Problem: Apple Scab
This plant disease is caused by a fungus (Venturia inaequalis). It is a serious problem on crabapples and apples in areas where spring weather is humid with temperatures ranging from 60° to 70°F. The fungus spends the winter in infected fallen leaves. In the spring, spore-producing structures in the dead leaves continuously discharge spores into the air. The spores are blown by the wind to new leaves and flower buds. If water is on the tissue surface, the fungus infects the tissue and a spot develops. More spores are produced from these spots and from twig infections from the previous year. The spores are splashed by the rain to infect new leaf and fruit surfaces. As temperatures increase during the summer, the fungus becomes less active.
Rake up and destroy infected leaves and fruit in the fall. When planting new trees, use resistant varieties. Apply a fungicide labeled for apple scab, following label directions.