The fungus causing black knot is Apiosporina morbosa (Dibotryon morbosum). Black knot affects plums and cherries (wild, ornamental and edible varieties) as well as other members of the Prunus genus.
Generally found on twigs, branches and fruit spurs. Trunks can also be infected. infections usually start on young tissue. abnormal growth starts as light brown swellings that later enlarge, rupture and become black. During the second year, the growth continues to elongate along the branch and encircles it. Several knots may merge to make a large knot. In the second year the fungus in the knot dies except at the margins. The black knot is then invaded by secondary fungi that may change the color to a whitish or pinkish color. Insects frequently colonize older galls. With time and the spread of the disease, the tree loses vigor and dies
Buy only certified clean stock. Prune and destroy infected wood during late winter to very early spring. Make cuts at least six to eight inches below any swollen or knotty tissue. On trunks, chisel out diseased areas; removing at least one inch of healthy tissue around the gall. Eradicate severely infected plants. Control insects. Grow resistant varieties.
Current fungicides are not very effective against this disease. The amount of control does not justify the cost.
Written by James Schuster, Extension Educator, Horticulture, and reviewed by Nancy Pataky, Extension Specialist, Plant Pathology, Department of Crop Sciences University of Illinois at Urbana-Champaign.