Black Knot Plant Pathology Black Knot Black knot is a fungal disease that occurs on wild and cultivated cherries and plums. Signs of the disease are easy to recognize. These swellings, 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.
The knots are soft when newly formed and later become hard and black with age. 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 and least 2-3 inches below the swelling because the fungus may have extended beyond the swelling.