Black knot (Apiosporina morbosa) is a fungal disease that causes greenish brown to black swellings in the stems of cherry trees (Prunus spp.). Its spores are released following periods of warm, wet weather and are spread by splashing water, wind, birds, and insects.
Black knot deforms branches and reduces their growth. Heavily infected trees often become stunted and may eventually die from the disease.
What You Can Do
Regularly monitor your cherry tree(s) for symptoms of black knot. Early on, the disease appears as small light brown swellings, usually found on new green stems. As the knots mature, the swellings will appear olive green with a velvety texture. Eventually the knots darken and harden. If you detect black knot, follow the guidelines below:
- prune out the infected branches between late fall and early spring when the plants dormant and the knots are easier to see
- remove the infected branches to at least 15-20 cm (6-8 inches) below the knot
- it is best to prune an infected branch further back to a suitable location, such as a healthy collar, rather than leave a stub
- for knots on the trunk or scaffold branches (main branches growing directly from the trunk) that can’t be removed, cut away the diseased material down to good tissue and at least 1 cm (1/2 inch) beyond the edge of the knot
- sterilize your cutting tools between each cut using bleach to prevent further spread of the disease
- destroy infected prunings immediately, as they can continue to produce spores for months after being removed (galls can be placed into a plastic bag for regular household garbage pickup)