Black Knot

What Is It?
Black knot (Apiosporina morbosa) is a fungal disease that causes greenish brown to black swellings in the stems of cherry trees (Prunus spp). It's spores are released following periods of warm, wet weather and are spread by splashing water, wind, birds, and insects.

What's the Problem?
Black knot deforms branches and reduces their growth. Heavily infected trees often become stunted and may eventually die from the disease.
Black knot on a tree
What Can I 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 the succulent 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 are dormant and the knots are easier to see.
  • Remove the infected branches to at least 25 cm (10 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 a disinfectant to prevent further spread of the disease.
  • Destroy infected prunings immediately by burning or burying, as they can continue to produce spores for months after being removed.