Most of the commonly-grown apples and crab apples are susceptible to scab. The disease causes losses by reducing the amount and quality of fruit and by reducing tree vigor. A reduction in tree vigor can result in increased winter injury and susceptibility to secondary diseases and insects. Symptoms Scab first appears on young leaves as olive-green spots with fuzzy margins. As the disease progresses, the spots develop more distinct margins and become greenish-black in color.
Infected leaves are often misshapen, and severe infection can result in defoliation.
Fruit is susceptible to infection by the scab fungus anytime during its development. Early infections can result in dropping of the blossoms and young fruit. Lesions on the young fruit resemble those on leaves. As the fruit matures, the affected areas become brown, corky and often cracked. Late summer and fall fruit infection show up as small black specks called pinpoint scab.