Forest Health: Oak Wilt

Oak Wilt is a vascular disease caused by the fungal pathogen Bretziella fagacearum. The first known case of Oak Wilt in Canada was confirmed in Niagara Falls in May 2023. Although all oak species are vulnerable, the red oak group (red, black, and pin) are most susceptible, frequently resulting in death within 2-12 months of infection. Species in the white oak group (burr, swamp white, and white) are more tolerant of the disease and may survive infection for one or more years while displaying declining symptoms.  

The fungus grows on the outer sapwood beneath the bark of infected oaks, restricting the flow of water and nutrients within the tree. The fungus is spread from infected to healthy trees by two means: insects carrying the fungal spores and through root graphs. In addition, the fungus can be spread by people moving infected wood products, firewood, and nursery stock. 

When a diseased red oak dies, fungal mats develop on the trunk in the spring and fall (these mats are not produced on living trees or white oaks). The fungal mats, which emit a sweet aroma, attract Nitidulid beetles, also known as sap or picnic beetles. While feeding, the sticky spores adhere to the insect’s body, which they then carry from infected trees to fresh wounds on heathy trees.  

While insects are an important dispersal mechanism, it is estimated that 90 percent of new infections occur between neighbouring trees through root grafts. While the fungus can survive above-ground for a year after the tree has died, it can survive considerably longer on tree roots below-ground, and intertwining roots facilitate infection to adjacent oak trees. 

Signs and Symptoms of Oak Wilt: 

  • The leaves wilt and turn bronze or yellow, starting at the top of the tree. Discoloration begins at the leaf tip and moves towards its center. 
  • Early and sudden leaf drop.  
  • White, grey or black fungal mats form under the bark of dead trees and often have a fruity smell.  
  • Pressure pads split or crack the trunk as the fungal mat pushes outward. 
  • A black stain forms in the sapwood of the tree.  

Reduce the Spread

The many species of sap beetles are attracted to fresh wounds or pruning cuts during the active growing season. To reduce the spread of oak wilt, avoid pruning oak trees between April and August. If a tree is wounded or pruned during this period, cover the damaged area immediately with a latex paint or wound dressing.

MNRF Study

In 2019, the Ministry of Natural Resources and Forestry (MNRF) conducted a flight trap study to get a better handle on when the various species of sap beetles were flying. The three study locations were the University of Guelph Arboretum, the Royal Botanical Gardens (Hamilton/ Burlington), and the UTRCA’s Fanshawe Conservation Area (London). From March until November, UTRCA forestry staff monitored three flight traps weekly, and shipped sap beetles collected to the MNRF lab in Sault Ste. Marie for identification.

More Information

Follow these links to find out more about Oak Wilt:

Contact: Rob Davies, Forestry and Restoration Supervisor