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Black Knot

Black knot is a disease caused by the fungus Dibotyron morbosum, AKA Apiosporina morbosa. It affects primarily plums, but it is also sometimes found on cherries, peaches, and apricots. In spring, the fungus produces spores (acospores) that are forcibly ejected during rainy periods. Rain and wind carry the spores to nearby plant material. During rainfall, when temperatures are above 55°F, new infections can be established in as little as six hours. The disease is active from April through to June, and it overwinters in the stem of the infected host.

The visible symptom of this fungus is a wart-like, black swelling on the tree. When the growth first appears, during the first year of infection, it is an inconspicuous swelling. The next year, it will develop into a greenish, corky tumor, and in the third year it has a blackened, charred appearance. The growth can be anywhere from a few inches to a foot in length, and it deprives the branch of nutrition, eventually girdling it entirely and killing it.

Black knot can be controlled with sensible sanitary practices. Obviously, all knots should be pruned out before the fungus becomes active in April. Cuts should be made 3-4 inches beneath the infection and blades should be cleaned with alcohol between cuts. Infected material and other debris need to be removed from the growing area and burned if possible. Inspect any nearby wild plums; these are often black knot hosts, and should be removed before they contaminate your garden. Finally, a fungicide spray program may be necessary if your trees are heavily infected. Some varieties are reported to have resistance to black knot: Bluebyrd and President are classed as highly resistant, while Methley, Shiro, and Santa Rosa are moderately resistant. Stanley is considered highly susceptible.

For more information, see the Cornell IPM Factsheet on Black Knot and the Cornell Guide to Growing Fruit at Home.

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