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. The apple and flowering crabapple scab fungus also attacks hawthorn and mountain ash; closely related fungi cause scab diseases of pear and pyracantha.
Hartman, John R., "Apple Scab" (1996). Agriculture and Natural Resources Publications. Paper 11.