Scab is caused by the fungus Venturia inequalis. Infection takes place early in the season and dark olive green spots appear on the leaves. In late summer the infected leaves fall off when they turn yellow with black, spots. Infected fruits have black, slightly raised spots. Use resistant varieties.
Fire blight susceptible trees have blighted branch tips. The disease is caused by the bacterium Erwinia amylovora. Leaves on infected branch tips turn brown or black, droop, and hang on the branches. The leaves look scorched as by a fire. The trunk and main branches become infected when the bacteria are washed down the branches. Cankers form and are separated from adjacent healthy bark by a crack. The infected bark may be shredded. There is no satisfactory chemical control for fire blight. Use resistant cultivars when available.
Powdery mildew coats leaves with white powder and may be controlled with sprays of benomyl.
Rust causes brown to rusty-orange spots on the leaves. Red cedars are the alternate host. Badly spotted leaves fall prematurely.
Crab apples are subject to several canker diseases. Avoid unnecessary wounding, keep trees healthy, and prune out infected branches when seen.