Learn more about diseases that affect Toronto’s trees. Find important definitions to help you understand the following information.

Trees are resilient and can recover from most insect feeding and leaf-distorting diseases. These are some of the common issues urban trees face. The City will not remove a tree or use pesticides if the insect of concern is considered a nuisance or causes only cosmetic damage. Toronto uses pesticides as a last resort option for pathogens that cause serious tree damage.

If you are concerned about pests that the City will not treat, you may apply for permission to hire a tree care company to do work on City-owned trees to treat certain pests. Complete and submit the Agreement for Arborists Retained by Private Property Owners to Undertake Work on City Trees and Application for Arborists Retained by Private Property Owner to Undertake Work on City Trees forms to apply for permission and indicate that the application is for Integrated Pest Management (IPM) work. Qualified staff will review the request to ensure the proposal is reasonable and that the proposed pesticide and the timing of the treatment are appropriate. The best time to submit these requests is during the winter leading up to the season. Often, once the pests are visible, it is too late for treatment but a plan can be developed for the next season.

A green leaf with brown spots.
Symptoms on leaves appear in spring as small olive-green spots which darken over time.

Apple Scab is one of the most serious diseases of apple and ornamental crabapple trees.

This is a cosmetic problem and direct control isn’t recommended.

What it looks like

On leaves

  • Starts as small, olive-green spots that turn to black.
  • leaves distort, turn yellow and fall early in summer.

On fruit

  • Starts as small, raised dark areas.
  • These spots become large and corky
  • Fruit distort, crack and fall too early.

What you can do

  • Rake and dispose of leaves and fruit in the fall since the disease and live in them over the winter.
  • Compost leaves.
  • Avoid watering the top of the tree.
Brown spots on older leaves, mainly at the edges.
Example of Ash Anthracnose.

Ash anthracnose is a common disease of ash trees, caused by a fungus.

What it looks like

  • Water-soaked spots appear on young leaves and shoots.
  • Brown and black spots form on the edges of leaves and work their way inwards.
  • The lower canopy usually gets infected first and it works its way up.

What you can do

New leaves will grow again in spring. To prevent reinfection:

  • rake and dispose of leaves and fruit in the fall since the disease and live in them over the winter
  • compost leaves
  • avoid watering the top of the tree
  • prune the tree to remove diseased twigs and branches; you do not need a permit if you are pruning to preserve the health of a tree
A white coating on the bark of a tree.
An example of the white wax substance produced by scales.

Beech bark disease is a devastating disease of beech tree and is caused by the combination of a beech bark scale, an insect, and a type of fungi.

The beech bark scale weakens the tree by making wounds in the tree, and the fungus gets into these wounds, spreads and kills the tree.

What it looks like

Signs of scales:

  • white woolly wax will form where the bark is rough
  • the whole tree can be covered with wax

Signs of fungus:

  • discolouration of the white wax produced by the scales
  • the disappearance of the scales
  • brown slime will ooze out from the tree
  • leaves won’t reach their full size
  • leaves will turn yellow and brown and then stay on the tree through fall

What you can do

Once the fungus infects the tree, there is nothing that can be done. These trees should be removed. Call 311 or a tree professional.


  • can be controlled by using a dormant oil applied after the leaves drop in fall or before the buds bloom in spring
  • removed by using a hard jet of water from a hose
  • can travel on firewood, so don’t transport firewood from infested area between July and November
A black, swollen tree branch.
An example of an extreme case of black knot on a tree.

Black knot is a disease of certain species that causes twig and branch swelling and discolouration, resulting in girdling and dieback of branches and sometimes the trunk.

Species include:

  • almond trees
  • apricot trees
  • cherry trees
  • apricot trees

What it looks like

  • Knots start as light brown, warty swellings around 1cm long on new shoots
  • The knots turn coal black and can encase the whole limb
  • Boring insects might be seen on the knots

What you can do

  • Prune trees at least 10cm below any visible knot, since the infection can run deep.
  • Dispose of pruned branches in plastic bags or burn them.
  • Prune in spring before new growth to prevent spores from spreading in the wind.
Lesions of a tree, with black stippling.
An example of cankers on a tree. Photo credit.

There are many different types of cankers that affect a broad range of trees. They can be caused by fungal or bacterial infections.

They can girdle the tree or make the tree vulnerable to other diseases or pests.

What it looks like

  • Appear on branches, stems or trunks.
  • Look like dark lesions.
  • These lesions might ooze and stain the bark.
  • Flat bark patches with cracked edges may appear.
  • On deciduous trees, leaves may wilt.
  • On conifers, needles may brown and fall off and resin may ooze out and turn white.
Pine needles coated with white resin.
An example of cytospora of spruce.

Cytospora canker is one of the most common and damaging diseases of spruce. It is caused by a fungus and usually infects trees that are already weak and have wounds in the bark.

What it looks like

  • Needles turn purple, then brown and fall off:
    • this leaves the infected branches bare
  • Infected branches are usually covered with white resin.
  • The fungus shows itself with black, pinhead-sized fruiting bodies.

What you can do

  • There is no chemical control for this disease.
  • Avoid injuring the tree.
  • Water trees if there is a dry period.
  • Prune and remove affected branches back to the main stem.
  • Prune only when it is dry.
  • Disinfect pruning tools with rubbing alcohol between each cut.
  • Contact a professional if you’re unsure how to prune and prevent wounds that could cause infection.

Dutch elm disease is the most devastating disease of elm trees in North America. Beetles carry the fungus that causes this disease.

What it looks like

This disease kills the tree. Signs include leaves wilting, curling, and turning yellow. Eventually, no leaves will be left.

What you can do

  • Early detection is key.
  • In most cases, it’s best to contact 311 or a professional to help with pruning and treating the tree with a fungicide.
  • If you prune the tree yourself, don’t prune during the growing season from April to October.
  • If you have several elm trees, you can dig a trench 60 cm deep around the infected tree to cut the infects tree roots off from healthy tree roots.

Monitoring Program

The City of Toronto:

  • Takes inventory and evaluates elms trees over 30 cm in diameter.
  • Gives recommendations to homeowners to protect the tree.
  • Monitors the Elm Bark Beetle population by trapping them. Trapping does not control the beetle population.
  • Monitors susceptible trees from June to mid-October for symptoms.
  • Prunes and disposes of identified diseased trees through October to May.
  • Injects a fungicide as a preventative in trees that do not show symptoms (Arbotect 20-S is used and full-dosages last 3 years).
Black, oblong shaped lesions of a tree branch.
Eastern filbert blight on a tree. Credit: Joseph O’Brien, USDA Forest Service – bugwood.org

Eastern filbert blight is caused by a fungus and is indigenous to northeast North America.

What it looks like

  • Dieback is usually the first sign.
  • Small football shaped cankers, or lesions, can be seen on the bark of branches.
  • These cankers appear in rows and every year a new row appears. This will kill the branches and eventually the tree.
  • Leaves will wilt and turn brown.

What you can do

  • Prune the infected limbs and cut a distance much lower than the diseased area.
  • Remove the pruned branches and dispose of them or burn them.
Leaves that appear wilted or dead among a few healthy leaves.
An apple tree with fire blight typical crooked or “candy cane” shaped leaves. Photo credit.

Fire blight can be a serious disease that affects many trees and plants.

Species affected include:

  • apple tree
  • pear tree
  • hawthorn tree
  • serviceberry

What it looks like

  • Leaves turn brown and wilt and curl into a candy cane shape.
  • Droplets of ooze can form on twigs.
  • Dark cankers, or lesions, cam form.
  • Fruit may be shriveled.
  • The tree might look burned.

What you can do

  • Prune infected areas.
  • Remove the pruned parts of the tree and dispose of them or burn them.
  • Sterilize tools with rubbing alcohol between cuts.
  • Contact a professional to apply a bacterial spray.
Orange-yellow blotches on green leaves.
Leaf blotch on a leaf.

Leaf blotch of horse-chestnut is a leaf disease caused by fungi. It is not a serious threat to trees.

What it looks like

  • First appears as water soaked spots on young leaves in early summer.
  • These spots may merge and form large reddish brown blotches.
  • Tiny black spots might form on dead trees.

What you can do

There’s no effective control after infection. Prevent infection by:

  • raking and disposing of infected leaves in fall
  • this fungus loves wet conditions
    • thinning the top, or crown, of the tree to improve air circulation
    • avoiding watering the leaves of the tree
Oak anthracnose affecting leaves, causing deformed edges.
Credit: USDA Forest Service Archive – bugwood.org

Oak Anthracnose is a disease affecting oak trees caused by a fungus. It is not serious and doesn’t cause serious damage.

What it looks like

  • Young leaves might have brown, dead and deformed edges.
  • Some leaves might be completely deformed and curled.
  • Older leaves might only have a few brown spots on them.
  • Dieback on the lower parts of the tree.

What you can do

  • Rake and remove dead or dying leaves and branches.
  • Avoid watering the leaves of the tree.

Oak decline is a syndrome that affects older or mature oak trees. It is triggered by the interaction of multiple stressors which weakens the trees over time, causing them to die.

Many oaks struggle to recover the energy lost after heavy defoliation by insects that feed on their leaves. The combination of urban stresses like drought, compaction and pollution, allow other pathogens the opportunity to infect and further weaken these trees.  Observations have shown a steady decline in the health of mature oak trees due to various stressors including:

  • defoliation stress caused by Fall Cankerworm
  • defoliation stress caused by LDD moth (European gypsy moth)
  • Two Lined Chestnut Borer
  • Armillaria Root Rot fungus

What it looks like

Oak decline will appear as a deterioration in health or the death of mature oaks, either as individual trees or in stands.  Extreme situations, such as heavy defoliation by an insect or chronic weakening by environmental factors, can show up as dieback in the upper crown, root rot and stem rot which causes trees to lose branches, fall over or die.

Monitoring Program

The City of Toronto is monitoring oak decline with the help of:

  • Forest Health Care inspectors who monitor pockets of mature oak City trees annually surveying their health status and the rate of decline.
  • Implemented programs in light of this disease and the general maturity of oak trees in some regions of Toronto including:

These initiatives are helping to maintain the health of a number of oaks and have led to an observed increase in natural oak regeneration. Oak trees will continue to be monitored for oak decline and both environmental and biological stressors will be recorded in order to determine future forest health care initiatives.

Oak leaf with raised, oval to round-shaped, yellow to green blisters on upper leaf surface.Oak leaf blister is caused by the fungus Taphrina caerulescens. It is a common foliar disease of many oak species. The blisters or deformities on leaves are more noticeable in years with cool, wet springs.

Members of the red oak group are the most susceptible to infection. White oaks are rarely infected. Heavy infections may impair an oak’s appearance but will not endanger the tree’s health.

What it looks like

  • Yellow raised blisters in early summer.
  • The blisters turn from yellow to reddish brown in late summer.
  • Blisters may merge and cause the leaves to curl.

What you can do

  • Remove the fallen infected leaves around an oak by raking them up to reduce the chances of the fungus recurring next year. Leaves may be composted by city composting programs.
  • Oak leaf blister does not seriously affect the health of the tree and chemical control is not recommended.

Monitoring program in effect.

Oak wilt is a fungal vascular disease that kills all species of oak trees. Oak wilt has been detected in Ontario, but has not been confirmed in Toronto. Learn more about oak wilt and what you can do to slow the spread or report potential sightings.

Orange/coppery spots on a green leaf caused by orange trellis
Orange trellis spots.

Pear trellis rust is cause by fungus. The pear trellis rust fungus has been introduced to southern Ontario in recent years.

The fungus spends winter in the safety of the juniper tree, then hops to new leaves on pear trees in spring.

What it looks like

  • Pear leaves show small orange spots.
  • Over time, these spots enlarge and turn red.
  • In late summer, these spots become blisters which burst into spores.
  • Leaves can drop early and dieback can occur.

What you can do

  • Maintain at least 10-1000 meters between juniper and pear.
  • Prune and dispose infected branches from juniper in the fall and winter.

There is no registered pesticide in Ontario for control of this disease.

Clusters of white spots on a green leaf.
Powdery mildew on a leaf.

Powdery mildew is a common foliar disease of many tree species. This disease is cosmetic and doesn’t kill the tree.

What it looks like

  • Appears as a white powder on the upper surface of leaves.
  • Leaves may change colours earlier than normal.

What you can do

  • Thin the tree to improve air circulation since the disease loves wet conditions.
  • Clean up and dispose of infected leaves to prevent infection the next year.
Long black lesions in a tree trunk.
A canker caused by sudden oak death. Credit: Joseph O’Brien, USDA Forest Service – bugwood.org

Sudden oak death is a disease affecting species of oak trees caused by a soil borne, fungal-like organism.This disease is not in Ontario yet.

What it looks like

  • Cankers form and ooze red or black.
  • Leaves turn pale yellow or brown.

What you can do

Plants and trees coming into Ontario should be monitored to prevent infection.

A close-up of a leaf with a brown splotch.
An example of sycamore anthracnose affecting a leaf. Credit: Clemson University, USDA Cooperative Extension Slide Series – bugwood.org

Sycamore anthracnose is the most serious disease of sycamore trees.

What it looks like

This disease attacks the leaves.

  • Sudden browning and death of young shoots and new, unfolded leaves in spring.
  • Brown spots along the veins of developed leaves.
  • Cankers form, girdle and kill young twigs.

What you can do

  • Remove and destroy fallen leaves and twigs in the fall to prevent reinfection.
  • Prune out and destroy diseased branches.
  • Sterilize tools with rubbing alcohol in between cuts.
Black spots on green maple leaves.
Tar spot.

Tar spot is a fungal leaf disease that may occur on several plants, but it is most common on maple. It does not cause permanent damage to the tree.

What it looks like

  • Small yellow spots on leaves.
  • These spots grow and get darker in late August.
  • Eventually they look like spots of tar.

What you can do

Rake and remove infected leaves in the fall to prevent infection the next year.

A dead tree branch with clusters of wilted leaves.
An example of verticillium wilt.

Verticillium wilt is a vascular disease caused by a soil-borne fungus. It enters in by the roots of a tree.

What it looks like

  • Wilting leaves.
  • Dieback of branches.
  • The entire tree may die in one season.

What you can do

Avoid root injuries caused by digging or soil compaction.