Overview apple scab
Apple scab is the most common disease of crabapple in Iowa, but can also be found on many varieties of apple and pear.
Signs and symptoms of apple scab
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.
Disease cycle of apple scab
Apple scab is caused by the fungus Venturia inaequalis (in apples and crabapples). 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.
Type of Sample Needed for Diagnosis and Confirmation
The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents. If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us
Management of apple scab
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 overwinter in leaf litter sanitation is the primary measure to manage scab
Satiation means removing fallen leaves where the fungus that will overwinter. Removing and destroying fallen foliage (chopped and composted to proper temperatures or discarded as trash) in the autumn will dramatically reduce the number of this pathogen available to infect trees next year. The best line of defense against this disease would be to plant scab-resistant cultivars and pruning and training trees to allow good air circulation.
There are fungicides labeled for the control of apple scab. Spray scab-susceptible with preventive treatment (before the symptoms are observed), may help reduce this disease, but scab is usually not a deadly disease.
For more information on apple scab visit the APS article Apple scab.