Oak wilt fungus found in a residential area of Ontario’s Niagara region in May was the first confirmed detection of the fungus in Canada, according to the Invasive Species Centre.
The disease, which kills oak trees, is caused by a fungus that grows on the outer sapwood of oak trees, restricting the flow of water and nutrients. Some oak species are more vulnerable and can die within a year of infection. In severe cases, red oaks can die within two to six weeks following infection.
Oak wilt has been confirmed in 24 US states, some of which border Ontario. Its origin is unknown, but it is believed to have been in North America since the early 1900s.
Infections occur mainly in the spring. According to the Canadian Food Inspection Agency (CFIA), fungal spores can be transported through all parts of the red oak tree as the tree nears death. In white oaks, however, the distribution of fungal spores is restricted to the xylem of the current year’s growth.
The fungus can spread naturally above and below ground. Above-ground spread occurs after a diseased red oak dies. Then, the fungus produces sporulating mats on the dead tree (these mats are not produced on living trees or white oaks). Nitidulid beetles, or bark beetles, feed on these fungal mats and pick up spores on their bod- ies which they carry to wounds on healthy trees.
Below-ground spread occurs from infected trees to healthy trees through interconnected roots that have grafted together. According to the CFIA, the fungus tends to survive on the above-ground parts of the infected tree for up to one year after its death. It can survive considerably longer on the below-ground roots of the tree.
The fungus can also be spread through the transport of infected wood products or nursery stock.
Each case of oak wilt is different and symptoms among trees are variable and not always visible.
The disease can be identified by the following:
- Red oak leaf discolouration and wilting occurs very fast.
- Leaves begin to change colour near the upper portion of the crown. As the disease progresses, they turn bronze in colour.
- White oaks react slowly to the disease compared to red oaks, usually dying one branch at a time. The discolouration and wilting of leaves is similar to red oaks.
- Fungus (grey or tan mats) is found beneath the bark of infected or dying trees.
