6 Common Tree Diseases in Toronto

Trees are a beautiful addition to any landscape— and here in Toronto, that particularly rings true. There’s nothing quite like sitting in a tree’s shade during hot summer days and witnessing the beautiful fall foliage every year. 

If you’re adding a tree to your landscaping plans, there’s a specific amount of upkeep that goes into caring for them. It’s best to understand the work required to maintain these large trees and how to protect them against pests and diseases that often sprout up in plants in this area. 

Below are some of the most common diseases you’ll find in Canadian trees. 

Apple Scab

The Apple Scab disease is one of the most pervasive issues faced by apple and crabapple trees in Canada. On an apple tree, this issue will become noticeably visible on the leaves and apples in early spring and summer. You’ll see small spots on the leaves that eventually turn black; the tree may shed its leaves in the early summertime period under this condition. 

Apples will start exhibiting small, dark spots, as well. Eventually, the fruit will begin to crack open and fall early. 

Experts recommend that you rake and dispose of fruit and leaves from infected trees fully. The disease can continue to live in and infect the tree, which is why it’s best to remove as much as you can in the fall.

Black Knot

The black knot disease is a common issue for stone fruit trees. It often strikes for cherry, plum, apricot, and almond trees. You’ll visibly spot the effects of this disease after it’s begun to take hold of your tree, unfortunately. On tree limbs, you’ll start to see small, wart-like knots forming. Eventually, those knots will darken in color and deepen in size. The disease could spread entirely throughout a tree limb and even begin to affect the tree trunk itself.  

Infected spores from this disease typically spread through rain or the wind. Experts advise you to prune limbs when you see the knots and typically recommend pruning about 10cm below the growth. The infection may spread much further than the knot implies. Carefully dispose of infected limbs to not spread the disease further, and be sure to clean off your pruning tools.  


Cankers are a wider umbrella-term for a broad health issue faced by a variety of trees. Cankers manifest as a different visible issue depending on each type of tree. Most predominantly, though, cankers look like dark bruises on a tree’s branches or trunk. Those bruises can sometimes appear to ooze. The bark could begin to crack, needles could turn brown, and leaves could wilt— the effects of this disease varies from tree to tree.

This disease is typically a sign of fungal or bacterial infection. The primary issue with cankers is that they can belt your tree and cause it to distort. Cankers also expose your tree to other potentially harmful diseases. 

Dutch Elm Disease

This disease is brutal for Elm trees and could result in your tree’s death if it isn’t addressed quickly. Dutch Elm Disease is spread through Elm Bark Beetles, and the fungus can rapidly affect your tree. 

Eventually, all of the leaves on your tree will yellow or shrivel and then fall off entirely. Experts advise contacting a professional immediately when you begin to suspect or notice this disease. They’ll send an expert out to apply a fungicide and hopefully save your tree and surrounding plants. 

A fun fact— the Siberian Elm is one of the very few elm trees resistant to this infection.

Fire Blight

Fire Blight has a magic-sounding name for its mystical looking appearance. A tree touched by Fire Blight looks as though part of it was sprinkled with fire. 

This disease is a bacterial-based issue that affects a number of trees, including many species of pine. You can spot signs of fire blight if cankers begin popping up on your tree limbs or trunk, leaves begin to wilt and tilt into a curved shape, and if part of the tree appears as if it has caught on fire.  

An overly wet environment can cause Fire Blight. To treat it, contact a professional or prune away any infected areas. Be sure to dispose of the removed limbs and leaves carefully and to wash your gardening tools. 

Tar Spot

Tar Spot isn’t harmful to your tree in the long run, and it’s a disease that’s easy to treat. You’ll first notice tar spots forming in early summer; leaves will begin to sprout yellowish-looking dots that grow darker and larger as the season continues. 

After a while, those spots on the leaves will look like large tar spots throughout your tree. Experts recommend pruning and raking up these leaves. Once you’ve removed them entirely, it’ll help prevent the disease from returning to your tree next year. Again, this particular issue won’t cause long-term problems for your tree.