While the problem may not become evident until leaves of crabapples and apples turn yellow and drop, control needs to start early in the season as buds open and leaves start to unfold. If apple scab has not appeared on your crabapple or apple tree the past few years, chances are it is a resistant cultivar. Apple scab first appears as olive green to black spots or lesions on leaves. As the season advances, leaves turn yellow or brown and drop from the trees. Once the leaves are infected and dropping, there's nothing to do to stop the disease that season.
Infection actually occurs early in the season during cool, wet weather as leaves develop.
There are some options to consider for managing apple scab. Captan, chlorothalonil, copper, maneb, mancozeb, sulfur, thiophanate-methyl are among the fungicides suggested. Read and follow all label directions. In the backyard orchard, home fruit tree sprays containing fungicides need to be applied at about the same times as for crabapples to provide protection from scab. Captan is suggested.