Apple scab is a significant fungal disease of crabapple trees. A similar disease affects pears.
Apple scab appears on expanding leaves in the spring as olive-green spots without distinct borders. These lesions grow and darken to a greenish-black color and develop black borders.
Leaves may then turn brown and drop and defoliation is possible. Fruits develop brown, scab-like lesions, are often deformed, and may drop off before maturing.
The disease is favored by cool, wet springs and overwinters on fallen leaves. Numerous disease cycles occur each year so controlling this disease early in the spring will lessen disease symptoms through the season.
Prevention and control strategies include
Plant resistant crabapple varieties, such as Ames White, Autumn Glory, Baskatong, Beauty, Coral Cascade, Gibb's Golden Gage, Gwendolyn, Harvest Gold, Henning, Molten Lava, Narragansett, Prof. Sprenger, Red Snow, or Sparkler. Check nursery catalogs and local garden centers, as there are many other resistant varieties.
Rake up and discard all infected and fallen leaves.