Tree Diseases and Pests

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.

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.

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.

Learn more about the Asian Long-Horned Beetle.

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.

Learn more about Birch leafminers.

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.

Learn more about Black Knot of Cherry.

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.

Learn more about the Eastern tent caterpillars.

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.

Learn more about the EAB in Richmond Hill.

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.

Hemlock woolly adelgid or HWA, is an aphid-like insect that attacks and kills hemlock trees. HWA feeds at the base of the needles, extracting nutrients and sap; this causes discoloration and branches to die back. The insects produce white “woolly” egg sacs at the base of hemlock needles. In Ontario, Eastern Hemlock (Tsuga canadensis) trees are susceptible and at risk, and will die in as little as four years once infected.

Hemlock trees are not commonly planted as street and park trees, but are found in natural areas and play an important part of the forest ecosystem. They provide habitat and food for many birds, and can play an important role in regulating and maintaining water temperatures by providing shade to streams, making them habitable for fish. Losing hemlock trees to HWA can have negative effects on the ecosystems in which they live. This can also make forests more vulnerable to other invasive species, like dog-strangling vine or garlic mustard.

Where is hemlock woolly adelgid present?

To date, hemlock woolly adelgid has been found in the Niagara region (Niagara Falls, Fort Eerie, and Wainfleet) and most recently it was confirmed in Grafton, Ontario in 2022.

Signs and symptoms

• White “woolly” sacs at the base of hemlock needles on most recent twigs
• Premature bud and shoot dieback
• Premature needle loss
• Thinner, greyish-green crown
• Dieback of twigs and branches
• Discolouration of foliage
• Death within four to 15 years

How is hemlock woolly adelgid spread?

• Naturally – migrating birds, wind and mammals
• Humans – movement of wood products, such as firewood, and spreading infested nursery stock

How to protect our hemlock trees

• Avoid placing bird feeders near hemlock trees
• Don’t move firewood from one area to another
• In the spring, keep an eye out for signs, such as woolly egg masses, on your hemlock tree and report any potential sightings
• Promote the health of your hemlock trees to improve resilience to HWA and other pests. Visit RichmondHill.ca/Trees to learn more

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

• The Canadian Food Inspection Agency (CFIA)
• EDDMapS Ontario
• Invading Species Hotline – 1-800-563-7711

For more information, email naturalenvironment@richmondhill.ca.

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.

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.

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.  

For more information, visit the Oak Wilt page.

Spongy moth