Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES
Scab of Apple and Crabapple
Apple
scab is one of the most serious diseases of apple and ornamental crabapple. Disease
development is favored by wet, cool weather that generally occurs in spring and
early summer: Both leaves and fruit can be affected. Infected leaves may drop
resulting in unsightly trees, with poor fruit production. This early
defoliation may weaken trees and make them more susceptible to winter injury or
other pests. Infected fruits are blemished and often severely deformed. Infected fruits may
also drop early.
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Symptoms Fruit
symptoms are similar to those found on leaves. The margins of the spots,
however, are more distinct on the fruit. The lesions darken with age and
become black and "scabby." Scabs are unsightly: but are only skin
deep. Badly scabbed fruit becomes deformed and may fall before reaching
good size. Symptoms
first appear in the spring as spots (lesions) on the lower leaf surface, the
side first exposed to fungal spores as buds open. At first, the lesions are
usually small, velvety, olive green in color, and have unclear margins. On
some crabapples, infections may be reddish in color. As they age, the
infections become darker and more distinct in outline. Lesions may appear
more numerous closer to the mid-vein of the leaf, if heavily infected the
leaf becomes distorted and drops early in the summer. Trees of highly
susceptible varieties may be severely defoliated by mid to late summer. |
Apple scab
lesions on apple leaves. |
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Apple scab lesions on fruit. |
Casual Organism Apple
scab is caused by the fungus, Venturia inaequafis. It survives the winter in the previous
year’s diseased leaves that have fallen under the tree. In the spring, the
fungus in old diseased leaves produces millions of spores. These spores are
released into the air during rain periods in April, May and June. They are
then carried by the wind to young leaves, flower parts and fruits. Once in
contact with susceptible tissue, the spore germinates in a film of water and
the fungus penetrates into the plant. Depending upon weather conditions,
symptoms (lesions) will show up in 9 to 17 days. The
fungus produces a different kind of spore in these newly developed lesions.
These spores are carried arid spread by splashing rain to other leaves and
fruits where new infections occur. The disease may continue to develop and
spread throughout the summer. Because a film of water on leaves and fruit is
required for infection to occur, apple scab is most severe during years with
frequent spring rains. |
Control
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1. The use of resistant or scab immune varieties is the
ideal method for controlling scab. Currently there are several apple varieties that are totally resistant to
scab. Backyard growers are strongly encouraged to consider using these
resistant varieties in order to reduce or eliminate the need for fungicide
applications around the home. Scab resistant apple varieties include: Prima,
Priscilla, Sir Prize, Freedom, Liberty, Jonafree, EnterPrise, Goldrush, Redfree, Pristine, Williams Pride, Novamac
and Nova Easygro. All other varieties, including
most commercially grown varieties are susceptible to scab; however, they differ
in their degree of susceptibility, McIntosh, Cortland, Red Delicious and Rome
Beauty are all very susceptible to scab, Golden Delicious and Jonathan are
less susceptible. Lists of scab resistant ornamental crabapples are available
from many nurseries and garden centers. Some of the many crab apples with
both excellent scab resistance and superior horticultural characteristics for
Ohio include: Anne E, Bob White, Molten Lava, Ormiston
Roy, Prairifire, Red Jewel, Sargent,
Sentinel, Strawberry Parfait and Sugar Tyme. 2. Rake and destroy fallen leaves. This will reduce the
number of spores that can start the disease cycle the next year. 3. Where resistance to scab is not present, fungicide
application is the primary method of control. |
Information obtained through the Ohio State Extension Factsheet HYG-3003-94
Insect
and Disease Fact Sheet Compliments of New Century
1-877-79TREES