This is a disease that most commonly affects plum and cherry trees. Greenish swellings form on twigs and branches, in time these develop into black, cylindrical corky galls which will kill the infected part. Black knot spreads more quickly during warm, wet springs.
There is no chemical control for black knot. Prune infected branches late winter, cutting at least 4 inches beyond the infected area and spray with dormant oil. Sterilize pruning equipment with dilute bleach in between cuts. Repeat applications of fungicide every 7-10 days. This may help to slow down the spread of the disease.