Apple Scab

Need To Know

• Apple scab is the most common disease of crab apples in Iowa, but can also be found apple trees. 
• The fungus causes spots on leaves that appear black or olive-green, and infected leaves eventually turn yellow and fall off. Infected fruit has brown or black spots, and can result in split skin and misshapen fruit. 
• The fungus survives the winter on infected leaves. 
• Sanitation, proper pruning, and other Integrated Pest Management methods can successfully manage this disease.  

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.

Symptoms of apple scab 

The causal fungus causes spots on leaves. In the 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 a 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.

Signs of apple scab 

Olive, dusty residue, magnification is needed to observe fungal spores.

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 an 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 investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each state's 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.

Want to submit a sample? Follow the instructions at Submitting Trees and Shrubs. 

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 overwinters in leaf litter, sanitation is the primary measure to manage scab.

Satiation means removing fallen leaves where the fungus will overwinter. In the autumn, removing and destroying fallen foliage (chopped and composted to proper temperatures or discarded as trash) 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 prune and training trees to allow good air circulation.

There are fungicides labeled for the control of apple scab. Spraying scab-susceptible trees with preventive treatment (before the symptoms are observed) may help reduce this disease, but scab is usually not a deadly disease.

 

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.