Learn more about diseases that affect Toronto’s trees. Please see important definitions to help you understand the list below.
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.
Ash anthracnose is a common disease of ash trees, caused by a fungus.
New leaves will grow again in spring. To prevent reinfection:
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.
Signs of scales:
Signs of fungus:
Once the fungus infects the tree, there is nothing that can be done. These trees should be removed. Call 311 or a tree professional.
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.
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.
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.
Dutch elm disease is the most devastating disease of elm trees in North America. Beetles carry the fungus that causes this disease.
This disease kills the tree. Signs include leaves wilting, curling, and turning yellow. Eventually, no leaves will be left.
The City of Toronto:
Eastern filbert blight is caused by a fungus and is indigenous to northeast North America.
Fire blight can be a serious disease that affects many trees and plants.
Species affected include:
Leaf blotch of horse-chestnut is a leaf disease caused by fungi. It is not a serious threat to trees.
There’s no effective control after infection. Prevent infection by:
Oak Anthracnose is a disease affecting oak trees caused by a fungus. It is not serious and doesn’t cause serious damage.
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:
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.
The City of Toronto is monitoring oak decline with the help of:
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 Wilt is a fungal vascular disease affecting all species of oak trees. The disease spreads by root grafts with neighbouring oak trees and/or by sap feeding beetles. Oaks in the Red oak group (Red, Black and Pin Oaks) are highly susceptible where oaks in the White oak group (White, Bur, Swamp Oaks) show some degree of resistance to infection. Oak Wilt is currently detected near the Detroit/Windsor border.
This disease kills the tree. Symptoms include leaves turning yellow-bronzy colour and early leaf drop (July to August). Eventually, no leaves will be left on the tree. Fungal mats form under bark which at advanced stage causes bark to split.
The City of Toronto has been:
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.
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.
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.
Plants and trees coming into Ontario should be monitored to prevent infection.
Sycamore anthracnose is the most serious disease of sycamore trees.
This disease attacks the leaves.
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.
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.
Avoid root injuries caused by digging or soil compaction.