Common Tree Diseases in Calgary and How to Spot Them

Calgary's trees face a handful of diseases that show up consistently year after year. Knowing what to look for can mean the difference between catching a problem early, when it is manageable, and discovering it late, when the tree is beyond saving. Here are the most common tree diseases in our area, what they look like, and what you can do about them.

Black Knot

Affects: Chokecherry, Mayday, Schubert cherry, plum, and other Prunus species

Black knot is arguably the most visible tree disease in Calgary. It appears as hard, black, swollen growths on branches that look like lumps of charcoal. In early stages (first year), the growths are olive-green and soft. By the second year, they mature into the distinctive hard black knots that can range from a couple of centimetres to over 30 centimetres long.

What to Do

• Prune infected branches at least 15 to 20 centimetres below the visible knot during dormancy (late winter)
• Dispose of infected material in the garbage, never in compost
• Disinfect pruning tools between cuts
• Severely infected trees with knots on the main trunk may need to be removed
• There is no effective chemical treatment once the disease is established

Dutch Elm Disease (DED)

Affects: American elm and all other elm species

Dutch elm disease is caused by a fungus spread by elm bark beetles. Calgary has one of the largest and healthiest urban elm populations in North America, and the City works aggressively to keep it that way. DED has been found in Alberta, and vigilance is critical.

Symptoms

• Rapid wilting and yellowing of leaves on one or more branches during the growing season, typically starting in June or July
• Affected leaves curl, turn brown, and may stay on the tree
• Brown streaking in the wood just under the bark of affected branches
• Symptoms often start on one side of the tree and progress through the canopy

What to Do

If you suspect DED, contact the City of Calgary at 311 immediately. Do not attempt to prune or treat the tree yourself. The City has a comprehensive DED management program and will dispatch inspectors to assess the situation. Remember: elm pruning is banned from April 1 to September 30 to reduce the risk of beetle transmission.

Fire Blight

Affects: Apple, crabapple, mountain ash, cotoneaster, and other members of the rose family

Fire blight is a bacterial disease that thrives in warm, wet spring weather. It gets its name from the way infected branches look scorched, as if they were burned.

Symptoms

• Blackened, curled branch tips that bend into a distinctive "shepherd's crook" shape
• Leaves turn brown or black but remain attached to the branch
• Oozing, amber-coloured bacterial droplets on infected bark during wet weather
• Cankers (sunken, discoloured patches) on larger branches and the trunk

What to Do

• Prune infected branches at least 30 centimetres below the visible damage during dry weather
• Disinfect tools between every cut with 70 percent isopropyl alcohol
• Avoid pruning during wet weather, which can spread the bacteria
• Do not fertilize with high nitrogen, as lush new growth is the most susceptible
• For severe cases, copper-based sprays applied during bloom can reduce new infections

Cytospora Canker

Affects: Spruce (especially Colorado blue spruce), poplar, willow

This is the most common disease of mature spruce trees in Calgary. It is caused by a fungus that enters through wounds or stressed tissue and kills branches from the bottom of the tree upward.

Symptoms

• Needles on lower branches turn brown or purple and drop
• White or bluish-white crusty patches of dried sap on the bark of affected branches
• Dead branches, typically starting from the lower canopy and progressing upward
• The disease can take years to kill a tree, slowly working its way up the trunk

What to Do

• Remove dead and infected branches, cutting back to live wood or the trunk
• There is no fungicide treatment for cytospora canker
• Reduce stress on the tree through proper watering and mulching
• Avoid wounding the tree (lawn mower damage, pruning during wet weather)
• Colorado blue spruce is the most susceptible; consider replacing severely affected trees with more resistant species

Apple Scab

Affects: Apple and crabapple trees

Apple scab is a fungal disease that shows up nearly every year in Calgary, especially during wet springs. While it rarely kills trees, it is unsightly and can cause heavy leaf drop by midsummer.

Symptoms

• Olive-green to dark brown spots on leaves, starting in late spring
• Leaves may turn yellow and drop prematurely
• Dark, scabby spots on fruit
• Severely affected trees may lose most of their leaves by August

What to Do

• Rake and dispose of fallen leaves in autumn to reduce the fungal spore load for next year
• Improve air circulation by pruning the interior canopy
• Fungicide sprays (applied during bud break and early leaf expansion) can be effective but require precise timing
• When planting new crabapples, choose scab-resistant cultivars

Bronze Leaf Disease

Affects: Poplar, particularly Swedish columnar aspen

This relatively newer disease has been spreading through Calgary's poplar population. It causes premature browning and leaf drop and can kill affected trees over several years.

Symptoms

• Leaves develop reddish-bronze discolouration, starting at the edges
• Affected leaves drop early, often by mid-summer
• Branch dieback follows in subsequent years
• The disease progresses from the top of the tree downward

What to Do

There is currently no effective treatment for bronze leaf disease. Fungicide injections have shown some promise in research, but there is no commercially available cure. Maintaining overall tree health through proper watering and fertilization can slow the progression. Severely affected trees may need to be removed.

Early detection is the common thread across all tree diseases. Walk your property regularly, look at your trees with fresh eyes, and do not ignore changes. A problem caught in year one is almost always easier and cheaper to manage than one discovered in year three.