Oak wilt is a vascular disease that affects oak trees. The disease is caused by the fungus Bretziella fagacearum. This fungus grows in the sapwood of the trees and restricts the flow of water and nutrients throughout the tree.

Currently, there are three confirmed locations of Oak Wilt in Ontario. Two in the Niagara region, and one near Barrie.

All species of oak trees (Quercus sp.) are susceptible to Oak Wilt. Trees in the red oak family are more susceptible than the white oak family. Red oak trees can die quickly if infected.

The fungus grows on the outer sapwood of oak trees. As trees are nearing death, ‘sporulating mats’ can form under the bark of red oak trees. These can have a fruity smell and attract Nitidulid beetles (bark beetles) that get covered with the fungus and spread it to other trees. The fungus also spread below ground through interconnected living roots of trees. Above ground the spores tend to live for up to one-year, while below ground the fungus can survive much longer.

The fungus is also spread through the movement of infected wood products or nursery stock.

The signs and symptoms will vary depending on the species of oak infected. Red oak species are most susceptible and can die within a year of being infected. White oak species can tolerate it for longer periods.

Signs:

• White, grey, or black fungal mats (‘pressure pads’) just under the bark. These may have a fruity smell.
• Vascular streaking (dark rings) can be seen in cross-sections of infected branches.

Symptoms:

• Wilting and bronzing of leaves starting from the top of the tree and moving down.
• Leaf discolouration beginning at the edge and moving towards the middle.
• Premature leaf fall.
• Vertical cracks in the bark. These are caused by the fungal spore mats exerting outward pressure on the bark.

Oak wilt is expected to have devastating impacts on oak populations within Canada. While oak is not a main species within forests of the GSCA watersheds, acorns are a valuable source of food for various wildlife species. Oaks also help to stabilize slopes, reduce soil erosion, and reduce air pollution.