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Cornell University : Cornell Cooperative Extension
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| Yates Association |
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Black Knot in Cherry and Plum |
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Black knot disease occurs on numerous cultivated and wild plums, prunes, and cherries. The disease is characterized by the presence of warty, black galls which may vary in size from 1/2 inch to more than 1 foot in length. In some parts of the Northeast and Midwest, black knot causes serious losses to commercial plum and prune growers. More often, however, the grotesque galls draw attention from homeowners who want to improve the unsightly appearance of affected landscape trees. The first symptoms is small, light brown swellings of the current or last season's growth. These swellings are often not noticed and the disease becomes evident only after the knots have enlarged and are olive-green with a velvety texture the following spring. Soon after this stage the knots become darker and by fall they appear as the black hard knots which are the typical symptoms observed. So the disease may not be recognized until a year or more after infection. Knots will continue to grow until they girdle the branch and kill it. Black knot is caused by a fungus. Infections occur between April and June during warm, wet weather. The fungal spores are produced on living galls which are one to several years old. Usually young succulent twigs are infected. In managing this disease one should prune out all knots during the winter season. Disinfect the pruners before each new cut is made. Prunings should be destroyed or buried so that fungal spores may not spread from these to cause new infections. For most home grounds situations this should give enough control unless there are numerous infected wild Prunus, plums, prunes and cherries, in the neighborhood. Cornell
Cooperative Extension Yates Association |
