This disease is caused by the fungus Apiosporina morbosa and typically affects both wild and cultivated cherry, chokecherry, and plum species. Black cherry and pin cherry are not susceptible to black knot.
Black knot causes galls (or swellings) to occur on branches, twigs, and/or the main trunk. Galls typically appear soft and olive green when newly formed, but eventually become hard and black with age. Several galls can infect a single tree and usually cause branch death beyond the area of the infection. These galls, which are longer than wide, may extend up the branch from a few inches to a foot or more. They are especially noticeable before the leaves emerge in the spring.
Black knot is caused by the fungus Apiosporina morbosa. Fungal spores produced on year-old or older galls initiate infections on young, succulent twigs or wounded tissue during wet conditions in the spring. Often the knots become covered with a whitish secondary fungus or insects.
The disease is controlled primarily through good sanitation measures. All galls should be pruned out and destroyed, preferably when the tree is dormant. It is important to make cuts at least 2-3 inches below the swelling because the fungus may extend beyond the swelling.
Sanitation measures usually control the disease adequately, but fungicides such as lime sulfur or tribasic copper sulfate may aid in the control of black knot. Several applications need to be made beginning before bloom, and continuing until after fruit set. Fungicide use alone will NOT control the disease.
It is also helpful to remove any diseased wild plum or cherries in the vicinity, such as in fence rows or nearby wooded areas.