What is that black glob on my cherry tree? If you've noticed black galls or growths on the branches of your cherry or plum trees, you could have black knot. 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.
Usually noticed in spring before leaves emerge, black knot causes galls or swelling to occur on twigs, branches, and/or the trunk. Galls can be several inches in length and 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.
Black knot fungal spores are produced on older galls and infect young, succulent twigs or wounded tissue during wet conditions in the spring. After infection, a small swelling occurs, which is practically unnoticeable. The following spring, sizable velvety olive green galls appear and, later in the season, the galls become hard and black.
Several methods are recommended to help prevent infection. Before planting new trees, inspect nearby cherry and plum trees for infection. If black knot is present, do not plant susceptible species in the vicinity. In addition, remove infected wild cherry and plum trees from the area before planting new trees.
Black knot can be managed by pruning out and destroying all galls infecting the tree. Prune trees 2-3 inches below the gall, preferably when the tree is dormant. 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.