The Most Common Tree Diseases in Calgary
Calgary’s trees are a defining part of our neighborhoods, adding shade, beauty, and value to every property. But like all living things, trees are vulnerable to illness. Pests, fungi, and bacterial infections can spread quickly through communities if left untreated.
Recognizing the early symptoms of these problems is the key to saving your trees. By acting fast—and with the guidance of a Certified Arborist—you can prevent small issues from turning into costly removals.
Major Tree Diseases in Calgary
Several serious diseases affect Calgary’s trees. Some are well-known threats that require citywide management, while others quietly weaken trees over time.
Dutch Elm Disease (DED)
Alberta is one of the few places in North America still free of widespread Dutch Elm Disease, but the risk is always present.
Symptoms include sudden yellowing, wilting, and curling of leaves in mid-summer, often starting at the top of the tree.
Because elms are so important to Calgary’s canopy, strict laws regulate elm pruning (only permitted between October 1 and March 31) to prevent spread.
Black Knot Fungus
A common fungal infection affecting mayday, Schubert chokecherry, and other ornamental cherries.
Appears as dark, swollen, tar-like growths on branches.
If left untreated, it spreads rapidly, girdling branches and killing large sections of the canopy. Entire neighborhoods can be affected if infestations go unchecked.
Fire Blight
A bacterial disease that affects fruit trees such as apples, pears, and crabapples.
Leaves and shoots look “scorched,” turning black or brown while clinging to branches.
Highly contagious, it spreads through pruning tools, insects, and even wind-driven rain.
Needle Cast in Evergreens
A fungal disease common in spruces and pines.
Causes older needles to turn yellow, brown, and fall off prematurely, thinning the tree’s appearance.
Often worsened by poor airflow or prolonged wet conditions.
Root Rot
Caused by waterlogged or compacted soils that limit oxygen around roots.
Symptoms include thinning canopies, stunted growth, yellowing leaves, or mushrooms near the base.
Severely affected trees may lean or fail during storms, creating hazardous conditions.
Common Pests in Calgary
Alongside diseases, several insect pests weaken Calgary’s trees:
Aphids – Small insects that cause curled leaves and secrete sticky honeydew, often followed by sooty mold.
Spider Mites – Thrive in Calgary’s dry summers; leave fine webbing and speckled foliage.
Leaf Miners – Tunnel inside leaves, leaving visible trails.
Emerald Ash Borer (EAB) – Not yet established in Alberta but present in other provinces. EAB has killed tens of millions of ash trees across North America and represents one of the biggest threats on the horizon.
Left unmanaged, pests weaken trees, reduce growth, and make them more susceptible to disease.
Why DIY Solutions Often Make Problems Worse
It may be tempting to grab pruning shears or chemical sprays from the hardware store when you notice symptoms, but DIY approaches often cause more harm than good.
Spreading Infections – Using unsterilized tools can carry fungal spores or bacteria from one tree to another.
Improper Timing – Pruning at the wrong time (e.g., outside the elm pruning window) can increase disease spread.
Misidentification – Mistaking one disease for another can lead to the wrong treatment—or no treatment at all.
Harming Beneficial Insects – Over-the-counter pesticides may kill pollinators and natural pest predators along with harmful insects.
Tree diseases and pests require precise identification and targeted solutions. Guesswork rarely works and often worsens the problem.
How a Certified Arborist Helps
An ISA Certified Arborist brings the training and tools needed to diagnose, treat, and prevent tree diseases effectively.
Certified professionals can:
Accurately Diagnose Issues – Identifying the exact cause of symptoms rather than relying on guesswork.
Prune Safely – Removing diseased branches with sterilized tools and proper cuts to minimize spread.
Recommend Targeted Treatments – From soil amendments to selective sprays, treatments are chosen carefully for effectiveness and environmental safety.
Monitor Long-Term Health – Scheduling inspections to catch early warning signs before problems escalate.
Educate Homeowners – Helping you understand what to watch for and how to support tree resilience.
By hiring a Certified Arborist, you protect both your trees and the larger Calgary urban forest.
Prevention is the Best Medicine
The best way to manage pests and diseases is to prevent them in the first place. Homeowners can reduce risks with a few simple steps:
Schedule Regular Inspections – Annual or bi-annual checkups ensure problems are caught early.
Water and Mulch Properly – Healthy trees are more resilient to pests and infections.
Prune at the Right Time – Follow Alberta’s elm pruning laws and seek professional advice for species-specific timing.
Avoid Stress – Prevent soil compaction, overwatering, or root damage from construction.
Plant Diverse Species – A variety of trees reduces the risk of neighborhood-wide infestations.
Strong, healthy trees are your best defense against pests and disease.
The Bottom Line: Early Action Saves Trees
From Dutch Elm Disease to black knot fungus, Calgary’s trees face serious threats—but early detection and professional care make all the difference.
If you spot suspicious symptoms—wilting leaves, black swellings, fungal growth, or unusual dieback—don’t wait. Call an ISA Certified Arborist. With proper diagnosis and treatment, many trees can be saved, protecting both your property and Calgary’s urban forest.
