Apple scab is caused by the fungus Venturia inaequalis. The causal fungus causes spots on leaves. In early stages, spots appear as small black or olive-green, velvety lesions with irregular margins. Later the spots become more distinct. Infected leaves eventually turns yellow and falls prematurely. Infected fruit show distinct brown or black spots with margins that are often irregular. When severe, the skin splits open and irregularly shaped fruit results.
Cultural methods of disease management produce better results in relatively dry years and, in some situations, may even eliminate the need for fungicides. A properly pruned open tree canopy has better ventilation which allows for faster drying of leaves, reducing the number of infections. Because the apple scab fungus overwinters in apples leaves on the ground, raking and removing fallen leaves is recommended.
Selection of high quality, disease resistant varieties is desirable since there are differences in susceptibility.
There are fungicides labeled for the control of apple scab. Spray scab-susceptible cultivars before bloom, and continue through the growing season at regular intervals. Daconil 2787 is labeled for control of scab on crabapples.