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 is an increasingly common disease. Early detection and sterile pruning is the most effective way of dealing with this problem.
The problem, Black knot deforms branches and reduces their growth. Heavily infected trees often become stunted and may eventually die.
Black knot is not regulated through any legislation. It is a native plant disease that is well established throughout the province.
Plant Species
| Amur Cherry |
Black Cherry |
Apricot |
| Flowering Almond |
Chokecherry |
Flowering Plum |
| Dropmore Cherry |
Japanese Plum |
Korean Cherry |
| Mayday Tree |
Mondgolian Cherry |
Cultivate Plum |
| Nanking Cherry |
Wild Plum |
Pin Cherry |
| Prunus Hybrids |
Sand Cherry |
Sour Cherry |

What does the disease look like?
Initially, a small, olive green growth will develop at a succulent growing point or a short twig that bears fruit buds. This swelling will grow until it is mature after 2 to 3 years. The mature growths are hard, black, 10 to 15 cm and may be somewhat ruptured. They will produce and release a vast amount of spores during the bloom period, resulting in a rapid increase of infections. The fungus continues to grow internally and externally, with the branch eventually becoming covered all around and dying.
What the Town is doing
- Town crews will continue the ongoing monitoring and pruning program for susceptible trees on public property
- Black knot pruning rotation is broken into 5 zones. Crews will start with zone 1 in fall 2023. To ensure the completion of this operational work plan, no pruning will be done outside the current zone of focus
- 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
- Only Town crews are authorized to prune infected trees on public property
- Pruning tools and equipment are sterilized between each cut using an approved environmentally-friendly solution which prevents the spread of the disease
- While black knot is managed and pruned from Open Space and Boulevard trees. Black knot in Natural Areas will only be managed if the tree presents a safety risk
What can you do to control the disease?
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:
- You can help to maximize the impact of the Town’s black knot pruning efforts by ensuring trees on private property are pruned for black knot
- Prune out the infected branches between late fall and early spring when the plants is dormant and knots are easier to see
- Remove 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 the next branch or to the trunk, rather than leave a stub
- 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
- All diseased wood should be burned or buried. Do not place wood in the organic bins.
- A list of tree pruning companies is available on the Better Business Bureau website.