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Apple scab

Venturia inaequalis (Cooke) G. Wint.

On leaves, young lesions are velvety brown to olive green with indistinct margins, and will often not be readily noticeable until after petal fall in commercial orchards (A). The number of lesions can vary from few to several hundred per leaf, depending on the season and varietal susceptibility. Older leaf lesions are typically raised, with a corresponding cupping on the underside of the leaf, and dark green to gray to brown in color, with distinct margins (B, C). Leaves that are heavily infected tend to curl, shrivel, and fall from the tree. On the fruit, young lesions appear similar to those on leaves (D). Although the entire surface of the fruit is susceptible to infection, lesions often cluster around the calyx end of the fruit. As lesions get older, they become brown and corky and take on a "scabby" appearance (E).

Distribution

Common to all fruit-growing regions in eastern North America. Often less severe and easier to control in arid or warmer climates, and in dry years.

Similar Species

Apple scab is one of the common diseases of apple and is usually easily distinguishable from other diseases. However, older, exhausted lesions in late summer and fall, particularly those in sprayed orchards, may be confused with any number of maladies due to the absence of visible, olive-green sporulation.

Management

Apple scab overwinters in leaf litter as small black fruiting structures called pseudothecia. Ascospores, the primary inocula, form in the spring and are discharged in response to wetting events. Ascospores can be released as early as green tip, but peak ascospore release generally coincides with bloom. Apple scab is managed primarily through the application of fungicides from green tip through midsummer. The destruction of leaf litter in the fall by flail mowing or through the application of chemicals that hasten leaf litter degrading, such as urea, helps reduce primary inoculum. The severity of infection depends on a variety’s inherent resistance. Cortland, McIntosh, Paula Red, and Crispin are extremely susceptible; Delicious, Empire, Gala, Golden Delicious, Ida Red and Spartan are moderately susceptible. Resistant varieties include Goldrush, Enterprise, Florina, Liberty, Jonafree, Macfree, Novamac, Nova Easygro, Prima, Priscilla, Sundance, Scarlet OHara, and William's Pride.

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