The fungus Plowrightia morbosa (Apiosporina) causes the formation of dark brown to black, long, swollen galls or knots on the branches of most types of cultivated plums, prunes and cherries.

The disease is very destructive, killing twigs and limbs, and occasionally whole trees. Although there are no cherry varieties that are reliably resistant to black knot, there are some plums with resistance.

Symptoms

• Small, light brown swellings on the twigs or branches form during the first fall or spring after infection has taken place.
• Young knots are olive-green early in the season but later become hard, brittle and coal-black.
• Branches and sometimes the main trunk are girdled and killed.

Life History

The spores of this fungus are spread by wind and rain to the young twigs in the spring. After infection, a light brown swelling develops late the same year or the following spring. During the following growing season, spores are produced on the olive-green surface of the gall. Wild plums and cherries are important sites where the fungus persists in the landscape.

Management

• Prune and destroy branches with knots during fall and winter or before new growth starts in the spring. At this time, cuts can be made close to the knot if desired. Or, the knots can be surgically removed without removing the entire limb if necessary. Check again in the spring and prune any knots that may have been missed. In the spring, the cut should be made at least 6 to 8 inches below the lowest part of the knot.
• Destroy nearby wild cherry and plum trees or prune and destroy all knots on them as above.
• Never purchase nursery stock that shows visible knots on the twigs and branches.
• Apply fungicide just as green tissue is seen in the spring and again just before flowering.
• Plum varieties with some resistance include Methley, Milton, Early Italian, Brodshow, Fellenberg, Formosa, Shiro, Santa Rosa, and President.
• It is reported that tart cherry varieties, such as Evans Cherry, are less susceptible than other cherries.

Prepared by Gary W. Moorman, Professor of Plant Pathology