Apple scab occurs wherever apples are grown and is one of the most serious diseases of apple and ornamental crabapple trees. 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.
Symptoms appear first in the spring as spots (lesions) on the leaf's underside which is the side first exposed to fungal spores as buds open. These lesions are usually small, velvety, olive green in color, and have unclear margins. On some crabapples, infections may be reddish in color. As the lesions 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.
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 looking. Scabs are unsightly, but are only skin deep. Badly scabbed fruit becomes deformed and may fall before reaching maturity.
Apple scab is caused by a fungus, Venturia inaequalis. This fungus survives over the winter in the previous year's fallen diseased leaves. In the spring, the fungus on the old leaves produce millions of spores which are released into the air during spring rains. Spores are then carried by the wind to new young leaves, flower parts and fruits. Once in contact with plant tissue, the spore germinates in a film of water and the fungus penetrates into the plant. Depending upon weather conditions, symptoms will show in 9 — 17 days.
Once established on the new leaves, the fungus produces a different type spore. These spores are carried and spread by splashing rain to other leaves and fruits where new infections occur. The disease may continue spreading 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.
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 crabapples with both excellent scab resistance and superior horticultural characteristics include: Anne E, Bob White, Molten Lava, Ormiston Roy, Prairifire, Red Jewel, Sargent, Sentinel, Strawberry Parfait and Sugar Tyme.
Rake and destroy fallen leaves as they occur. In the fall, carefully remove all fallen leaves from around the tree and destroy, do not compost. This will reduce the number of spores that can start the disease cycle the next year.
Where resistance to scab is not present, fungicide application is the primary method of control.