University of Maryland Extension

Black Knot- Trees

black galls on twigs
Black knot galls on a twig

Black knot, caused by the fungus Apiosporina morbosa, forms galls on plants in the genus Prunus, including plum, cherry, flowering almond, apricot, and blackthorn. The first symptoms appear in the fall as swellings on twigs. The swellings enlarge the next season, gradually encircle the stem, and form black galls. The surface typically splits or cracks, giving a roughened or crusty appearance. The stem beyond the gall typically dies back and fails to leaf out.

black knotManagement

Chemical controls are not very effective. Prune out infected stems and branches at least 4 inches below the galls and remove the clippings from the landscape before April 1. Infected wild plum and black cherry are often a source of new infections.

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