Threats to Trees: Diseases
Learn more about diseases that affect Toronto’s trees. Please see important definitions to help you understand the list below.
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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.
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
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.
Scales:
- 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
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.
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.
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).
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
