| Apple scab is caused by the fungus Venturia
inaequalis. It is common on susceptible apples and
crabapples in Missouri and causes leaves to yellow
and drop prematurely in midsummer. In years with
moist and cool spring weather, infection can cause
widespread defoliation. In spite of severe leaf drop,
apple scab rarely kills a tree. If severe defoliation
occurs for several years the tree could be weakened,
making it vulnerable to other problems. Scab can
cause cosmetic blemishes on the fruit. Fruit with
apple scab is still edible. Apple scab can also infect
hawthorn and firethorn.
Symptoms and Diagnosis
The first signs of apple scab often are not noticed.
These signs include twisted and puckered leaves that
have black, circular scabby spots on the underside.
On the upper surface the spots look velvety and
have an olive-green, sooty appearance. The spots
may fuse and cover the whole leaf. As the disease
progresses, the leaves yellow and drop. The fruit
develops scabby spots that are tan and sunken.
Apple scab may be confused with cedar-apple rust,
also a fungal disease. Cedar-apple rust can also
cause yellowing leaves, but the spots are bright
yellow with a dark center of black fruiting bodies.
Life Cycle
Fungal spores overwinter on infected fallen leaves
and fruit. Spring wind and rains carry the spores on
to the young leaves. Cool, moist springs favor
infection. Ten to twenty days after infection, new
spores are released which infect new leaves. These
secondary infections can continue throughout the
summer until conditions become too hot and dry.
Wet leaves are needed for the fungal spores to
germinate and infect the leaf.
Integrated Pest Management Strategies
1. Live with the disease. Apple scab rarely kills trees
and the severity of infection will vary from year to
year depending upon climatic conditions. Give
more attention to young trees that may not
establish well if prematurely defoliated. Also take
note of any trees that become weakened through
several years of defoliation and act accordingly.
2. Keep plants healthy. A tree that is watered during
dry periods will be better able to withstand
periodic defoliation.
3. Clean up and destroy infected leaves. The fungus
overwinters on infected leaves; therefore, it is
important to rake up and destroy leaves in the fall.
Since the spores also travel on the wind, cleaning
up leaves may be only partially effective.
4. Prune trees to allow good air circulation.
Allowing for better air movement will allow leaves
to dry more quickly and help prevent infection
from occurring. Do not use overhead watering.
5. Apply fungicidal sprays to susceptible trees if
clean foliage is necessary or if defoliation reduces
fruiting on eating apples. sulfur can be used but it
must be applied before the fungal spores
germinate. Chemicals registered for use include
chlorothalonil (Daconil), copper, ferbam, fosetyl-
Al, mancozeb, maneb, thiram, triflumizole, and
ziram. Fungicidal sprays must be applied before
infection as a preventive to be effective.
6. Replace the tree with a variety resistant to apple
scab. Resistant varieties of eating apples include:
‘Florina’, ‘Freedom’, ‘Gavin’, ‘Jonafree’, ‘Liberty’,
‘Macfree’, ‘Nova Easygro’, ‘Novamac’, ‘Priam’,
‘Prima’, ‘Priscilla’, ‘Redfree’, and ‘Sir Prize’. Many
resistant crabapple varieties are available at
nurseries and garden centers or through mail
order sources. |
