Apple scab is a disease caused by a fungus and is most prevalent in areas with cool, moist summers, and in cool, wet years. The disease spores can spread quite rapidly under those conditions. The disease usually results in a heavy reduction in yield and quality of the fruit, but it can also blister and rupture the bark on twigs, defoliate the tree, and can result in the eventual loss of the tree. Apple, flowering crab apple, mountain ash and hawthorn are susceptible to this disease.
Symptoms occur on both the leaves and the fruit. Look for pale yellow or olive-green spots on the leaves. The margins of these spots are not distinct, but gradually blend into healthy tissue. The discolored areas can darken nearly to black. Severely infected leaves may become curled, cracked, and distorted. On fruit, symptoms first occur as small, dark, raised areas. Later, the skin ruptures and the exposed fruit tissue has a brown or black velvety appearance. As these spots enlarge and become older, the center turns brown and corky. Fruit becomes distorted and cracked as it grows, and young fruit drops. The disease spores pass the winter in fallen leaves and in the soil. In early spring, wind picks up the spores and blows them around. During damp periods, the opening leaves of the fruit tree are easily infected. The longer the leaves are wet, the more severe the infection will be. Once the disease has infected the tree, damage to the crop may be minimized but not cured.
Preventative spray programs must begin early in the season, since the spores will infect leaves from the time the cluster buds break until the leaves are fully expanded. Regular applications of mancozeb or lime sulfur fungicides throughout the season applied according to label instructions may prevent the disease. Lime sulfur can sometimes cause fruit injury on 'Delicious' apples. Preventative spray programs entail regular application of a fungicide throughout the season.
Additional ways to help control the disease include picking up and destroying fallen fruit that is infected, and raking and disposal of leaves and debris this fall. In early spring of the coming year, implement a preventative spray program if apple scab is a problem this year, or if it is historically a problem in your area. When using any pesticide, whether it is natural or synthetic, be sure to read and follow label instructions carefully to ensure personal safety, and safety to others and the environment. When planting new apple trees, try to select resistant varieties. Ask your local plant nursery or supplier for names of locally adapted resistant varieties.
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