As the season progresses the swellings harden and become coal black in color. By this time the knots have enlarged considerably and may be as long as 6 inches. Often only one side of a limb is affected, however, in some cases the limbs may become completely encircled. Knots continue to expand in the following years until the branch is girdled and dies. Old knots may become covered with a white or pink parasite.
Infections can occur in elongating twig tissues from bud break to shuck split.
Pruning is best accomplished in the fall or winter when the lack of foliage makes it easier to locate branches with galls. Cuts should be made 4 to 8 inches below the knot. Surgical removal of the black swollen tissue may be necessary if the trunk and large limbs have become infected. Examine infected trees before the buds open in April and prune out all newly formed knots. The removal of wild plum and cherry trees in the area of valuable cultivated susceptible trees is also advised wherever practical.