Apple scab is the most common disease of crabapple in Iowa, but can also be found on many varieties of apple and pear.
The causal fungus causes spots on leaves. In early stages, spots appear as small black or olive-green, velvety lesions with irregular margins. Later the spots become more distinct and may grow up to one-half inch in diameter. Infected leaves eventually turn yellow and fall prematurely. Infected fruit show distinct brown or black spots with margins that are often irregular. When severe, the skin splits open and irregularly shaped fruit results.
Apple scab is caused by the fungus Venturia inaequalis. This fungus survives the winter in infected leaves on the ground. In the spring, the fungus produces sexual spores (ascospores) that can travel by wind to infect newly-developing leaves. Once infection has begun, the fungus on the new leaves develops asexual spores (conidia) to reinfect the leaf and initiate infection of other nearby leaves. Cool, wet conditions in the spring favor apple scab, so the severity of disease seen in a given year can vary with the weather.
Cultural methods of disease management produce better results in relatively dry years and, in some situations, may even eliminate the need for fungicides. A properly pruned open tree canopy has better ventilation which allows for faster drying of leaves, reducing the number of infections. Because the apple scab fungus overwinters in apples leaves, raking and removing fallen leaves is recommended.
Selection of high quality, disease resistant varieties is desirable since there are differences in susceptibility. However, since crabapple cultivars vary in their susceptibility to apple scab, some have virtually no problems and others can be defoliated nearly every year. Note that even moderately resistant varieties can still develop scab in years when the weather is especially favorable for disease.
There are fungicides labeled for the control of apple scab. Spray scab-susceptible cultivars before bloom, and continue through the growing season at regular intervals until about the middle of June. Infections are less likely to occur with the arrival of warmer, drier weather in early summer. Daconil 2787 is labeled for control of apple scab on crabapples.