In years with moist, cool spring weather infection can cause widespread defoliation. In spite of severe leaf drop, apple scab rarely kills a tree. If severe defoliation occurs for several years the tree could be weakened, making it vulnerable to other pathogens. Scab can also cause cosmetic blemishes on the fruit. Fruit with apple scab is still edible.
Apple scab can also infect hawthorn and firethorn.
The first signs of apple scab often are not noticed by the homeowner. These signs include twisted and puckered leaves that have black, circular scabby spots on the underside. On the upper surface the spots look velvety and have an olive green appearance. The spots may fuse and cover the whole leaf. As the disease progresses the leaves yellow and drop.