Black Knot Fungus in Calgary: What You Need to Know
Calgary homeowners are no strangers to tree challenges—harsh winters, strong winds, and dry summers already put stress on urban forests. But one of the most destructive and widespread threats comes from Black Knot Fungus, a disease that primarily affects mayday and chokecherry trees.
This aggressive fungus not only damages infected trees but also spreads quickly through neighborhoods. If left untreated, it can kill entire trees and weaken Calgary’s tree canopy. Recognizing the signs early and responding with proper treatment is the key to managing this problem.
What is Black Knot Fungus?
Black Knot Fungus (Apiosporina morbosa) is a fungal disease that attacks trees in the Prunus family, especially mayday, Schubert chokecherry, and ornamental cherries.
Appearance – The disease is easy to recognize once it develops. It forms swollen, hard black growths along branches that resemble burnt charcoal or tar.
Progression – Initially, the knots may appear as small olive-green swellings. Over time, they darken and expand, girdling branches and cutting off water and nutrient flow.
Spread – The fungus produces spores that travel through the wind, rain, or contaminated tools, infecting nearby trees.
Because it spreads so easily, one untreated tree can quickly put entire blocks of trees at risk.
Why Black Knot is a Serious Threat in Calgary
Black Knot is especially concerning for Calgary homeowners for several reasons:
Rapid Spread – Once spores are released, they can travel long distances, infecting trees across neighborhoods.
Tree Mortality – Left unchecked, Black Knot eventually kills infected branches and, over time, the entire tree.
Loss of Property Value – Mayday and chokecherries are common ornamentals in Calgary. Their decline reduces curb appeal and property value.
Community Impact – Because the fungus spreads easily, one neglected tree can create widespread issues, harming community landscapes.
Without intervention, Black Knot can significantly reduce the health and beauty of Calgary’s urban forest.
Identifying Symptoms of Black Knot
Early detection is critical. Signs of Black Knot include:
Swollen, olive-green growths on young branches.
Black, rough, elongated knots resembling burnt wood on older branches.
Branch dieback beyond the infected area.
Multiple knots appearing along the same branch or tree.
If you see these symptoms, act quickly—delaying treatment only increases the risk of spread.
How Black Knot is Treated
Unfortunately, there are no sprays or chemical treatments that eliminate Black Knot. The only effective solution is pruning out infected branches.
Pruning Technique – Infected branches must be cut at least 6–8 inches below the visible knot, as the fungus extends beyond what’s visible.
Timing – The best time to prune is late winter to early spring, while trees are dormant and spore activity is low.
Disposal – Infected wood should never be composted. It must be bagged and taken to landfill or burned where permitted, to prevent spores from spreading further.
Sterilization – Pruning tools must be sterilized between cuts with bleach or alcohol to avoid transferring spores.
In cases where Black Knot has spread extensively through a tree, removal may be necessary to protect surrounding trees.
Why DIY Pruning Often Fails
Many homeowners attempt to manage Black Knot on their own but unintentionally make the problem worse. Common mistakes include:
Cutting Too Close – Leaving stubs or cutting directly into the knot allows the fungus to remain active.
Improper Disposal – Composting or leaving infected wood nearby gives spores a chance to spread.
Unsterilized Tools – Carrying spores from one branch to another spreads the disease within the same tree.
Pruning at the Wrong Time – Cutting during spring or summer increases the risk of spore release.
These errors often result in re-infection or faster spread, turning a manageable problem into a costly removal.
Why Hire an ISA Certified Arborist?
Managing Black Knot correctly requires precision, timing, and knowledge. An ISA Certified Arborist ensures:
Accurate identification of Black Knot and other tree diseases.
Proper pruning techniques to completely remove infected tissue.
Sterilization of tools to prevent contamination.
Safe disposal of infected material according to city guidelines.
Recommendations for long-term tree health and prevention strategies.
Certified arborists can also determine when a tree is too far gone and guide you on safe removal and replacement options.
Preventing Black Knot in Calgary
While no tree is completely immune, you can reduce risks with proactive care:
Regular Inspections – Have your trees checked annually for early signs of disease.
Timely Pruning – Remove infected branches during dormancy, before spores spread.
Species Selection – When planting new trees, consider varieties less susceptible to Black Knot.
Neighborhood Cooperation – Encourage neighbors to treat infected trees; untreated trees nearby can re-infect yours.
Prevention is always cheaper and more effective than removal.
The Bottom Line: Don’t Wait to Act
Black Knot Fungus is one of the most destructive tree diseases in Calgary, and once it takes hold, it spreads quickly. Early detection and professional pruning are the only reliable ways to manage it.
If you notice black, swollen growths on your mayday or chokecherry trees, don’t delay. Call an ISA Certified Arborist to assess, prune, and protect your trees—before the fungus spreads further.
Your proactive care not only saves your trees but also helps preserve Calgary’s urban forest for generations.
