Apple scab has been a serious disease problem on crabapples and apples the past few seasons as cool, wet spring weather is ideal for this disease to develop. During the past few seasons, leaves on susceptible trees dropped by mid-summer. Scab affects both ornamental crabapples and apples grown in orchards. Cultivars will vary in susceptibility. If your crabapple or apple did not get this disease the past few seasons, either you have a resistant cultivar or were spraying the tree with a fungicide to prevent scab.
Apple scab appears as olive green to black spots or lesions on leaves.
Heavy infection, as happened the past few seasons, makes leaves turn yellow or brown and drop from the trees. Once the leaves are infected and dropping, there's nothing to spray on the tree. Controlling apple scab involves several steps. If a susceptible crab already exists in the landscape, the only way to prevent scab is to apply protective fungicide sprays to newly developing foliage. Captan, chlorothalonil, copper, maneb, mancozeb, sulfur, and thiophanate-methyl are among the fungicides listed in the 1998 control guide from the University of Illinois.