results in similar
injury which lays the tree open to attack. Rank, ill-ripened
wood is probably particularly liable to be injured in this way.

Peaches. — Twigs killed by the disease have at first a dark
purplish skin which later becomes leathery and shades into
scarlet and purple. The affected areas finally change to a drab
colour and the skin becomes loose and wrinkled. Later the
spore-pustules of the fungus appear on the affected areas and
these exude their spores in minute, tendril-like threads as
described above. In Canada the Cytospora disease causes
severe cankers on the wood, but this type of injury has not been
observed in England.

Apples. — In the case of apple trees the disease may start
on the branches and spread downwards producing somewhat
similar effects to those described for peach trees. The colour
of the diseased branches varies from a lightish to a dark purple.
Not infrequently, however, in the case of apples, the fungus
first attacks the main branches, or even the trunk of the tree
near the ground, and produces cankerous wounds. This form
of attack is very much more serious than the ordinary form of
die-back since the disease extends more rapidly in the tissues,
and if the trees are not immediately attended to, they may be
completely killed within a year or eighteen months.

* Both on plums and other trees another form of fructification, tke
ascigerous stage, sometimes develops on the large branches which have
been dead for a considerable time. The ascigerous stage belongs either
to the genus Valsa or Eutypella according to the species of Cytospora
which is present. The previous edition of this leaflet was confined to
the two forms. of the fungus Eutypella prunastri. It is now known that
this species is only one of several which cause die-back of fruit trees.

42

Cherries. — Cherry trees attacked by die-back usually show
a general yellowing or withering of the leaves, commencing
at the tips of the shoots. This may commence in spring and
may spread so rapidly (see Fig. 2) that by autumn the whole
of the upper part of the tree may be dead. When this takes
place cracks, from which a viscid gum oozes, often appear in
the bark (usually at the crown or at the base of the trunk) and
at the same time young shoots arise from the base. On the
dead wood the fungus produces spore-pustules abundantly,
and in moist weather reddish, tendril-like threads of spores are
extruded as in the case of other varieties of fruit trees. The
varieties " Noble " and " Napoleon " suffer the most
seriously.*

Measures of Control. — 1. Die-back Form. — If tackled
promptly the disease can to a certain extent be controlled. In
the case of a die-back, it is of the utmost importance that the
diseased portion should be cut away and burned as soon as it is
noticed. The longer it is left, the greater is the amount of
damage done. The shoot should be cut back to the still healthy
region and the wood painted over with tar.

2. Canker Form. — Where the fungus has formed cankerous
wounds on the main branch or on the trunk, the affected part,
if observed early enough, may be cut out with a knife or chisel,
taking care to remove all the diseased tissue. As the fungus
is a wound parasite, the trees should be damaged as little as
possible during these operations; all rough wounds should
be pared over with a knife and protected immediately with a
coating of tar. In order to prevent further spore-liberation
great care should be exercised to collect and burn all diseased
wood.

3. Badly Attacked Trees. — If the disease once obtains a firm
hold and the case is beyond recovery, the trees should be taken1
up at once and the branches cut off and burned to prevent the
liberation of further spores.

PEACH LEAF-CURL.

(Exoascus deformans.)

Peach leaf -curl attacks peaches, nectarines and, less fre-
quently, almonds; it occurs constantly in this ^ country
wherever these plants are grown in the open, but is much
less prevalent on trees under glass. The disease is caused by
a fungus and, in seasons which favour the fungus, the attack
may prove very destructive. It is now known that Peach
Leaf-curl can be controlled effectively by spraying. There is,
therefore, no necessity for the extensive outbreaks which in
the past have so often annoyed and baffled the grower of out-
door peaches.

* For details of the disease on cherries see Wormald, H., Jour. South
Eastern Agric. College, Wye, Kent, No. 21, 1912.

43

Description of Symptoms. — The disease appears in the
spring shortly after the leaves begin to emerge from the bud.
The leaf-blade becomes thickened and puckered along the
midrib, causing the leaf to become curled and twisted. The
diseased part remains yellowish in colour with a tinge of red
in it. As the leaves become older, this curling and crumpling
of their surface becomes more pronounced, then* substance
becomes fleshy and the coloration darker; finally the upper
surfaces of the diseased leaves become covered with a delicate
" bloom " due to the fungus passing into its spore-bearing
stage. Affected leaves finally die and drop from the tree, and
in severe cases the entire tree may become defoliated. New
sets of leaves, however, usually develop and replace those that
have fallen.

The fungus not only attacks the leaves but also invades the
young shoots and, more rarely, the flowers and fruits. Young
shoots infected with the fungus become swollen and twisted
and the diseased leaves usually form a tuft on a stunted shoot,
owing to the internodes failing to elongate.

In winter, symptoms of fungal activity are visible on the
young growth, brown patches being here and there present;
these patches increase in size, until finally the whole length
of the lateral beyond this point withers and a number of dead
ends are left.

The injury caused by the disease consists not only in the
distortion of the leaves and premature defoliation but in the
dropping of the fruit at an early stage and in the strain on
the tree due to the development of a second crop of leaves.
In the case of nursery stock, consecutive attacks for three or
four seasons usually kill the tree or stunt its growth to such
an extent that it is practically valueless.

Cause of the Disease. — Leaf-curl is caused by an attack
of the fungus Exoascus deformans. The fungus enters the
young leaves early in the spring when the buds are just
commencing to expand. The mycelium (or system of fine
fungus threads) develops between the cells of the leaf, robbing
them of nourishment, destroying the green colouring matter
and causing the leaf to become deformed.

After a time the fungus mycelium forms a layer just
beneath the skin of the leaf and from this layer a number of
spore-sacs (termed asci) are developed; it is their presence
which causes the " bloom " upon a diseased leaf. Within
each of these spore- sacs eight spores are produced and these
spores usually bud off a number of secondary spores, all of
which are capable of germination and reproducing the disease.
The spores are produced in great abundance upon the diseased
leaves during the spring and early summer.

Commencement of Attack in Spring. — It was long
believed that the fungus mycelium hibernated in the tissues

44

of affected shoots and during the succeeding spring grew up
into the expanding leaflets and produced disease. Later
observation, however, has shown that this is of comparatively
rare occurrence. Such facts as are at present available tend
to show that fresh infection of the leaves in the spring always
takes place by means of spores which have lain dormant
during the intervening months, entangled among the scales
upon the buds.

The establishment of a new outbreak of disease by these
spores is intimately connected with the weather conditions
which prevail at the time. Cold, wet weather, when the
leaves are expanding, causes them to become abnormally
gorged with water and much more susceptible to an attack by
the fungus. It is a widely observed fact that leaf -curl is very
much less prevalent in a uniformly warm and dry spring; it
has also been noticed that where infection has occurred a
return of warm, dry weather, or even the occurrence of a
hot, dry wind will check the development of the fungus
within the tissues. Peach trees planted near large expanses
of water, where the atmosphere is moister and cooler than
elsewhere, have been found to be especially liable to the
disease.

Measures of Control. — There is only one method of
controlling leaf-curl, that is, by spraying. The fact that, if
the affected leaves are picked off, a crop of new, clean growth
is produced, has led some persons to assume that the removal
of the curled foliage will eradicate the disease. This is not
the case. The summer growth is always clean and, unless the
plants are sprayed in February cr March as described below,
leaf-curl will develop again in the following season.

The secret of success is to spray early, immediately before
the buds begin to swell. This time will range between the
middle or latter part of February and the beginning of March,
according to the locality and the nature of the season. Bur-
gundy or Bordeaux mixtures may be used.

1. Burgundy Mixture. — A Burgundy mixture of the fol-
lowing composition has been used with excellent results at the
Eoyal Horticultural Society Gardens, Wisley* : —

Copper Sulphate 2| Ib.

Sodium Carbonate 2f Ib.

Water 12 gallons.!

* See Home, A. S., Jour. Roy. HorL Soc., Vol. XLI (1915),
pp. 110-115.

f If a smaller quantity of the mixture be required, the proportions
would be : —

Copper Sulphate 9£ oz.

Sodium Carbonate 11 oz.

Water 3 gallons.

At Wisley, the addition of a small quantity of milk to the above mix-
ture was found to be of advantage in facilitating the adherence of the
spray to the buds.

Photo.]

44—1

PEACH LEAF-CURL.

\_F. 0. Mosley.

AMERICAN GOOSEBERRY MILDEW.

FIGS. 1 and 2. — White or summer stage on young shoot and leaves.
FIG. 3. — Summer stage on berries, passing gradually into the brown

stage.

J?IG. 4. — Shoot showing brown felt of mycelium or winter stage.
FIG. 5. — Portion of the same, magnified about four times, showing

minute spore cases embedded in the felt of mycelium.

45—1

45

The copper sulphate (98 per cent, purity) and the sodium
carbonate should be dissolved in separate wooden vessels,
and mixed together when completely dissolved.* One
spraying, just before the buds open, is usually sufficient, but
a second spraying a few days later gives additional assurance
of success. Great care should be exercised to see that the
buds are completely coated with a film of the mixture. Before
spraying, all dead twigs and any obviously diseased shoots
should be cut away.

2. Bordeaux Mixture. — Bordeaux gives more trouble to pre-
pare than Burgundy mixture, but if preferred the following
proportions are recommended : —

Copper Sulphate 13 oz.

Best Quicklime 13 oz.

Water 10 gallons.

The copper sulphate should be dissolved in water at the rate
of about 13 oz. to 1 gallon. This should be done in a wooden
receptacle. The lime should be slaked to a fine powder with
a little water in another receptacle. Water should be added
gradually to make a " milk " and then diluted to the amount
required (9 gallons), stirring well throughout. Finally, the
copper sulphate should be poured into the diluted milk of lime
and stirred thoroughly for 5 minutes. The mixture should be
used within 24 hours and, preferably, as soon as made.

AMERICAN GOOSEBERRY MILDEW.

(Sphaerotheca mors-uvae, Berk.)

The serious disease known as American Gooseberry Mildew
was introduced into Europe in 1900, and has since spread,
throughout the Continent. This mildew, which is caused by
the fungus Sphaerotheca mors-uvae, has greatly interfered with
gooseberry growing, and in some countries it has rendered the
cultivation of this fruit quite unprofitable. Few diseases have
been more carefully investigated in this country, and, thougn
much remains to be learned, it is now possible very largely to
control its ravages.

Description of the Disease. — There are two well-marked
stages in the life of the mildew fungus which it is important
to distinguish, namely, a white and a brown stage. The white
stage always, comes first, but it is often of short duration and
passes into the brown stage so rapidly that it is the latter that
sometimes first attracts attention.

* For fuller details regarding the methode of preparing Burgundy or
Bordeaux mixtures see Leaflet No. 23 (Potato Disease " Blight " : and
its Prevention), obtainable from the Ministry and also to be included in
Sectional Volume No. 8.

46

The white form appears as glistening white patches o$
mildew on the berries, shoots and under-sides of the young
leaves (see Figs. 1, 2 and 3).* This stage may occur at
any time from May to October, and is consequently often
referred to as the summer stage. Whilst in this form the
mildew produces vast quantities of minute spores (conidia)
which are readily spread, and, germinating at once, produce
fresh spots of mildew on any young growth on which they
may alight. In this way the mildew is very rapidly distri-
buted, especially in warm and moist weather. Suckers or
soft, quick-growing shoots on bushes which have been highly
manured or grown upon deep porous soils containing an
abundance of moisture are particularly liable to be attacked.

The brown stage is a subsequent development of the white
stage, and it occurs abundantly on the shoots and berries
and, less often, on the leaves (see Figs. 3 and 4). It takes the
form of a thin felt which can be peeled off easily with a knife
or with the finger nail. The felt consists of fine threads of
fungus mycelium, and in it will be found embedded minute
spore-cases known as perithecia, which contain the resting or
winter-spores. The perithecia- are just visible to the naked eye
as minute black specks (see Fig. 5).

As in the case of other mildew fungi, these bodies are
produced in late summer and autumn, and normally remain
dormant till spring, when they discharge their spores and
bring about the infection of the new foliage. It is known,
however, that a certain number of spores are discharged
from the perithecia the same season that they are produced.
Notwithstanding this autumn spore-discharge, it is of the>
utmost importance that as many perithecia as possible should
be destroyed by the practice known as " tipping," or cutting
away of affected shoots, in September. There is, however,
another point of importance to remember in connection with
tipping, namely, that many of the perithecia do not remain
attached to the brown felty mycelium till spring, but fall
out on the ground during the autumn and winter. The
necessity, therefore, of early tipping as insisted upon in the
paragraph on " Tipping " is clear.

Measures of Control. — There are two principal lines of
treatment against American Gooseberry Mildew : (1) spraying,
to prevent and to destroy the white or summer stage ; and (2) tip-
ping, to eliminate the brown stage with its resting: spores. In
addition, it need hardly be said, that all possible measures
should be taken to prevent the disease from gaining access to
areas not affected.

* This serves to distinguish it from the very much less serious disease
known as European Gooseberry Mildew, in which almost invariably the
leaves only, and chiefly the old ones, are affected. (See Leaflet No. 52.)

47

1. Spraying. — For spring and summer spraying lime-
sulphur is the most convenient and satisfactory substance.
It can be bought ready-made in drums at a moderate price,
and for use only needs diluting to the proper strength. In
purchasing, however, a guarantee should be obtained that
the lime-sulphur is of 1/3 specific gravity. The strength
usually employed is 1 gallon of lime-sulphur to 29 gallons
of water. When required for use, the lime-sulphur should
be poured slowly into the water, and after being well stirred
the mixture should be applied at once.*

Three, or at least two, sprayings should be given. The
first, which is intended to give the bushes a fungicidal
covering to prevent infection, should be made about the
first week in April. The remaining two applications, which
are either preventive or have as their object the actual destruc-
tion of the summer stage, should be given at intervals of three
or four weeks. Every care should be taken to wet the leaves
thoroughly on both sides with the spray. If only two applica-
tions are given, the last of the three sprayings should be
omitted.

It has been found that the following varieties of goose-
berries are apt to be damaged by lime-sulphur if used at the
strength mentioned above, and in these cases a wash of one-
half the usual strength should be employed: —

Berry's Early (Keepsake).
Cousin's Seedling.
Lancashire Lad.
Crown Bob.

A few kinds cannot safely be sprayed with lime-sulphur
at all. These are chiefly the sulphur varieties, which include
Golden Drop (Yellow Eough).

An objection to the use of lime-sulphur for the later
sprayings is that it leaves an adhesive deposit on the berries
and, although this is not poisonous, it detracts from the com-
mercial value of the fruit. This deposit may be removed by
mechanically rubbing the berries against each other in water,
or by passing them through a ' ' Gooseberry Cleaner. ' '

As an alternative to lime-sulphur, Messrs. Eyre and Salmon
recommend an ammonium polysulphide wash, especially for
the last, spraying.! This wash leaves no visible deposit on the
sprayed parts, and is, therefore, useful for application to
dessert varieties. The stock solution of ammonium polysul-
phide, which should be bought ready-made, is dilated wifh
water as directed so as to contain O'll per cent, of polysulphide

* Tt should be remembered that the sulphides in the lime-sulphur and
ammonium polysulphide washes react with copper, hence spraying
machines with copper parts should never be used with these mixtures

f See Journal, Ministry of Agriculture, Vol. XXU, Feb. 1016,
pp. 1118-1125; Vol. XXIII, Feb., 1917, pp. 1098-1100; Vol. XX?
Mar., 1919, pp. 1494-1497; Vol. XXVI, Nov. 1919, pp. 821, 822.

48

sulphur. This wash, which is known as the A.P.S. wash,
should be used with a 0*5 per cent, soft soap ( = 5 Ib. of soft
soap to 100 gallons of wash).

2. Tipping.— ' Tipping" consists in the cutting away in
autumn of all shoots which show signs of the presence of
mildew. The operation should be carried out as soon as
possible after the wood is ripened, since, as explained already,
many perithecia fall off the shoots during late autumn andl
winter. On the other hand, it should not be commenced
before active growth ceases, as otherwise the bushes will
produce fresh shoots, the soft tips of which are particularly
liable to mildew. Late August or early September is usually
the most suitable time, but the exact date varies with the
season, the age of the bush and the locality. On no account
should the tippings be allowed to fall to the ground; as the
work proceeds they should be carefully collected and burned.
If tipping is thoroughly carried out, the outbreak in spring
is very greatly reduced. It need hardly be added that there is
no need to tip parts of bushes that are not diseased or to tip
healthy bushes.

3. Preventive Measures. — Gooseberry bushes should not
be purchased unless a guarantee is given that they are free
from American Gooseberry Mildew.

If the disease is known to be present in the neighbourhood,
special precautions should be taken to prevent its spread.
Baskets, packing-cases, or empties likely to be contaminated
with spores should not be allowed on the premises, unless
they have been disinfected by washing in a solution of copper
sulphate (1 Ib. copper sulphate to 20 gallons of water). The
clothes of workers and pickers are also liable to be con-
taminated with spores; if pickers, therefore, have worked ii
gardens where the mildew exists in its summer stage, at least
a week should elapse before they are permitted to pick in clean
plantations. Periodical inspections of the bushes should be
made once a month from May to September.

NOTE. — Under the American Gooseberry Mildew Order of
1919, notification of the disease is required only from persons
who grow gooseberry or currant bushes for sale, but the
Ministry retains power under the Order to deal with fruit
growers and private owners who fail to take proper steps to
check the disease. Such notification must be made to the
Ministry of Agriculture and Fisheries, or to an Inspector of
the Ministry. It is illegal to sell gooseberry or currant bushes
affected with the disease, but the bushes may be sold after
notification if all the diseased shoots are cut away.

There are no other restrictions on the movements of goose-
berry and currant bushes.

49

Article 6 of this Order and all previous Orders relating to the
sale of gooseberries (fruit), affected with this disease, are now
revoked, so that any gooseberries fit for human consumption
may be sold in all markets and shops.

On and after 1st October, 1921, the importation of goose-
berry and currant bushes and of gooseberries (fruit) into
England and Wales, from any place abroad other than the
Channel Islands, is permitted .provided they are accompanied
by a certificate issued by the exporting country to the effect
that they are healthy and free from disease. (Destructive
Insects and Pests Order of 1921.)

Further information with regard to either of these Orders
can be obtained on application to the Ministry.

EUROPEAN GOOSEBERRY MILDEW.

(Microsphaera* grossulariae, Lev.)

The gooseberry plant is attacked by two distinct mildews,
termed respectively the European Gooseberry Mildew and
the American Gooseberry Mildew. The former, caused by
the fungus Microsphaera grossulariae is the only one dealt
with in the present leaflet. TEe latter, caused by an entirely
different fungus, is a very much more serious pest and will
be found described in Leaflet No. 195.

The two mildews, though somewhat similar in certain
stages, can with a little experience usually be distinguished
^ven with the naked eye. The European Gooseberry Mil-
dew occurs on the upper side of the leaves in the form of a
very delicate white mould or mildew. In the early stages it
is coated with a distinctly mealy substance. As is the case
with all true mildews the mycelium (spawn) is quite super-
ficial, although, in order to obtain the nourishment it requires,
the fungus sends down minute suckers into the cells of the
attacked leaf. The white mealy substance consists of masses
of spores. These spores are termed conidia or " summer
spores," and, being carried by insects. or being blown about
by the wind, bring about the infection of other leaves. As a
rule the mildew is confined to the upper surface of the leal,
but occasionally it is found on the under surface and also on
the berries.

Later in the season the fungus produces a second form of
fruit. These consist of minute spherical spore-cases (peri-
thecia) which contain the " winter spores "or " resting
spores." They occur on the mildewed patches and appear
first as yellow specks just visible to the naked eye^ but
mamalely become black. In autumn the spore-cases either
fnll off the leaves or fall to the ground with the leaves. They

50

remain dormant during the whole winter. In spring the spores
are liberated and shot out into the air. They are blown by
the wind on to the new leaves just unfolding and infection
results. The delicate white mould soon appears and a crop
of summer spores follows in a few days. From these
primary Y infections, new or " secondary " infections take
place and thus in favourable weather the mildew spreads
rapidly.

European Gooseberry Mildew may occur as early as May
and lasts throughout the season. It is widely distributed
and as a rule causes but little damage; it is mostly found
under heavy shade and on old bushes. When the attack
is severe the leaves die, and fall off early in the season; the
fruit as a result is checked in its growth and remains small.
If a severe attack follows for several seasons in succession
the bushes become stunted and may even be killed. Red
currants are also occasionally attacked by European Goose-
berry Mildew.

The most obvious differences between the present disease
and the American Gooseberry Mildew may be summed up
as follows. Whereas in the European Gooseberry Mildew the
fungus occurs as a very delicate mould or mildew on the
leaves, the American Mildew forms a dense white woolly
mould which, though found on the leaves, attacks chiefly the
shoots and berries. As the growth of the American Mildew
continues the white stage gives place to a light brown woolly
phase and finally to a thin dark-brown felted mat, which is
very conspicuous on the berries and shoots. This condition
is never found in European Gooseberry Mildew, where the
mildew always remains thin and scanty. The resting-spore-
fruits, moreover, are in the European Mildew found exposed
on ihe. leaves, whereas in the American Mildew they are
embedded in the felt on the shoots and berries. Occasionally
the two mildews are found on the same plant.

Measures of Control. — European Gooseberry Mildew can

be controlled by spraying. If the amount present is small it-
will hardly be necessary to resort to this treatment, but if the
attack. is severe, or if from previous experience a bad outbreak
of mildew may be expected, it will be advisable to spray
occasionally with lime sulphur. The first application should be
given early in May and at the rate of 1 part lime sulphur to
60 parts water.

The ground under diseased bushes should be dug over in
winter in order to bury the winter spores, and if onlv a few
bushes are grown the additional precaution may be taken-
of collecting and burning in autumn the dead and fallen
diseased leaves.

51

CORAL SPOT.

(Nectria cinnabarina.)

No fungus is perhaps more plentiful on dead twigs and
branches and more familiar to every gardener than the Coral
Spot fungus, Nectria cinnabarina. It is abundant and con-
spicuous in damp weather on dead branches of elm, lime,
poplar, sycamore and many other trees, and is always to be
noted plentifully on old pea sticks, particularly hazel, in
autumn and winter. In these dead branches the fungus
occurs in the condition known as saprophytic, i.e., living on
dead or decaying material, but though it usually occurs as a
saprophyte it not infrequently becomes parasitic, that is, it
invades living tissues. In this form it attacks trees of many
kinds, ornamental shrubs and bush fruits, of which lime,
sycamore, horse chestnut, apple and currant may be mentioned
as being more particularly susceptible. The present leaflet
deals specially with its attack on fruit trees, though the general
remedial measures recouimended apply equally to other plants.

The Coral Spot fungus may be recognised by the small
pink or flesh-coloured warts which are thickly scattered over
the surface of the dead and dying branches. The warts are
in reality spore-pustules which consist of fungus filaments
bearing masses of spores. In damp weather the pustules are
slimy and salmon coloured, in dry weather they are powdery
;ind pale pink in colour.

Method of Attack.— On fruit trees Coral Spot is particu-
larly common on red and black currant and gooseberry, but
it also attacks apples and pears. In all these cases it attacks
the plants in one or other of two ways ; either by commencing
on the dead twigs and working from them into the living
tissues or, more rarely, by gaining entrance by means of a-
wound to the stems or branches lower down.

In red currants the evidence suggests that, by far the largest
amount of infection occurs through small branches which
have died back. The fungus spreads from these dead shoots
into the main healthy branches, causing them to wilt. This
wilting occurs in May and June and is a very conspicuous
feature of badly affected plants.* The fungus probably obtains
entrance in a similar way in the case of gooseberries.

In young apples cases have been observed where, owing
to rank grass and weeds having been allowed to grow round
the stems, the Coral Spot fungus gained an entrance through
dead branches or wounds near the ground level and caused
the death of the entire trees.

* For information as to Ked Currants the Ministry is indebted to
Mr. F. T. Brooks, of Cambridge University, who has this disease under
investigation.

52

The effect of the fungus filaments within the tissues is to
cause the gradual death of the branch. In the case of its
ally, the Apple Canker fungus, Nectria galligena (see Leaflet
No. 56), the tree resists the parasite, forming a callus, and a,
canker- wound is the result. No callus formation takes place
in Coral Spot. The mycelium invades the woody portion of
the stem and kills and blocks up the water-conducting tissues.
It is not surprising, therefore, that in bush fruits such as
currants the death of the attacked branches should follow
sooner or later. Wood which has been destroyed by the Coral
Spot fungus turns first dark green or black and then brown.
In the case of apples (as also in trees such .as sycamore and
horse chestnut) larger branches may be attacked with equally
fatal consequences. Soon after the tissues are killed the fungus
commences to form spores, these being borne in amazing
quantities on the pink pustules above referred to. These
spores are very minute and are termed conidia (conidia meaning
dust-like) and are developed all the year round. Another
form of spore is, however, also produced, chiefly during spring,
and especially on the larger branches. These spores are formed
in dark red bodies named perithecia and are usually borne
on the site of old conidial pustules. The perithecia are com-
parable in function to those so well-known in American Goose-
berry Mildew.

Measures of Control. — 1. Prevention. — It has been shown
that the fungus gains entrance in two ways, viz., through
dead branches and through wounds. All dead shoots and
branches should therefore be removed during pruning and
care taken that no snags which will die back are left. Injury
to bushes should also be avoided whether in pruning or in
cultivation with the hoe. Wounded surfaces should be
protected by tar.

2. Treatment of Attacked Trees. — The disease may often be
arrested by the rigorous use of the knife. Diseased branches
should be cut clean out, taking care to cut back to healthy
wood and to protect the wound. In the case of large boughs,
if the attack is dealt with early, it may be possible to save
the branch by removing the infected area with a knife or
chisel. Great care must be taken to remove all the affected
tissue and to tar the wounds.

3. Sanitation. — Accumulations of dead wood and sticks
should never be allowed in an orchard. On such debris fungi
of all sorts flourish, including those causing Coral Spot, Silver1
Leaf, Canker and other dangerous diseases, and from these
fungi millions of spores are liberated into the air and blown
amongst the trees. The better the sanitary condition of the
orchard, the less the chances of infection by fungus parasites*

THE DIE-BACK DISEASE OF GOOSEBERRIES.

FIG. 1. — Portion of stem (just above ground level) of a gooseberry bush
attacked by Botrytis ; fructifications of the fungus can be seen appear-
ing between the cracks of the bark.

FIG. 2. — Part of an old diseased branch ; compact cushion-like tufts of
Botrytis (x) have been formed on its surface.

52-1

53-1

53

THE DIE-BACK DISEASE OF GOOSEBERRIES

(Botrytis cinerea.)

The Die-back of Gooseberries is a disease very widespread
in England and one liable to occur wherever these bushes
are grown, whether in large plantations or in small gardens.
Although gooseberries may die-back from various causes, in
the majority of cases the serious attacks which occur are'
iound to be due to the minute fungus Botrytis cinerea. This
fungus is exceedingly common as a saprophyte on decaying
vegetation, but, under certain conditions, it becomes parasitic
and attacks living plants. The present leaflet is confined
10 the disease caused by Botrytis.

Description of the Disease. — The gooseberry bush may be
attacked by Botrytis in four distinct places, viz., (a) the main
stem and base of the branches, (b) the young wood of the
current year, (c) the leaves, or (d) the berries.

(a) The Main Stem. — As regards the main stem, the
mycelium (spawn) of the fungus penetrates the outer tissues,
and at the end of the season causes the bark to crack and
peel off, often in large pieces. The part of the stem first
attacked is usually that portion situated at the ground level or
a little above it; the fungus kills the wood and eventually
" rings " the stem at this place and the whole bush is killed.
Before this occurs, however — and, in the case of a well-grown
bush, death does not as a rule result for several seasons — the
mycelium of the fungus spreads upward in the stem to the
base of the branches. Here it frequently attacks some of the
branches so severely that they die. The presence of dead
branches in a few bushes, or the death of half the bush,
is a characteristic sign of the first appearance of the Botrytis
disease in a plantation.

.Renewed growth of the mycelium of the fungus in the stem
takes place every spring, and it is then that the manner
in which the fungus exists and spreads can be most
easily seen. If a diseased stem be examined during a warm
and damp spell of weather in the spring, the appearance
shown in Pig. 1 will be observed. The bark will be found
to be peeling or cracking off, while greyish, fluffy patches1 of
a " mould " have appeared at the edges of the peeling bark
or in the fissures where the bark is cracked. If there is a>
dead branch on the bush, as a rule, small greyish tufts or
little cushions — which soon develop in suitable weather into
fluffy patches such as are shown in Fig. 2 — will be found

This leaflet is based on work carried out by Mr. E. S. Salmon, of
Wye, and the illustrations are leproduced from the Journal of the
South Eastern Agricultural College, Wye, Kent, No. 18, (1909),' by kind
permission of the College authorities.

54

scattered here and there over its surface. These tufts of the
fungus occur both on the main branches nearly down to their
base (see Fig. 3), and frequently also on the younger wood.

Small hard, blackish bodies, of irregular shape, named
sclerotia are also produced in the bark. They are the resting
bodies of the fungus and on subsequent germination give rise
to multitudes of spores which spread the disease. They are
extremely resistant to climatic conditions such as frost, drought,
&c. , and serve to carry the fungus through all vicissitudes from
one growing season to the next.

(b) The Young Wood. — In the case of young bushes
especially, a considerable proportion of the young shoots
may be affected by Botrytis and much weakened or killed —
a fact which has caused growers to speak of the disease as
" die-back "'(see Fig. 4). The dead shoots may constitute a
prolific source of infection. Prunings of Bo£n/£ts-afiected
bushes left lying in a heap in a corner of the plantation or
garden may develop during the following spring an
abundant crop of powdery tufts of Botrytis, the spores
of which, carried by the wind in countless numbers, will
spread the disease through the plantation. There is also
the danger of cuttings having been taken from Botrytis -infected
bushes, when many of the young bushes resulting therefrom
will become diseased.

(c) The Leaves. — Very commonly Botrytis infects the
leaves, which soon become discoloured at their edges, at
first turning yellowish, and finally ashy-grey or whitish
(see Fig. 3). If the attack extends from the edge of the
leaf inwards until the greater part of the leaf is affected
the fall of the leaf soon takes place ; if, however, as is often
the case, the injury remains restricted to the edges of the
leaves, they remain on the bush until they drop in the normal
way. Whebher the injury spreads over the leaf to such an
extent as to make it fall prematurely seems to depend on the
climatic conditions which prevail at the time.

(d) The Berries. — Lastly, the fungus occasionally attacks
the berries and causes them to rot. The first sign of disease
on the berry is the browning of the skin; this browning
gradually extends until one side of the berry shows obvious
signs of softening and of being badly diseased. The Botrytis
fructification, in the form of the characteristic ashy-grey
" mould," then appears on the surface of the discoloured
portions and in about a week the berry becomes completely
rotten.

Measures of Control — 1. Sanitation. — The most effective
treatment is to remove and burn all dead bushes or branches
in the plantation. As soon as the edges of the leaves show
si^ns of discoloration, all the bushes in the plantation should
be examined and any found with the main stem diseased

55

should be grubbed up and burned. This treatment, carried
out for a. few seasons, has proved more efficacious than
spraying.

It must not be forgotten that the fungus is capable of
developing vigorously on dead parts of the bush. In nearly
all cases the disease appears first either on single plants
scattered here and there through the plantation, or on several
bushes over a small patch of ground, while the surrounding
bushes are healthy. If the disease on its first appearance is
dealt with summarily by the burning of all dead bushes and
branches, there is no need to spray or to take any other remedial
measure.

2. Spraying. — Where the disease is severe and widespread
.spraying must be resorted to in addition. A heavy spraying
with a solution of copper sulphate (4 Ib dissolved in 100
gallons of water) should be given just before the buds burst,
care being taken to spray thoroughly the main stems of the
bushes. The infection of the leaves may be prevented by
spraying, directly the fruit is set, with Bordeaux mixture
composed of 8 Ib. copper sulphate, 8 Ib. quicklime, and 100
gallons of water (for the preparation of this mixture, see
Leaflet No. 131). It is essential that the under-surface of
the leaves should be reached as much as possible. No injury
follows the appl: cation of Bordeaux mixture of this strength,
and if the spraying be done at the time indicated, no spotting
of the berries occurs.

3. Cultivation*. — It may finally be noted that any treat-
ment which induces the bushes to make vigorous growth
tends to stop the attacks of the fungus. This treatment
will chiefly consist in the keeping down of weeds, the cutting-
out of old and superfluous growth and attention to soil con-
ditions. The gooseberry responds to good treatment in the
shape of manuring but is very dependent on the presence of
lime in the soU, and strongly resents soil acidity. Lime, there-
fore, should be applied if necessary. If there is any evidence
tint potash is deficient, a dressing of 1 cwt. sulphate of potash
per acre should be given. It is well known that diseases make
more headway where there is an excessive supply of nitrogen
or lack of potash.

POWDERY MILDEW OF THE VINE.

(Uncinula necator, Burr.)

The vine is subject to two very serious forms of Mildew,
the one a member of the group of Powdery Mildews caused
by the fungus Uncinula necator, and the other, one of the

* See Leaflet No. 346 (Gooseberry Growing), obtainable from the
Ministry and also to be included in Sectional Volume No. 4.

56

Downy Mildews, induced by the fungus Plasmopora viticola.
Both these fungi have been recorded for Britain, but the latter
is exceedingly rare and is not dealt with in this leaflet.

The Powdery Mildew fungus is now believed to be a native
of Eastern Asia. It was first observed in Europe in 1845;
appearing in England near Margate that year, and very soon
after in the vineyards of France, Spain and Italy, where it
spread rapidly and caused enormous devastation. Since then it
has spread to practically all the large vine-growing countries
and has caused much damage to cultivated vines both in the
Old and New Worlds.

Powdery Mildew is the common vine mildew familiar to
every vine grower in England as giving trouble both under
glass and in the open. It caD now be completely controlled
under glass, and with due care and attention can be very
largely held in check on outdoor vines.

Description. — The appearance of the disease being so
well known hardly requires description. The fungus attacks
the young shoots forming white patches of fungus threads
(mycelium) on young leaves and shoots. As in the case of
other Powdery Mildews the mycelium is superficial, and merely
sends down minute suckers in order to extract nourishment.
In spite of this, however, great damage is done, and the foliage
is often crippled and sometimes killed. On this same mycelium
spores are produced in enormous numbers. They are exceed-
ingly minute and are readily blown about. They occur in such
profusion that the masses of spores are visible on the leaf as a.
white mealy powder.

In addition to the foliage the fungus also attacks the flowers
and fruits. This causes the fall of the flowers and also of the
fruits if still quite young. If the fruits are attacked at a
later stage the fungus may cause distortion and cracking of
the berries, thus rendering them useless. No new infection
takes place after the grapes have begun to ripen.

The mildew usually commences about the time the grapes
are in flower, and continues to spread through the whole
growing season. In America and certain other countries it
forms resting spores in autumn which provide for the fresh
outbreak of the Mildew in spring, but in Europe this form
of over-wintering is practically non-existent and the fungus
hibernates by means of mycelium in the buds.

Measures of Control 1. Outside. — Where vines are grown

extensively in the open the selection of a well-drained site and
of one in which the vines have abundance of sun and air will do
much to minimise mildew attacks. Another point of great im-
portance is to plant the vines well apart and to pinch the
laterals back carefully in summer to prevent crowding.

Both on the Continent and in America Powdery Mildew
in the open is largely controlled by the use of flowers of

POWDKRY MIT.DKW OF THE VINE.— Diseased Leaf and Grapes.

" REVERSION " IN BLACK CURRANTS.

FIGS. 1 to 7. — Progressive stages of the disease, from a normal leaf in
Fig. 1, to the oak-leaved form in Fig. 7. The main veins are marked
A, B, C, D, E, and the sub-main veins, which spring from C and
are the important ones for purposes of identification, are indicated
by numbers. ,.,

fi7— 1

57

sulphur or other fine sulphur-powder. This should be applied
by means of a sulphur bellows or sulphurator and is best put
on early in the morning when the foliage is damp. It is
possible that a lime sulphur solution such as is used for
American Gooseberry Mildew (see Leaflet 195) may be useful,
but no records as to the use of this fungicide for vine mildew
are available. Bordeaux Mixture, although used with success
on the Continent for the other vine mildew (Downy Mildew),
is not regarded as satisfactory for keeping down Powdery
Mildew.

2. Under glass. — Mildew may very largely be prevented by
careful attention to cultural details. The atmospheric c ndi-
tions of the house, which to> a great extent can be controlled
by careful ventilation, are most important. Air should be
given early and gradually increased as the temperature rises;
draughts should always be avoided. During dull, wet periods
a buoyant atmosphere should be maintained by increased fire
heat and by giving some ventilation for at least a short perio4
of the day. The same or even more care should be exercised
as in the open with regard to the thinning out and pinching
back of laterals so that plenty of light and air are admitted.
The maintenance of a steady temperature and the avoidance of
dryness at the root also tend to prevent mildew from obtaining
a footing.

If mildew appears sulphur should at once be applied. The
safest method is to blow a fine sulphur powder on to the foliage
by means of a sulphurator. The old fashioned plan of painting
the hot water pipes with a mixture of milk and sulphur and
then heating them steadily for a few hours is effective, but the
work should be entrusted only to an experienced hand. By
far the most effective method is the use of the apparatus known
as a Sulphur Vaporizer. With this apparatus Vine Mildew
can be completely suppressed, but here again great caution is
needed and the instructions accompanving the vaporizer should
be strictly attended to. In the case of these last two methods,
any ferns and other plants with delicate foliage which are
present in the house should be removed during the vaporizing
operation.

« REVERSION »' OR NETTLEHEAD OF BLACK
CURRANTS.

(Cause Unknown.)

Introduction. — One of the most serious diseases of Black
Currants at the present time is that termed " Nettlehead " or
" Eeversion." Owing to its ravages hundreds of acres of Black
Currants have ceased to bear as they did formerly and many
have become so unprofitable that the bushes have been grubbed
up, The precise nature and cause of the disease are still

58

obscure. It does not represent reversion in the strict sense of
reverting to a previous type. It is apparently not caused by any
fungus, and insects are not directly responsible, though possibly
their attacks may not be altogether unconnected with it. In
spite of this unsatisfactory state of knowledge, it is felt that
the issue of a leaflet on the subject is justified, not only to
enlighten those not fully aware of the existence of " Rever-
sion," but to point out measures which will help to prevent-
further spread of the trouble. The disease has been studied
for many years by Mr. A. H. Lees of the Long Ashton
Research Station, and much of the information given in this
leaflet is based on an article by him which appeared in the
Ministry's Journal for March, 1921. It should be clearly
understood that the account given below does not profess to be
final, and that with continued research further information
may be early expected.

Description of the Disease. — General Characters. —
" Reversion " shows itself in several ways. (1) The most
obvious symptom is the change in the character of the leaves,
which become elongated and narrow and often assume the
" nettlehead " form, as is described more fully below. (2)
There is a progressive change, as the trouble develops, in the
form of flower- truss and in the flowers themselves. (3) There
is ultimately an almost complete failure to produce fruit. It
should be clearly understood, however, that " Reversion " is
quite distinct from the trouble known as " Running-off" which
occurs on perfectly healthy bushes. This is a very widely-
spread phenomenon and is often mistaken for " Reversion."
In an important paper recently published* it is shown that
" running-off " on healthy bushes is due to ineffective pollina-
tion. When such bushes are effectively pollinated, either by
artificial means or by natural agencies, a full crop of fruit
is produced, given normal conditions. In " Reversion " the
flowers become more or less abnormal in structure and artificial
pollination completely fails to produce a crop.

Leaf Characters. — With regard to the leaves, although the
general appearance of " reverted " foliage was familiar to all
growers, the precise symptoms by which one could detect
them with certainty were not clearly grasped. The method
of identification described by Mr. Lees is based on the fact
that the number of sub-main veins and marginal teeth are
fewer in " reverted " leaves, the decrease being small in
slightly " reverted " leaves and great in those which are badly
attacked. The examination should be confined to strong new
shoots, as spur leaves are apt to be unreliable. The successive
stages are found in the illustrations, No. 1 being a normal
leaf and No. 7 a very badly diseased one. It will be seen that

* The " Eunning-off " of Black Currants : K. Wellington, E. G.
Hatton and «T. Amos, Jour. Pomcloqy, Vol. II, No. 3.

59

in the normal leaf there are five main, veins, A, i5, C, JD, &r
and these run from the top of the leaf stalk, to a point at the
margin.* From the middle main vein, or midrib C, a series of
sub-main veins run to points on the margin and it is the
number of these sub-main veins which provides one indication
of the degree of " Reversion." If less than five are present
the leaf is " reverted," if five or more the leaf is probably
n6nii;\l. For. further details the reader is referred to the paper
quoted.

The second indication is- afforded by the leaf margin. If
finely-toothed with 4 to 8 teeth between C and X not receiving
a sub- main vein, the leaf is almost certainly normal, but if
coarsely-toothed, with less than four fine teeth not receiving
sub-main veins, the leaf is to some degree " reverted." (Ihe
maximum number of unveined teeth on a healthy leaf probably
varies with the variety.) In leaf No. 4, which shows a medium
stage of ' ' Reversion ' ' and one which is of common occurrence,
the sub-main veins are reduced to three and many of the teeth
have disappeared. In No. 5 there is a further reduction and
in No. 7 the extreme or " Oak-leaved " type is reached.

Identification by shape and size is less certain. " Reverted "
leaves are usually small and often long and narrow, but not
ahvuys. so. If, for instance, a " reverted " bush grown urfder
good cultural conditions be cut to the ground, the following
season's growth has quite large leaves which at first sight,
appear to be also almost normal in shape, but their " re-
verted " character is quickly revealed by the method outlined
above. Conversely, small leaves are not necessarily " re-
verted," such leaves, quite normal in character, being produced ;
frequently, both at the beginning and at the end of the growing;
period.

Date at which Inspection should be made. — It is important
to note the season during which ' * Reversion ' ' may be best
detected. A diseased bush may start growth by producing-
normal leaves; but soon after (about the middle of May at
Long Ashton) the ' ' reverted ' ' leaves begin to appear and
they continue to develop to the end of June. Additional leaves
of this type may develop later, but very often the foliage
produced in July and August is very much less " reverted '
and, by overtopping the badly affected leaves, is more con-
spicuous. No attempt to identify the disease should be made
early in the season. May and June are the best months and
if it is desired to do the work after this period, the shoots
should be turned back and the May and June leaves examined.

It should be noted finally that the whole bush does not
necessarily become affected at the same time. The bushes may
" revert " by degrees, branch by branch becoming affected.

* For the sake of convenience only half of the leaf is shown in theee
illustrations, but the amount of reduction is the same on both halves
of the leaf.

60

This fact is of great importance when selecting bushes for
propagation as described in the next paragraph. To be abso-
lutely safe every branch should be examined.

Measures of Control. — Two methods only can be recom-
mended at present, namely, care in propagating from sound
stock and rogueing of the beds. By strict attention to these
precautions at least one large grower has practically eliminated
the disease from his plantations.

1. Propagation from Sound Stock. — To secure this the stock
should be gone over in June and those plants marked which
are absolutely healthy. Plants should not be marked in groups,
but an individual examination of each plant should be made.
It is best to err on the safe side by marking only bushes which
are beyond suspicion.

2. Rogueing. — In June each year the cutting beds should
be gone over and every affected bush grubbed up and burned.
The same procedure should be adopted in the plantation except
that, as the bushes are fruiting, they should be marked and
reserved for destruction after the fruit is picked.

CONCLUSION.

In addition to the diseases dealt with in the twelve preceding
leaflets fruit trees are attacked by a large number of other fungi
and also by certain diseases the cause of which is unknown.
Some of these latter such as Bitter Pit are very serious but
up to the present so little is known as to their cause and
nature and the means required to control their attack, that no
leaflets have been issued. Others, again, are not considered
of sufficient importance in this country to justify the publica-
tion of special leaflets.

The principal diseases of fruit trees not included in this
volume are listed below, and such information as is available on
any of them will be supplied on application to the Secretary,
Ministry of Agriculture and Fisheries, 10, Whitehall Place,
London, S.W.I.

Apple — Gooseberry —

Silver Leaf. Leaf Spot.

Bitter Bot. Bust.

Bitter Pit. Cluster Cup.

Glassiness. Peach —

Scorch. Mildew.

Chlorosis. Pear-

Cherry— Canker.

Blossom Wilt. Brown Bot.

Brown Bot (see Plum Brown Leaf Spot.

Bot). Cluster Cup.

Leaf Scorch. Plum-

Currants — Bust.

Leaf Spot. Strawberry —

Bust. Leaf Spot.

Fig — , Mildew.

Canker

IX

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