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Help prevent Oak wilt spread

ABCA Forestry and Land Stewardship Specialist Ian Jean holds a branch with healthy Red Oak leaves.


Ausable Bayfield Conservation urges people to help prevent Oak wilt spread
People can help prevent spread of fungal disease by not pruning Oak trees between April and November, and by not transporting firewood

A fungal disease called Oak wilt has been detected in southern Ontario. Ausable Bayfield Conservation Authority (ABCA) staff urge you to help prevent the spread of Oak wilt. People can help by not transporting firewood and by avoiding the pruning of Oak trees between April and November.

Oak wilt grows in the sapwood of Oak trees. Recently, the Canadian Food Inspection Agency (CFIA) announced Oak wilt has been detected in Niagara Falls and in the Township of Springwater, near Barrie. While the disease is new to Ontario, it has been known from the Midwest United States since the 1940s. Red Oak trees are particularly susceptible, resulting in tree death within a single season. All Oak species can get the disease but in White and Bur Oak trees the disease is not always fatal and is sometimes limited to branch dieback.  

Experience from the U.S. Midwest provides us with knowledge on how to reduce the spread and impact of the disease. Oak wilt moves slowly on its own. However, Oak wilt can be moved long distances when people move infected firewood from one location to another.  

“Transporting firewood is the main way that Oak wilt is introduced to a new location,” said Ian Jean, ABCA Forestry Specialist. “We need to get the message out to not move firewood,” he said. “When visiting or vacationing please use local sources of firewood.” Transporting firewood from other areas of Ontario, to campgrounds or cottages, increases the risk of introducing the disease locally.

Oak wilt may spread short distances when fungal spores are moved by sap feeding insects from an infected tree to a freshly pruned or injured Oak tree. The current CFIA guidelines are to avoid pruning Oak trees between April and November, except for emergency purposes. The Ontario Chapter of the International Society of Arboriculture has relevant guidelines for Certified Arborists and tree service providers.

Once a tree is infected, the fungus moves very efficiently from an infected Oak tree to a healthy Oak tree through grafted root systems. “It is common for trees to form root connections with nearby trees of the same species,” Ian said. “Unfortunately, this trait enables the fungus to spread through root grafts such that any Oak tree with a root connection to a diseased Oak tree will get the disease.”  

According to the Invasive Species Centre, symptoms of Oak wilt usually appear in July and include leaf drop that starts at the top of the tree and progresses to the bottom. Similarly, the dying leaves show browning that starts at the tip of the leaf and progresses toward the base or stem of the leaf. Suspected cases of Oak wilt can be reported to Invasive Species Hotline 1-800-563-7711 or to the Canadian Food Inspection Agency via their online form.  

It is important to note that in areas of the United States, where Oak wilt has been present for several decades, it has not resulted in total loss of Oak. There are still healthy Oak trees, and Oak continues to be a major proportion of natural and urban forests throughout the U.S. Midwest. However, in local areas where Oak wilt becomes established, considerable effort and expense must be undertaken to remove and destroy infected trees and to sever underground root grafts to prevent the disease from spreading from tree to tree. 

Forests make up just more than 14 per cent of the land cover across the Ausable Bayfield watershed according the Ausable Bayfield Watershed Report Card (2023). Oaks are an important component of local forests, which contribute towards watershed health and resilience. Not only do they produce acorns, a high value food source for wildlife, Oaks also host many leaf-eating caterpillars and insects that, in turn, feed birds and small mammals. “Fortunately, we do have ways to prevent the spread of Oak wilt,” Ian said. “Please leave your firewood at home,” he said. “There is a supply of firewood available when you arrive at your destination.”

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