It's the time of year to start seeing apple scab. Apple scab is the most common disease of crabapple in Iowa. Brown to olive-green, velvety spots appear on leaves, often concentrated along the veins. The spots may grow up to one-half inch in diameter, with feathery edges when young and more distinct edges when older. Affected leaves often turn yellow and fall off prematurely. A heavily infected crabapple may be completely defoliated by mid-July. Besides crabapples, scab can also be a problem on apple, pear, hawthorn, and mountain ash. It is the most economically important disease for apple production in most parts of the world.
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. You can help to minimize problems by keeping trees well spaced and pruned to promote airflow through the canopy. Raking up and destroying fallen infected leaves can help to reduce the level of initial inoculum. Even with these efforts, susceptible trees may still become diseased every year. Some homeowners choose to spray fungicides every spring to manage this disease on susceptible trees. Fungicides need to be applied early and repeatedly.
Crabapple cultivars vary in their susceptibility to apple scab, with some having virtually no problems and others being defoliated nearly every year. When selecting a crabapple variety, choose one with good resistance to scab. See Crabapples for Midwestern Landscapes for disease ratings of many common varieties. The Morton Arboretum also keeps a good list of susceptibility ratings. Note that even moderately resistant varieties can still develop scab in years when the weather is especially favorable for disease.
Apple scab on crabapple leaves.