As winter approaches, dormant-season pruning is a good opportunity to inspect trees and shrubs for black knot. Black knot (Apiosporina morbosa) is a fungal disease commonly found in cherry trees (Prunus spp.) including Shubert chokecherry and Mayday trees.
Black knot is identified by rough black galls that can encircle entire branches. Infections occur during spring when spores are released and spread by splashing water, wind, birds, and insects. However, symptoms can be difficult to spot until the next season. Black knot deforms branches and reduces growth. Severe infections can stunt a tree’s overall growth and may eventually cause the tree to die.
Take proper measures to minimize the spread of black knot. If black knot is detected, use the following guidelines:
- Prune black knot between late fall and early spring when plants are dormant.
- Remove infected branches at least 25 cm (10 inches) below the knot. If infections are on the trunk or main branches that cannot be removed, cut away the diseased material down to healthy tissue and at least 1 cm (½ inch) beyond the edge of the knot.
- It is best to prune back to a suitable location, such as a healthy branch collar, rather than leaving a stub.
- Sterilize your cutting tools between each cut using a disinfectant to prevent further spread of the disease.
- Destroy infected branches immediately by burning or burying them, as they can continue to produce spores for months after removal.
References
County of Wetaskiwin No.10. (2026). Black knot. Wetaskiwin.ab.ca. https://www.county.wetaskiwin.ab.ca/205/Black-Knot
The Canadian Forest Service. (2024). Black knot. Nrcan.gc.ca; Government of Canada. https://tidcf.nrcan.gc.ca/en/diseases-caused-by-pathogens/factsheet/4