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Prevention and Treatment of Oak Wilt

September 13, 2023

Oak Wilt, a devastating disease of oak trees, has been recently detected in southern Ontario. Oak wilt is caused by the fungal pathogen, Bretziella fagacearum. The pathogen infects the water-conducting tissues of oak trees, eventually causing the tree’s death. Oak Wilt is a devastating disease that affects all species of oaks (Quercus spp.) but is most severe in species in the red oak group. Red oaks will often die within weeks or months of infection while white oaks may last between 1 and 3 years after infection.

This article will delve into the cause of Oak Wilt, its symptoms, where it has been detected and the various strategies for managing and preventing this disease.

Oak Wilt Detections

Oak wilt has been found in the following locations:

City of Niagara Falls, Ontario (first confirmed detection of oak wilt in Canada);
Township of Springwater, Ontario;
Town of Niagara-on-the-Lake, Ontario.

When an official Ontario Oak Wilt map is available, we will link to it HERE.

Oak Wilt has been actively expanding across many parts of the United States. Oak wilt has been confirmed in 24 states, some of which border Ontario (e.g. Michigan, New York).

Symptoms of Oak Wilt

Oak wilt is a vascular disease of oak trees (Quercus sp.). The fungus grows on the outer sapwood (water-conducting tissues called “xylem vessels”) of oak trees. The tree responds by walling off the infected water-conducting tissues, in an effort to compartmentalize the invading fungus, but together the tree and fungus end up cutting off the flow of water and nutrients through the tree.

What to look for:

Wilting, yellowing and bronzing of oak leaves, starting from the top of the tree and moving down.
Leaves may wilt, turn brown at the edges and fall off the tree prematurely (July to August).
Vertical bark cracks in the trunk and large branches (from the fungal spore mat exerting outward pressure on the bark).
Greyish-white to black fungal mats, also referred to as “pressure pads”, just under the bark that may emit a fruity smell making them attractive to insects (sap beetles)
Affected trees (especially those oaks in the red oak group) often die quickly, within a few weeks to six months of the first visible symptoms.

Spread of Oak Wilt

Overland Spread

The overland spread of Oak Wilt is primarily through sap beetles (Family:Nitidulidae) that are attracted to the disease’s fungal mats growing under the bark.
These sap beetles carry spores from infected trees to healthy ones, particularly those with fresh wounds from storm damage (or pruning).
The fungus can also spread from moving diseased firewood

Underground Spread

The fungus can move from an infected tree to a healthy one through interconnected root systems (called “root grafts”).
Root grafts between healthy and diseased trees are common, especially within the same species.
The underground spread is the most common method of transmission in established oak stands.

Oak Wilt Treatment

Oak wilt prevention and treatment in Ontario consists mainly of preventative management strategies which can be found in the section below
Preventative Fungicides are used in the United States for high-value oak trees to help prevent the infection of landscape trees. Fungicides are injected directly into the water conducting tissues, just under the bark, to travel throughout the plant and stop any fungal infections that it comes in contact with. Preventative fungicides can help reduce inspection but their success depends on the amount of disease present in the tree.
Currently, there are no fungicides registered for Oak Wilt in Ontario but efforts are underway to find safe, legal & effective fungicide treatments here.
Check this article regularly join our Mail List to be notified when a preventative Oak Wilt Fungicide Treatment is available.

Prevention of Oak Wilt

The most effective strategy for managing Oak Wilt is prevention. Here’s what you can do:
- Avoid wounding oaks during the high-risk period (April through July in the northern hemisphere), when the beetles that spread the disease are most active. However, most arborists avoid pruning of oak trees from April to October to avoid creating entry wounds for this devastating fungus.
- Painting pruning cuts (a.k.a. oak wilt paint) is no longer a recommended practice in arboriculture. Arborists will use a sterilant, such as a solution of rubbing alcohol, to seal off fresh wounds and accelerate drying in order to make wounds less attractive to the fungus.
- Avoid moving Oak firewood. Oak Wilt can be spread with the movement of infected oak firewood. Assume any firewood from a dead oak tree is a potential vector of Oak Wilt disease.
- If a tree is infected, it should be removed and properly disposed of to prevent the disease from spreading (according to the Canadian Food Inspection Agency guidelines). Do not stockpile wood from infected trees.
- In areas where Oak Wilt is common (United States), the prevention of oak wilt may include the creation a buffer zone by severing the roots around infected trees.
- If you suspect that a tree may be infected with Oak Wilt, it’s important to contact the Canadian Food Inspection Agency.
- Our Certified Arborists are trained to recognize oak pests and can assist in evaluating your beautiful oak trees for Oak Wilt.

Oak Wilt is a serious disease that can have devastating effects on oak populations. However, with regular inspections and proper management, it is possible to protect our valuable oak resources. Applying good arboricultural practices to improve the general health of your trees, including watering, mulching, fertilizing, pruning for maintenance, and preventing injury can go a long way in the prevention of oak wilt.

Contact Us for your free on-site assessment to help keep your beautiful oak trees healthy and thriving.