What Are the Treatments for Black Knot on Plum Trees?

The fungal disease black knot attacks plum trees (Prunus sp.), and produces galls that can potentially kill the tree. The galls can girdle tree trunks if the infection is severe. Black knot disease on plum trees requires some vigilance to identify, as it can be covered by leaves in the early stages and thus overlooked until the symptoms of the infection are firmly established.

Pruning Treatment

Irregular swelling on twigs is one of the disease's first symptoms. Carefully look at the tree's twigs in order to catch and treat the problem. The following season, swellings turn olive-green and have a velvety texture. If allowed to grow during this season, they darken and develop a hard, brittle texture. Well-established knots are 1/2 inch to 12 inches long and 2 inches in diameter. Older knots may become white or pink because of the fungal parasite Trichothecium roseum. Treat the disease by removing the galls, and then burn or bury them. Prune branches 6 to 8 inches below the gall. If it is necessary to remove galls from the tree trunk, use a chisel and cut at least 1 inch beneath the infected area. The tools should be cleaned with a 10-percent bleach solution.

The black knot disease is less likely to attack healthy plum trees, so fertilize each spring with an all-purpose fertilizer, and water the tree during dry weather. Inspect both wild cherry trees and other plum trees within 500 feet of your plum tree; if galls are present, remove them, and then burn or bury the branches. Remove any source of the disease from hedgerows or woodlots.

Some commercially available fungicides can control and treat black knot disease on plum trees. Fungicides should be used if the infection is severe with many large galls that girdle the tree's limbs and trunks. Captan is a wettable powder, chlorothalonil is a liquid (flowable) fungicide and myclobutanil and thiophanate-methyl are locally systemic fungicides. When applying fungicides, thoroughly cover the tree following the label's directions. Your choice of fungicide should follow the spray schedule: spray the tree just before blooms open in the spring, spray again after the petals have fallen, then continue spraying at 10- to 14-day intervals throughout the growing season.