Flowering Prunus species, such as purple-leaved plum and many backyard fruiting plums, are susceptible to black knot disease, which is caused by the fungus Apiosporina morbosa. Plum twigs are infected in early spring and by fall may begin to show evidence of elongated swellings. By the second year, these swellings become rough and black and, where they occur, twigs and branches begin to die back.

The disease is managed mainly by pruning out and destroying the knots as they appear. Select resistant cultivars when possible.

Apiosporina morbosa is widespread in cultivated and wild plums, prunes, and cherries (Prunus spp.). Control is highly dependent on early identification and removal of infected twigs, stems, and branches. Material that cannot be burned, bag and send to a landfill. Also, destroy mummified and diseased fruit from branches and ground underneath. Wounds to trees by wildlife, such as deer and voles, are a point of disease entry.