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
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 (
Types of Samples Needed for Diagnosis and Confirmation
Collect 3 to 5 twigs with leaves attached and fruits. For further information go to Fruits.
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
Selection of high quality, disease resistant varieties is desirable since there are differences in susceptibility. However, since crabapple
There are fungicides labeled for the control of apple scab. Spray scab-susceptible