Apple Scab is one of the most prevalent and serious diseases affecting apple trees throughout the world. Crabapple and wild apple trees are also attacked. This is another fungus related tree disease and is caused by the fungus venturia inaequalis. Wet and cool conditions in the spring favor the disease. The fungus attacks the leaves and buds causing premature defoliation and poor and deformed fruit production. The fungus will survive through winter on fallen leaves. In spring weather the fungus will produce millions of spores, which are released by rain and spread by the winds.
Symptoms: Include leaf spotting and lesions on the underside. Fruit develop dark blotches and may be disfigured.
More information: Purdue University Extension, Ohio State University Extension