Oak wilt is a vascular wilt disease that affects all species of oaks (Quercus spp.). It is caused by an invasive fungus (Bretziella fagacearum), which attacks and kills healthy and stressed trees. Oak wilt has been present in the United States since at least the 1940s and was first detected in Canada in 2023.

Once the oak wilt fungus has infected a tree, it grows through the tree’s vascular tissues, which become blocked. The tree is unable to transport water and nutrients between the canopy and roots, and the tree declines and dies.

As of summer 2023, three oak wilt infestations have been reported in Ontario.

The first was in the city of Niagara Falls. The infected trees at this site were removed and monitoring is ongoing to determine if any additional trees were infected. The second was in the township of Springwater, outside Barrie. The third was in the town of Niagara-on-the-Lake. Movement restrictions are currently in place at all three locations to help prevent spread.

Signs & Symptoms

Leaf bronzing, premature leaf drop of partially green or bronzing leaves, spore mat underneath the bark, vertical cracks and sweet, fruity smell emited from the spore mat.

Diagnosing Symptoms

Trees infected with oak wilt usually exhibit leaf bronzing and premature leaf drop, beginning in the upper canopy. Leaves turn brown at the tip, progressing toward the leaf base. Leaves can drop while still partially green, as early as June. Trees that have died of oak wilt develop a spore mat underneath the bark, which forms pressure pads that push against the bark and cause vertical cracks. The spore mat also emits a sweet, fruity smell sometimes compared to Juicy Fruit gum.

Oak wilt spreads by two primary means:

  • Aboveground spread occurs when small sap beetles (Nitidulidae) transfer spores from the spore mats of an infected tree to a healthy tree via openings in the bark. These openings could be pruning wounds, cracks, or woodpecker holes. Once the fungal spores have entered the new host tree, the fungus grows in the tree’s vascular tissue. There is no evidence to suggest that oak wilt is spread by acorns.
  • Belowground spread occurs when the roots of neighbouring trees fuse together. These root grafts allow the trees to exchange nutrients, but they also provide a pathway for the fungus to travel from an infected tree to a healthy tree.
Trees in the red oak group (red oak, black oak, pin oak, etc.) succumb to the disease quickly, often dying in one season, sometimes in as little as a few weeks after infection. Trees in the white oak group (white oak, swamp white oak, bur oak, etc.) can live for years with oak wilt, declining more slowly. Some trees in this group may eventually die, but others may recover from the disease.
Some varieties of apple, American and European chestnuts, species of chinquapin, and tanoak were shown to be susceptible to B. fagacearum after artificial inoculation.

Treatment Strategy

Currently, there are no chemical treatments registered in Canada for oak wilt. Propiconazole – a fungistat – is being used in the United States to reduce the impact of oak wilt. This product does not cure a tree with oak wilt and does not halt the dispersal of the fungus through the root system. Therefore, it is typically reserved for select, high-value trees, and used in conjunction with other control measures such as root graft disruption or root plate excavation. In the United States, root graft disruption is the most common treatment strategy.

Oaks should not be pruned unless absolutely necessary between April and November, which is the high-risk period for spore transfer by sap beetles. If pruning is required to reduce risk or for other emergencies, wound paint should be applied to avoid attracting sap beetles to the fresh wound.

Burn firewood where you buy it. Firewood can harbour a variety of insect and disease pests and when transported over large distances, it can introduce invasive pests like oak wilt into new areas.

Oaks should be monitored during the late spring and through the summer for any signs of oak wilt. Early detection and rapid response are key to preventing the spread of pests like oak wilt.

Yes, there are several native species of oaks in Ontario, and it is still a good idea to include oaks along with other species in a diverse planting program. Oaks can live a long time and deliver important ecosystem services through their life. While oak wilt is a serious disease, it is normally slow-moving and there are management tools available to control its spread.
At this time there is no evidence to suggest acorn stock can transmit oak wilt.
In Canada, oak wilt suspects should be reported immediately to the Canadian Food Inspection Agency (CFIA). This can be done online or by sending an email to CFIA ([email protected]). Note the address and take pictures of the leaves and entire crown. DO NOT collect any samples.

Oak Wilt in Woodlots: What Can We Do?

Learn about oak wilt preparation, prevention, and management options for landowners. Oak Wilt is a vascular disease of oak trees and has recently been found in Ontario. Oak wilt is caused by an invasive fungus and kills oak trees by disrupting the movement of water and nutrients. Infected oaks can die within a year of infection.