Black knot is a very destructive fungal disease on susceptible cultivars of plum and prune.
This fungal pathogen is also occasionally found on apricot, peach, sweet and tart cherry, and ornamental Prunus spp. First described in 1821 in Pennsylvania, black knot is now generally distributed throughout North America. It was one of the most destructive diseases of plum and tart cherry in northern and eastern fruit regions during the late 1800s, but today is considered of less importance in most commercial orchards, except in blocks where it is well established. The disease occurs only on the woody parts of trees, primarily on twigs and branches, and sometimes on trunks and scaffold limbs. The warty swellings first become visible on new shoots in late summer or the following spring.