What is it? Black knot fungus (Apiosporina morbosa) is a common disease that affect plum and wild cherry trees. It infects the stems and twigs, causing a black, swollen mass on the twig.
Should I worry? Don’t lose sleep on it. Black knot is most concerning in orchards where there are many high value trees. New growth and young twigs are most often infected. Young trees with bad infections will be stunted and can eventually be killed. Addressing the problem before it becomes widespread will prevent damage to your trees.
How do I treat it? Check in winter for knots when they are most visible. Knots should be pruned out of the tree to prevent further spread of the disease. Prune 4-6 inches below the affected area. The knots must be burned, buried, or at least removed from the area. If they are not, they will continue to release spots and infect other branches. Severely infected trees may need to be removed.