Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES
Apple Powdery
Mildew
Damage from powdery mildew
attack results in stunted growth. The foliage becomes distorted and twig growth
is reduced. Also, the fruit surface may become russetted
or discolored, and dwarfed. Heavily mildewed trees are weakened, and are more
susceptible to other pests and winter injury.
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Symptoms Powdery mildew maybe found
on buds, blossoms, leaves, twigs, and fruit. Symptoms first appear in the
spring on the lower surface of leaves, usually at the ends of branches.
Small, whitish felt-like patches of fungal growth appear and quickly cover
the entire leaf. Diseased leaves
become narrow, crinkled, stunted and brittle. By midsummer, tiny, black
round-specks show up on the lower leaf surface, but more commonly on the
twigs. These are fungal fruiting bodies, but their-importance in the disease
cycle is probably minimal. The fungus spreads rapidly
to twigs, which stop growing and become stunted. In some cases the twigs may
be killed back. Leaves and blossoms from infected buds will be diseased when
they open the next spring. Infected blossoms shrivel
and produce no fruit. Fruit symptoms are not usually seen unless the disease
has built up to high levels on susceptible cultivars. Diseased fruit has a
fine network type surface blemish called russetting. |
Malformation of apple leaves caused by powdery
mildew. |
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Netting pattern on apple fruit caused by powdery
mildew infection. |
Causal Organism and Disease Cycle Powdery mildew is caused by
the fungus, Podosphaera leucotricha. Powdery mildew over winters as fungal
strands (mycelium), in vegetative fruit buds which were infected the previous
season. Infected terminals may have a silvery gray color; stunted growth, and
a misshapen appearance and are more susceptible to winter kill than are
non-infected terminals. Temperatures near -18 degrees F kill a majority of
mildewed buds and the fungus within them. Even at lower temperatures,
however, some powdery mildew survives. |
As buds break dormancy, the
powdery mildew fungus resumes growth and colonizes developing shoots causing
primary infections. The powdery white appearance on infected shoots consists of
many thousands of spores which are responsible for spreading the fungus and
causing secondary infections later in the growing season. Secondary infections
are important because they produce the over wintering infected buds. .
Secondary infections usually
develop on leaves and buds before they harden off and may reduce the vigor of
the tree. Fruitlets may become infected shortly after
bloom, resulting in a web-like russetting on the
mature fruit.
Powdery mildew infections
occur when the relative humidity is greater than 90% and the temperature is
between 50-77 degrees F. The optimum temperature range for the fungus is 66-72
degrees F. Although high relative humidity is required for infection, the
spores will not geminate if immersed in water. Leaf wetting is, therefore, not
conducive powdery mildew development. This is quite different from most other
foliar and fruit diseases caused by fungi that require free water for their
spores to germinate and infect. Under optimum conditions, powdery mildew can be
obvious to the naked eye 48 hr after infection, About 5 days after infection, a
new crop of spores is produced, Non-germinated powdery mildew spores can
tolerate hot dry conditions and may persist until favorable conditions for
germination occur. It is important to remember that powdery mildew can be a
problem in drier growing seasons when other diseases are not a problem.
Control
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1. |
Apple varieties vary
greatly in their susceptibility to powdery mildew. Jonathan, Granny Smith, Mutsu, (Crispin), Rome, Cortland, Baldwin, Monroe,
and Idared are very susceptible and should be
avoided if powdery mildew is a problem.
Most other varieties may also be infected if inoculum is present and
conditions are favorable for infection.
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2. |
Plant trees in sunny
locations with good air drainage. This
reduces the humidity around trees and reduces the chances of disease. |
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3. |
Where powdery mildew is a
problem, a good fungicide spray program is generally required for
control. This is especially true in
commercial plantings. |
We
want to thank the New York State Agricultural Experiment Station for use of
figure 3, Taken from Tree Fruit IPM, Disease Identification Sheet No.4.
Information obtained through the Ohio State Extension Factsheet HYG-300l-94
Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES