Black knot is a common and often serious disease of plum and prune trees in New York. Once established, the disease becomes progressively more severe each year unless control measures are taken. Infected limbs and twigs lose vigor and may eventually die, and commercial plantings in which the disease becomes widespread are seldom economically viable to maintain. Black knot is occasionally found on cherries, peaches, and apricots but is seldom a problem on these trees.