Tree Diseases and Pests

There are a number of diseases and insects that affect trees. Residents in Richmond Hill should not perform any work to trees on City property. To report a damaged tree or request an inspection of a tree, contact Access Richmond Hill at 905-771-8800. Additional information is available on

Learn more about tree maintenance, tree planting and tree preservation in Richmond Hill.

Common tree diseases and pests

Some diseases and insects that affect the trees in Richmond Hill include:


Anthracnose is a fungal disease that affects many trees during damp weather, as the leaves are expanding from buds. It causes dark, sunken spots on the infected plants' leaves, stems, flowers and fruits. The disease causes the leaves to form dark lesions, which may cause the leaves to drop from the tree. As the tree puts out new growth later in the growing season, the leaves will not be affected if the weather tends to be drier and the spores that infect the leaves are not present.

Anthracnose of Maple.

Learn more about Anthracnose.


Aphids are small sap-sucking insects. These soft-bodied insects can survive in almost any area. Aphids multiply quickly and suck on the leaf surface, excreting a sticky substance, which will stick to the leaves and anything under the tree. Heavy rains and spraying the leaves with water can disrupt and cause them to disperse. Bees and wasps are often attracted to sticky leaves and surfaces, which tend to be more of an issue than the aphids themselves. As with most insects, populations fluctuate annually based on weather and other factors.

Learn more about Aphids.

Asian Long-Horned Beetle

The Asian Long-Horned Beetle (ALHB) is an invasive forest pest with no natural enemies in North America. It attacks most broadleaf trees, especially maple trees. There have been two local outbreaks of this pest in neighbouring municipalities in recent years. If this insect or damage it causes is observed, contact the Canadian Food Inspection Agency immediately.

Asian Long-Horned Beetle

Learn more about the Asian Long-Horned Beetle.

Birch leafminer

Birch leafminers are sawflies, closely related to bees and wasps. They are one of the most common insect pests affecting birch trees in North America. Birch leafminer larvae eat the areas between the layers of the leaves, affecting the leaves' ability to produce food.

Birch Leaf Miner damage.

Learn more about Birch leafminers.

Black Knot of Cherry

Black Knot is a common and often serious disease of plum, cherry and prune trees. The disease becomes worse every year and will eventually kill the tree. Richmond Hill will remove and replace the infected trees when there is infection on major limbs and/or trunks.

Black Knot on a major limb of an infected Plum tree.

Learn more about Black Knot of Cherry.

Eastern tent caterpillars

Eastern Tent Caterpillars form webbed nests most commonly on Schubert Cherry trees. They will eat the leaves causing defoliation. In some cases, the tree may be completely defoliated. Richmond Hill staff can remove the nest in several cases. Trees that are healthy will put out new leaves after infestation, Providing water to these trees if conditions are dry will help the tree during this process.

Eastern Tent Caterpillars form webs in the forks of branches.

Learn more about the Eastern tent caterpillars.

Emerald Ash Borer

The Emerald Ash Borer (EAB) is an invasive insect that attacks and kills ash trees. Once infected, an ash tree can be killed within 2 - 4 years. Richmond Hill has an EAB strategy in place to remove the ash trees in both parks and on streets on a priority basis. Richmond Hill will be replacing street trees and park trees in manicured areas on a tree-for-tree basis. Forest and woodlot replacements are part of a wide-scoped restoration program that will be focus on areas that are highly affected by EAB.

Emerald Ash Borer

Learn more about the EAB in Richmond Hill.

European gypsy moth/Lymantria dispar dispar (LDD moth)

Visit the LDD moth page for details.

General leaf diseases

General leaf diseases are dependent on weather conditions and plant locations. General leaf diseases spread blight, mildew, scabs and spots on leaves.

Learn more about general leaf diseases.


Leafhoppers (insects from the Cicadellidae family) are small insects that suck plant sap from trees. Trees affected in Richmond Hill tend to be the Honey Locust trees. In severe cases, they may cause heavy defoliation. Keeping affected trees well watered during this period will allow the trees to put out new growth.


Learn more about Leafhoppers.

Linden Borer

The Linden Borer is an insect found in northeastern North America. This pest attacks basswood and linden trees. In Richmond Hill, it will damage the layers beneath the bark, which causes trees to lose structural integrity and reduced movement of water and nutrients along the trunk. Damaged areas appear as the bark is broken off and the tunnels (or galleries) surround the trunk. The bark and underlying tissue breaks apart very easily.

Linden Borer

Learn more about the Linden Borer.

Oak Wilt

Oak wilt is a vascular disease of oak trees caused by a fungus. The fungus restricts the flow of water and nutrients through the tree causing the characteristic wilting and bronzing of the leaves. All oak species are susceptible and at risk. There is no cure for oak wilt infected trees.  

Where is oak wilt present?

To date, oak wilt has not been found in Canada. However, oak wilt has spread throughout the eastern United States. In 2016, oak wilt was confirmed on Belle Isle (United States), in the middle of the Detroit River, 579 meters from the shores of Windsor, Ontario.

Oak leaf with signs of oak wilt. Photo courtesy: F.A. Baker, Utah State UniversitySigns and symptoms

  • Leaves turn dull green, brown or yellow
  • Discoloration of leaves progressing from the edge of the leaf to the middle
  • Wilting and bronzing of foliage starting at top of the tree and moving downwards
  • Premature leaf fall (including green leaves)
  • White, grey or black fungal mats just under the bark that emit a fruity smell
  • Vertical bark cracks in the trunk and large branches as a result of the fungal spore mats (also referred to as pressure pads) exerting outward pressure on the bark 

How is oak wilt spread?

  • Roots - The disease spreads from infected oak trees to uninfected oak trees through root-to-root contact.
  • Insects - Sap-feeding insects, such as picnic beetles, can move spores from infected oak trees to healthy oak trees.
  • Humans - Movement of wood products, such as firewood can spread the pathogen over long distances. 

How to protect our oak trees

  • Do not prune oak trees during the growing season from April to July to avoid attracting picnic beetles. If pruning must occur, or if a tree is injured, apply a thin layer of wound paint immediately.
  • Do not move firewood from one area to another.
  • Be on the lookout for unusual changes. Oak trees should have their leaves in July. If leaves are on the ground in July that’s a very common sign of oak wilt. 

If you see signs and symptoms of oak wilt, report it to:

For more information, email