Apple scab is the most important fungal disease of apples in eastern growing regions of the United States, especially in the cooler climate regions. If not controlled, the disease can cause extensive losses (70 percent or greater) where humid, cool weather occurs during the spring months. Losses result directly from fruit or pedicel infections, or indirectly from repeated defoliation that can reduce tree growth and yield.
The disease can be observed on leaves, petioles, blossoms, sepals, fruit, pedicels, and less frequently, on young shoots and bud scales. The first lesions are often found on the lower surfaces of leaves as they emerge and are exposed to infection in the spring. Later, as the leaves unfold, both surfaces are exposed and can become infected. Young lesions are velvety brown to olive green and have feathery, indistinct margins. With time, the margins become distinct, but they may be obscured if several lesions coalesce. As an infected leaf ages, the tissues adjacent to the lesion thicken, and the leaf surface becomes deformed. Young leaves may become curled, dwarfed, and distorted when infections are numerous. The lesions may remain on the upper and lower leaf surface for the entire growing season; occasionally, the underlying cells turn brown and die, so that brown lesions are visible on both surfaces. The number of lesions per leaf may range from one or two to more than a hundred. The term "sheet scab" is often used to refer to leaves with their entire surfaces covered with scab. Young leaves with sheet scab often shrivel and fall from the tree. Infections of petioles and pedicels result in premature abscission of leaves and fruit, respectively. In late summer or early fall, lesions may appear whitish due to the growth of a secondary fungus on the lesion surface.
Lesions on young fruit appear similar to those on leaves, but as the infected fruit enlarge, the lesions become brown and corky. Infections early in the season can cause fruit to develop unevenly as non-infected portions continue to grow. Cracks then appear in the skin and flesh, or the fruit may become deformed. The entire fruit surface is susceptible to infection, but infections early in the season are generally clustered around the calyx end. Fruit infections that occur in late summer or early fall may not be visible until the fruit are in storage. This syndrome is called "pin-point" scab, with rough circular black lesions ranging from .004 to 0.16 inch in diameter.
The disease in managed with a combination of sanitation and chemical control. Use of resistant cultivars is recommended for home orchards and organic orchards east of the Rocky Mountains.
Apple scab lesions on leaf. Photo courtesy of Alan R. Biggs, West Virginia University.
Older scab lesions showing how the leaf becomes deformed. Photo courtesy of Alan R. Biggs, West Virginia University.
Apple scab lesions on young apple fruit. Photo courtesy of Alan R. Biggs, West Virginia University.
Fruit with apple scab may become corky and deformed. Photo courtesy of Alan R. Biggs, West Virginia University.
Alan R. Biggs, West Virginia University