Apple Scab by John R. Hartman Apple scab is the most consistently serious disease of apples and flowering crabapples in Kentucky. The most noticeable losses result from the reduced quality or premature dropping of infected fruit, but the disease also causes a general weakening of the tree when infected leaves are shed prematurely. For flowering crabapples, summer defoliation from scab invariably results in fewer flowers the next spring. The best control of scab is provided by use of scab-immune cultivars. A good spray program also can provide excellent control of apple scab, but failure to follow such a program can lead to substantial losses on susceptible varieties, especially in a wet year.
Spots range in size from that of a pinhead to slightly smaller than a dime.
Eventually, tissue near the lesions thickens and bulges upward. Mature, well-outlined apple scab leaf lesions, covered with a dark, velvety growth. Fruit Symptoms on the fruit are similar to those on the leaves, although on fruit the spots may be more distinctly outlined. Uneven growth in the vicinity of the scabs can cause fruit to become severely deformed if infections occur while the fruit is still small. Heavily infected fruit may drop prematurely.