What Is Black Knot?
Black knot (Apiosporina morbosa) is a fungal disease that forms hard, black, swollen growths on branches of trees in the Prunus family. In cold climates, it's extremely common on Mayday trees, Schubert Chokecherries, and Pin Cherries — three of the most popular ornamental trees planted in cold-climate yards.
Left untreated, black knot spreads rapidly — girdling branches, killing limbs, and eventually making tree removal the only option. The fungus releases spores in spring during warm, wet weather, infecting new growth on the same tree and neighbouring trees within a 500-metre radius.
Black knot fungal growths on a Mayday tree — each knot produces thousands of spores that infect nearby trees.
Municipal Enforcement
Under Alberta's Agricultural Pests Act and municipal bylaws, property owners are responsible for controlling black knot on their trees. Your municipality can issue enforcement notices requiring treatment or removal. Failure to comply can result in fines and the city removing the tree at the owner's expense.
How Black Knot Spreads
Black knot spores are released during spring rains when temperatures reach 16–27°C. Late spring (May–June) provides ideal conditions for infection. New infections appear as light brown swellings on young wood, turning olive-green by fall, and hardening into the characteristic black knots by the following year.
The disease cycle means that by the time you see black knots, the infection is at least a year old — and the tree has already been producing spores. This is why early identification and aggressive pruning are critical. A single untreated tree can infect every susceptible tree on a block within 2–3 years.
In many Canadian neighbourhoods where Mayday trees were planted heavily in the 2000s and 2010s, black knot outbreaks are now widespread. Many homeowners don't realize their trees are infected until the disease is advanced.
Our Pruning Process
Full tree assessment to identify all infected branches — including early-stage infections
Cuts made 15-20cm below visible infection per ISA (International Society of Arboriculture) standards
Sterilization of pruning tools between each cut with 70% isopropyl alcohol
Proper disposal of all infected material — bagged and removed, never chipped on site
Follow-up inspection recommended in 6-12 months to catch new infections
Removal recommendations if infection exceeds 50% of canopy
Neighbour notification when adjacent trees show signs of infection
Best Time to Prune
Late winter through early spring (February–April) when trees are dormant and infections are easiest to spot against bare branches. Dormant pruning also minimizes stress on the tree and reduces spore exposure risk.
Prevention Program
After pruning, we recommend annual inspections and preventive fungicide applications during the spring infection window. Early detection and treatment is far cheaper than dealing with advanced infection or full tree removal.
When Removal Is the Better Option
If more than 50% of the canopy is infected, or if large structural branches show infection close to the trunk, removal is often more cost-effective than repeated pruning. A heavily infected tree will continue to spread spores to healthy neighbouring trees regardless of how much you prune.
PWW provides honest assessments — we'll tell you when a tree can be saved and when it's time to remove and replant with a resistant species. Consider replacing removed Mayday or Chokecherry trees with resistant alternatives like Green Ash, Amur Maple, or Swedish Columnar Aspen that thrive in our cold climate.
Pricing Guide
Black knot pruning is quoted per tree based on size, severity, and accessibility:
- • Light infection (1–5 knots): $150–$250 per tree
- • Moderate infection (5–15 knots): $250–$450 per tree
- • Heavy infection / full removal: $500–$1,500+ depending on tree size
- • Multi-tree discount: available for 3+ trees on the same property