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. Disease Cycle Apple scab is incited by the fungus, Venturia inaequalis, which overwinters in apple leaves on the ground. During rainy periods in the spring, just as apple buds are beginning to open, fungus spores produced in the fallen leaves are forcibly discharged.