The best time to prune crabapples is in winter. Coat all pruning cuts that are an inch or more across with clear shellac. Inspect your crabapple trees frequently for signs of diseases and insects described in this section. See your county agricultural agent or State agricultural experiment station for information on how to control diseases and insects. Diseases Powdery Mildew Powdery mildew is a common fungus disease, particularly in humid areas.
Crabapples usually survive it without much damage.
The disease is characterized by a white moldy growth that disfigures the leaves and stems. Cedar Apple Rust Cedar apple rust is a fungus disease that originates on cedar trees and infects flowering crabapples. The disease appears first as yellow spots on the upper surfaces of leaves. These spots enlarge, turn orange, and develop small black specks in their centers. Blisters form in the leaf tissue under the spots.