Common Tree Diseases in Calgary and How to Spot Them
Calgary’s trees face a unique set of challenges. Harsh winters, chinook winds, and alkaline soils already stress our urban forest—but pests and diseases add another layer of risk. Without early detection, tree diseases can spread quickly through entire neighborhoods, leading to costly removals and long-term canopy loss.
For homeowners, recognizing the warning signs is key. But proper diagnosis is rarely straightforward. Many issues look similar—what appears to be drought stress may actually be disease, and the wrong treatment can make the problem worse. That’s why working with an ISA Certified Arborist is essential to protecting both your property and Calgary’s larger urban forest.
Black Knot Fungus
Host Trees: Mayday, chokecherry, and other members of the Prunus family.
Symptoms:
Swollen, black growths on branches that look like burnt charcoal.
Early stages show as olive-green swellings that turn dark and hard.
Over time, branches become girdled and die back.
Risks:
Black Knot spreads through airborne spores and can move rapidly between trees in a neighborhood. Left untreated, it will eventually kill the host tree.
Treatment:
The only effective management is pruning. Infected branches must be removed 6–8 inches below the knot during the dormant season (late winter). All infected wood should be destroyed—never composted—to prevent further spread.
Fire Blight
Host Trees: Apple, pear, crabapple, mountain ash, and other fruit-bearing ornamentals.
Symptoms:
Leaves and blossoms appear scorched, as if burned by fire.
Twigs and branches may curl into a distinctive “shepherd’s crook.”
Sticky, amber-colored bacterial ooze may be visible.
Risks:
This bacterial disease spreads rapidly during warm, wet weather and can kill entire branches within weeks. It is especially destructive in fruit trees prized for both beauty and productivity.
Treatment:
Infected branches must be pruned well below the visible symptoms, and pruning tools sterilized between cuts. Severe infections may require full removal of the tree to prevent spread to nearby plants.
Cytospora Canker
Host Trees: Common in poplars, willows, and spruce trees.
Symptoms:
Sunken, discolored patches on bark.
Resin flow (sap “bleeding”) on conifers like spruce.
Dieback of branches, often starting on stressed or older trees.
Risks:
Cytospora is opportunistic, attacking trees already weakened by drought, root damage, or other stressors. Once established, it can slowly spread through the trunk, compromising the tree’s structure and health.
Treatment:
There is no cure, but infected branches can be pruned out, and overall tree health can be improved through watering, mulching, and soil care. Preventative maintenance is the best defense.
Why Professional Diagnosis Matters
Many of these symptoms—yellowing leaves, dieback, or bark damage—can also result from environmental stress, nutrient deficiencies, or pests. Homeowners often misdiagnose tree problems and apply the wrong treatments, wasting time and money. Worse, DIY pruning with unsterilized tools can spread disease further.
An ISA Certified Arborist can:
Accurately identify whether the issue is fungal, bacterial, pest-related, or environmental.
Recommend the correct treatment, from pruning and sanitation to fertilization and soil amendment.
Advise whether removal is the safest option to protect other trees on your property.
The Role of TRAQ in Disease-Compromised Trees
When diseases progress, they can weaken a tree’s structure, increasing the risk of falling during storms or heavy snow. A TRAQ-qualified arborist (Tree Risk Assessment Qualification) provides a science-based evaluation of whether a diseased tree is still safe to retain.
This process considers:
The tree’s structural integrity.
The likelihood of failure.
Potential consequences for property, vehicles, or people if the tree fails.
This professional insight helps homeowners make informed, confident decisions about treatment versus removal.
Protecting Calgary’s Urban Forest
Tree diseases aren’t just personal problems—they’re community issues. Black Knot, Fire Blight, and Cytospora Canker can spread from one yard to another, weakening entire neighborhoods. By catching problems early and hiring certified professionals, you’re not just protecting your property—you’re also contributing to the resilience of Calgary’s urban forest.
The Bottom Line: Early Action Saves Trees
In Calgary, tree diseases like Black Knot Fungus, Fire Blight, and Cytospora Canker are common and destructive. Left untreated, they can kill valuable trees and spread through neighborhoods, costing homeowners thousands in removals and replacements.
The key is early detection and professional care. By hiring an ISA Certified Arborist—ideally one with TRAQ certification—you ensure your trees receive accurate diagnosis, effective treatment, and long-term protection.
Healthy trees add shade, beauty, and value to your property. Don’t wait until disease takes hold—take action early to preserve your living investments.
