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 The City Is Doing
City crews will continue the ongoing monitoring and pruning program for susceptible trees and monitor areas impacted by black knot
Currently, the City is completing an annual focus on a specific quadrant of the city; this year, the northwest quadrant is being reviewed and pruned for black knot
During the fall/winter of 2021/2022, crews will be focused on the southeast quadrant
To ensure the completion of this operational work plan, the focus will remain on these specific quadrants
Only City crews are authorized to prune infected trees on public property
Pruning for black knot takes place between late fall to early spring when the tree and spores are dormant and when the knots are easier to see
The pruning cycle for Prunus spp. is approximately every 4 years
Pruning tools and equipment are sterilized between each cut using an approved environmentally-friendly solution which prevents the spread of the disease
Diseased wood chips and logs are disposed of at the Edmonton Waste Management Centre or the Northland Recycling Centre, this ensures that the wood chips are not reused for tree planting or in shrub beds
While black knot is managed and pruned from park, boulevard, and ornamental trees on City property, our natural areas are not managed for this disease as the overall risk to species affected by black knot is low. Black knot will only be managed if the tree presents a safety risk to a target zone.
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, the 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 sealed in a plastic bag and placed in your black garbage cart for disposal
Black knot is a native plant disease well established throughout the province and is not regulated through any legislation. Creating a local bylaw therefore would be impractical, however, citizens are encouraged to help manage the pest on their own property.