Many American, Japanese and European plums are susceptible. A major source of infection are the native American wild cherries that are common along the roadside.
Severe infections can result in the death of branches and, eventually, the entire tree.
The black knot fungus causes elongated swellings or knots on infected branches. At first the knots, sometimes called galls, are greenish to light brown. Upon aging, they turn black and become quite hard. Initially, the knots are about an inch in diameter but, because they grow annually, they will eventually girdle the stem. This will result in the death of the upper portion of the stem.