Plant Disease Clinic Highlights Plant Pathology Plant Disease Clinic Highlights Black Knot Black knot occurs on cultivated and wild cherries and plum. The disease is caused by the fungus Apiosporina morbosa . Black knot is characterized by the presence of rough, warty black galls which may extend up the branch from a few inches to a foot or more.
Fungal spores produced on year-old or older galls initiate infections on young, succulent twigs.
Sanitation measures usually control the disease adequately, but may be supplemented with a fungicide program if the disease is unusually difficult to control. Prune out and destroy all galls while trees are dormant. Cuts should be made 6-8 inches below the visible infection. Use resistant cultivars in new plantings. Fungicides such as lime sulfer or tribasic copper sulfate will aid in control of black knot.