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. For backyard apple trees, home orchard sprays containing fungicides need to be applied at about the same times as crabapples to provide protection from scab. Captan is suggested. Dormant oil sprays do not control scab.
Resistant varieties are the best way to manage scab, especially for ornamental crabapples.
There are many resistant cultivars available, so be sure to ask for them when selecting crabapples. Consider replacing extremely susceptible trees with resistant cultivars. Some apples also have resistance.