As you look outside at your favorite plum or cherry tree, you may notice something quite shocking (disgusting to some) on the branches of the tree. Large black swellings can sometimes take over most of the woody parts of the tree. What could these unsightly swellings be? The answer is most likely black knot.

Black knot is caused by the fungus Apiosporina morbosa. This fungus attacks a wide variety of trees in the Prunus genus, including sweet and sour cherry, many cultivated varieties of plums and prunes, and wild plum and cherry trees. Usually just an eyesore to most, it can become widespread in the tree, causing death of limbs, and even death of the tree. These knots also make the tree more susceptible to a variety of borer-type insects.

You can find these large black swellings on any woody part of the tree. These swellings (knots) are first visible on new shoots in the summer or late spring. They first appear olive-green in color, but then age and become blackish, hard and brittle.

There are a variety of ways to control black knot. Sanitation is the most important control measure. During the fall and winter months, prune the infected trees to remove all visible knots. When pruning, be sure to remove at least 2-4 inches below the lowest part of the knot to ensure that all disease is removed. Sometimes the infection is already too far along, and if most of the tree is covered with the "knots," you may have to remove the tree. Infected branches should be removed and burned (if feasible) to prevent further spread of the disease. You also should make sure to inspect trees for disease problems before purchasing them. In addition to sanitation, fungicides also may be used in the spring when green tissue starts to appear. Sprays should continue just before and after bloom, and you may need to continue a preventive treatment until growth stops. Planting trees with resistance can also be helpful.

This article originally appeared in the 2/13/2004 issue.