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Black Knot Disease of Plums and Chokecherries

Source: NDSU Extension Service Horticulturist

Black misshapen swellings on the branches of plums, cherries, chokecherries and Mayday trees is usually a sign of the black knot disease. Black knot is caused by a fungus that infected the branch one or two years before the knot showed up. In the spring, these knots are a velvety olive-green in color and millions of spores are produced that help the fungus spread to healthy branches. Later in the season the knots become hard and brittle, and black by fall. The rest of the branch from the knot to the branch tip, may die. Often the branch is twisted or distorted at the knot. To control this disease, prune out all diseased branches. The pruning cut should be made about 3 inches below the knot. All of the diseased branches should be destroyed by burning, burying or sending to a sanitary landfill. After pruning, the cut surfaces of large branches should be covered with a wound dressing such as asphalt tree coating. Effective control of black knot requires elimination of nearby sources of infection - this includes diseased branches in other plum and cherry trees as well as nearby wild plum and cherry trees. Either the wild trees must be eradicated, or else the diseased knots must by pruned out.

Since the knots take one or two years to develop, a moderate number of new knots may appear the first year after the control program was initiated. By the second and third years, only an occasional black knot should appear. If there are many new knots after this period of time, then some nearby source of infection has not been eliminated. This source should be located and eliminated. Annual spraying during early spring (while branches are still dormant) with lime-sulfur will also help to control the spread of this disease.

For future reference, you may want to obtain a copy of Extension bulletin PP-689, "Disease Control in Cherries, Plums and Other Stone Fruits," which is available at your county office of the NDSU Extension Service.

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