The fungus that causes apple scab on apples and crabapples is Venturia inaequalis. There are related fungi that cause scab on other plants in the rose family.
Symptoms usually start on the undersides of leaves. Spots, at first, are small, irregular lesions that are light brown to olive green in color. As infection continues, lesions become more circular and velvety olive green in color. Spots eventually turn dark brown to black. Infected tissue thickens, causing the upper surface to bulge upwards and the lower surface to depress. Leaves may curl and scorch at the margins. If the leaf petioles become infected, the leaves drop early. If the pedicels become infected, the fruits may drop early.
Scab on the fruit appears as nearly circular, velvety dark green lesions. The skin of the apple near the infected area margin ruptures. Older lesions are black, scabby, and cracked.
Infections occur during moist conditions (rain, dew or constant irrigation). The temperature affects the severity of infections. In order for infection to occur in cool weather, the plants must remain wet relatively longer than in warm weather.
Keep plants vigorous. Avoid stressing the apples and crabapples. Follow good sanitation practices. Remove and destroy infected leaves, flowers, and fruit as soon as possible. Grow resistant varieties when ever possible.
If necessary apply suggested fungicides according to all label directions and precautions.
The battle against scab is won or lost during late April through early June (from bud break to fruit set). This is when scab gets started. Thus, a fungicide spray schedule should be followed on more susceptible apple and crabapple varieties, if complete disease control is the goal.
During rainy, wet weather, make applications according to label directions and precautions. Thorough and uniform covering of all leaves and developing fruits is required for control. Most fungicides are used to prevent infection. Using a commercial spreader-sticker according to label directions will help ensure thorough coverage of the foliage with the fungicide.
Once leaves start to yellow and fall off the tree, it is too late to spray for control during the current growing season.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Nancy Pataky, Extension Specialist, Plant Pathology, Department of Crop Sciences University of Illinois at Urbana-Champaign.