Publication 519 June, 1953

Revision

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STUDIES IN FRUIT DISEASES

IX

APPLE SCAB

J. Fred Hockey

*** &»*

"Mature

6304

C212 DEPARTMENT OF AGRICULTURE

P 51 9 OTTAWA, CANADA

1953

APPLE SCAB

By J. Fred Hockey

Laboratory of Plant Pathology, Kentville, N.S.

The most prevalent and destructive disease affecting the apple is that
known as scab or black spot. With the possible exception of the irrigated
orchards in British Columbia, no commercial orchard area in Canada is free
from the disease. Orchardists in Ontario, Quebec and the Maritime Provin
spend hundreds of thousands of dollars annually in the purchase and applica-
tion of chemicals to their apple trees in an effort to prevent the damage caused
by apple scab. The disease attacks the leaves, blossoms and fruit and fre-
quently causes a drop of blossoms or defoliation of the trees or severe spotting
of the fruit to make it unmarketable. Losses of 200,000 to 400,000 bushels
of fruit have been experienced annually in Nova Scotia during the 1950, 1951
and 1952 crop years.

Orchards in Eastern Canada require spray protection every year as weather
conditions in this area favour the disease. When poor spray protection is given
apples the disease may assume epidemic proportions and result in the loss of
the current season's crop as well as materially weakening the trees for the
production of the crop the next year.

This pamphlet has been prepared to acquaint growers with the cause of
the disease and the relation of weather conditions to it, in the hope that a
better understanding of their relationship will lead to a more efficient applica-
tion of control measures.

Figure 1' Under surface of leaves showing
primary infections.

Description of the disease

In the spring the disease is first found on the under surface of the leaves
(Figure 1) as dark lesions. It appears later on the upper surface of the foliage
(Figure 2) in mould-like patches. The scab lesions are dark olive green,

velvety, of indefinite form and frequently cause a puckering of the leaf surface.
The disease is initiated each season by spores formed in the overwintering
leaves on the ground, concerning which a fuller discussion will be given in
a later section of this pamphlet.

Exposed parts of the blossoms are susceptible to infection and it is not
uncommon to find the disease girdling the stems and causing a heavy drop of
blossoms and young fruit. The lesions on the flower parts are not so conspicuous
as those on the leaves. They are lighter in colour and sparse.

Figure 2: The common form of scab on the foliage.

Figure 3: Scab spots on apples. Cracking is common on the older spots.

The twigs of very susceptible varieties of apples may be infected with
scab. Infection takes place when they are still quite green and succulent.
The disease appears as small round to oblong lesions, seldom over one-eighth
of an inch in diameter. These are most noticeable in the fall of the year.

The spots on the fruit (Figure 3) are well known to the majority of
orchardists. During the early part of the season they are most prevalent al
the calyx end of the apple but before harvest time may be found on any pari
73734

of the fruit. The spots are irregular in outline and like those on the leaves
have a dark olive-greeD colour. They sometimes enlarge and unite to form
extensive sca.b areas which may cause a dwarfing and cracking of the fruit.
The young fruit is frequently deformed or when the stems are affected and
girdled it may drop. Because of their small size late-season lesions on the
iiuii are sometimes known as "pin head" or "pin point" scab. These appear
shortly before harvest. They are frequently followed by "storage" scab
(Figure 1) occasionally called "ink spot" on account of the colour. The spote
are very dark, lack the olivaceous velvety appearance of earlier fruit infections
and may not develop until after ten to twelve weeks in storage.

Figure 4: Storage scab or "ink spot" on Golden Delicious. The infections
undoubtedly spread from the larger spots near the stem end.

Susceptibility of Varieties

Most commercial varieties are susceptible to apple scab. Mcintosh,
Fameuse, Delicious, Northern Spy and others are well known for their lack
of resistance. Practically all the seedlings and sports from these varieties,
which have been used commercially, can be placed in the same category.
Apparent resistance in some standard varieties has been found to be due to
escape from, rather than resistance to, infection. Apple breeding work now in
progress would suggest that varieties may be developed with resistance to scab.

Cause of the Disease

Apple scab is caused by a fungus named Venturia inaequalis (Cke) Wint.
The fungus overwinters in the leaves which fell to the orchard floor the pre-
vious autumn. From the time of leaf fall until early spring the fungus slowly
develops fruiting bodies which break through the top layer of the leaf tissue
as they approach maturity. They may be seen with the aid of a pocket lens
to be small, black round bodies scattered or in clusters. These fungus bodies
are known as perithecia, a Latin term used to describe similar fungous growths
which contain ascospores, spores developed in an ascus or sac. At the time
the perithecia are breaking through the leaf surface they are developing spores

internally and by the time the spores are mature there may be several hundred
perithecia on a leaf, each containing hundreds of spores. Since one spore pro-
duces a scab spot it will be realized that a few hundred leaves under a tree
may produce sufficient spores to cause a severe epidemic of the disease in an
orchard. When mature, the spores are readily discharged at any time the old
leaves are thoroughly wetted by rain, and are carried by air currents to the
trees. The first discharge occurs about the time the buds are in the "green tip"
or "mouse ear" stage of development. Subsequent spore discharges occur over
a period of six to eight weeks. The spores may infect any of the susceptible
tissue on the tree and are the cause of the primary infections. When the
spores reach the wet leaves, blossoms or fruit, they germinate and the germ
tube penetrates the outermost layers of the tissue. Here the penetrating germ
tubes develop into fungous growths which break through the surface in approxi-
mately two weeks, causing the dark olive-green, velvety lesions mentioned above.
When the primary scab lesions are readily visible on the foliage or fruit
they produce large numbers of summer spores frequently called conidia, to
distinguish them from the ascospores. These are splashed by rain to other
parts of the tree and cause secondary infections which may continue the
spread of the disease throughout the season ending with "pin head" scab at
harvest or the "ink spot" in storage.

Weather conditions

The fungus causing apple scab is dependent on weather conditions to
favour its development. The perithecia will not mature spores unless the
fallen leaves in which they grow are occasionally wetted. Ascospores will not
be discharged from the perithecia unless the leaves are moist. Rainfall is
essential to the spread of spores during the season as it not only liberates the
spores from spore-bearing structures but provides a medium in which germina-
tion of the spores may take place. Continuous foliage wetting for a period of
nine to fifteen hours accompanied by temperatures between 50° and 70° are
most favourable for spore germination. Such environmental conditions consti-
tute an "infection period". It is an advantage for growers to be able to identify
infection periods as this may have a bearing on the choice of the fungicide
to be used. Protectant materials must be applied before infection periods,
whereas eradicant fungicides may be used to good effect immediately after-
wards. The identification of infection periods can be made by a glance at the
diagram (Figure 5). For instance, infection may be expected to follow a
fifteen-hour wet spell with a temperature of 49° F or a ten-hour wet spell
at 60° F. and so on. It may require from ten to eighteen days after an infec-
tion for the new scab lesions to show, with the exception of storage scab lesions
which may not appear until several weeks after harvest.

Control of apple scab

From the above life history of the fungus it is apparent that there are
two main sources of infection: (a) Ascospores from the old leaves and
(6) conidia from the current season's lesions. Spores from twig lesions may
be considered a minor source of infection in Canada. It is, therefore, necessity
to prevent either the formation or discharge of ascospores or to protect the
foliage and fruit from primary infection.

Ploughing under or burning the leaves will destroy a tremendous source
of primary infection. Some chemicals applied as sprays to the leaves on the
orchard floor will prevent maturation and discharge of ascospores in the leaves
which are thoroughly covered with spray. The difficulties encountered in the
practical application of binning, ploughing under or spraying of the Leaves
in an orchard are such that these practices are of limited value in the com-
mercial orchards.

The control of scab in commercial orchards independent largely on the
use of fungicides, thoroughly applied a1 the proper dilution and the correcl
time. Thoroughness of application ran be determined by the operator and
the recommended dilution for each material is usually stated on the spray
calendar or on the fungicide container. The correct liming of applications
requires a further explanation as it involves some knowledge of the function
of the materials to be used. Fungicides, such as bordeaux mixtures, wettable
or paste sulphurs and organics (e.g. ferbam, Crag,) are protective materials
and to be effective must be applied before infection periods. Such materials
as lime sulphur, the organic mercurials (e.g. Tag, Puratized Apple Spray,
I j ad,) and the organic non-metals (e.g. Phygon) are eradicant fungicides.
These materials have given satisfactory control of scab when applied within
Iniiy-eight hours after the conclusion of an infection period.

DEGREES
FAHRENHEIT

75°

70

C± -,

65

60"

55'

50

45'

40'

HOURS OF

CONTINUOUS

WETTING

> 9 HRS.

- 10 HRS.

- II HRS.

- \2 HRS.

- 15 HRS,

- 20 HRS.

O

30 HRS,
46 HRS,

Figure 5: Apple scab infection periods. The diagram shows the approximate minimum number of hours
of continuously wet foliage necessary for scab infection. The longer the period of wet weather, the
more severe the infection. For example, 10 hours at 60° F. will cause a light infection, but 20 hours
may cause a severe infection. Scab spots do not appear until from 10 to 18 days after a given infection
period. (Diagram adapted after Dr. W. D. Mills, Cornell University).

Since the fungus may attack any of the new current season's growth the
spraying must be done thoroughly. Unsprayed parts of the leaves or fruit are
liable to infection. Clean foliage and fruit in the early season usually result
in clean fruit at harvest. Through the establishment of spray services in many
districts, growers may receive advice on the proper time to make specific
applications and the materials to use. Special weather forecasts are made for
the benefit of growers so that they may be warned of critical periods and may
time their sprays accordingly. Extra applications are sometimes necessary in
the spring during prolonged periods of wet weather. On the other hand it is
possible, frequently, to delay or eliminate applications during prolonged dry
periods. However, changes in the spraying program should not be made
without consulting the spray service in the locality concerned.

Spray Calendars and Materials

Spray calendars are published in all the provinces where apple production
is carried on commercially. They are issued as a guide to the control of
diseases and insects in those districts and are the result of extensive investi-
gational work. The sequence of mixing and the dilution strengths for recom-
mended materials should be closely followed.

Recent developments in spray materials make it possible for growers
requiring several days to spray their orchards, to put confidence in the
emergency use of eradicant fungicides. When an infection period interrupts
the application of a protectant fungicide, a grower may spray with an eradicant
immediately after the infection period in order to complete the program planned.
The eradicant will arrest infections that may have started and provide pro-
tection for about one week. The use of an eradicant is recommended for such
an emergency as indicated above and has been found in similar cases to be
very satisfactory under experimental and commercial conditions.

Before a decision is made on what fungicide is to be used, it is advisable
to inquire of the nearest plant pathological laboratory to secure a copy of the
spray calendar or schedule recommended for the district concerned. Some ma-
terials are more effective and can be used with greater safety in some districts
than in others.

It is generally accepted that sprays are more economical to use than dusts,
especially in districts where scab is a serious factor. Dusts may be used for
supplementary or emergency applications. Efficient dusting is just as essential
as thorough spraying.

Equipment

The selection of equipment for the application of the fungicide is important.
A large assortment of dusting and spraying equipment is available on the
market, some of which is designed for the home garden. The commercial
orchardist has a choice of power sprayers, speed sprayers and mist blowers.
The power sprayers have been standard orchard equipment for many years.
The speed sprayers are a more recent development and depend on a strong
air blast for the distribution of the fungicide. The mist blower or concentrate
sprayer is a modification of the air blast sprayers which will distribute a more
concentrated fungicide mixture as a fine mist. Each type of sprayer has its
advantages and disadvantages as well as its proponents. The size of the
orchard area and the availability of water and labour are important considera-
tions in the selection of suitable equipment.

Bordeaux mixture. — This fungicide, the oldest of the spray mixtures, still
remains the standard copper fungicide. Many formulae are in use to satisfy
local conditions. It is a good fungicide but, if used for either the "pink,"
"calyx" or first cover application may cause a severe russeting of the fruit.
Bordeaux should be applied only when it will be able to dry quickly, otherwise
severe leaf injury may result.

Copper Fungicides. — There are a number of "fixed" coppers available on
the market as alternative fungicides to bordeaux mixture. Recommendations
for their use are given by the manufacturers.

Lime Sulphur. — Lime sulphur has been in use for many years and is still
the recognized standard sulphur fungicide in some districts. It should no! be
used under weather conditions that will prevent it from drying within half an
hour after being applied. Applications of lime sulphur made during hot, humid
weather may cause foliage injury and dropping of fruit. Russeting of fruit
may follow the use of this material during very hoi weather. Excellent results
may be obtained with lime sulphur if used under suitable weather conditions.

s

fj(^j|9A OTTAWA K 1 A 0C5
3 9673

Sulphur Fungicides. Several forms of wet table and paste sulphurs are on
1 he market under various trade names. The paste sulphurs have the greater
adhesiveness. The manufacturers of these materials provide instructions for
their use Inn they should not be used when the temperature is above 85° F.

Organic Fungicides. — During recent years some very useful organic chemi-
cals have found their place as fungicides. Some are strictly protectants, such
as Ferbam, Crag, etc., while others, such as Tag, Puratized, etc., arc eradicants.
The eradicant fungicides may be used in place of lime sulphur after infection
periods with little or no fear of injury. Spray calendars frequently recommend
organic fungicides.

Dusts. — These materials have very limited use at the present time. Various
fungicide mixtures may be obtained for orchard use. Dusts may be used to
advantage as emergency applications during severe scab infection periods.

EDMOND CLOUTIER, C.M.G., O.A., D.S.P.

QUEEN'S PRINTER AND CONTROLLER OF STATIONERY

OTTAWA, 1953