Oak wilt infects Ontario trees in Niagara Falls, Springwater

Oak leaves browning from oak wilt Photo by Hope Dorman/Shutterstock

A tree disease originating from the United States was detected in Canada for the first time this May. Experts say it could devastate Canadian forests. 

First detected in Wisconsin in the 1940s, oak wilt disease is caused by the spread of a fungus called Bretziella fagacearum. It can infect over 20 species of oak trees, but red oaks are especially vulnerable; oak wilt can defoliate and kill them within a single season, according to the Canadian Food Inspection Agency

Matthew Logan, president of the International Society of Arbor Culture, says oak wilt is now found in 24 U.S. states, including several against the border of Ontario. CFIA first detected the disease in Niagara Falls, Ont., then later in Springwater, Ont., which was unexpected based on previous studies.

“It’s been on our radar for approximately the last 10 years because it’s been ravaging south of the border in Michigan,” Logan says. “We originally thought that the point of access was going to be the Windsor-Sarnia area, so we were a little surprised.”

Logan says researchers don’t know how the disease ended up in Ontario, but wood chips or logs could be to blame. He adds that the best way to prevent spreading the disease is to not move firewood or mulch between a city house and a cottage, since it can be hard to tell when wood is infected.

“If the wood is already in your neighbourhood or in your yard, it’s okay to move,” Logan says. “But our concern is moving this disease from one region to another.”

A tree is infected with oak wilt if it has browning and wilting leaves, premature leaf drop, cracks in the bark, and white, grey, or black pads along cracks that smell like Juicy Fruit gum. Sap beetles and other bugs are attracted to the smell, which, according to Invasive Species Centre staff Madison Sturba, gives the fungus an opportunity to spread to other trees.

Sturba says there’s a high risk the spread will continue without proper preventative measures. If a tree is wounded or needs pruning for safety reasons, she and Logan recommend covering the tree with latex or pruning paint, which you can find at most hardware stores. Otherwise, it’s good practice not to prune oak trees that look infected, especially between April and August. 

“That’s when the disease is most active, and the oak trees are most susceptible,” Logan says.

Since oaks are an ecological keystone species in Canada, Sturba says their loss could reduce the country’s forest biodiversity and disrupt “environmental services.”

“Oak trees help stabilize slopes, limit soil erosion, and reduce air pollution,” she says. “They also make up a significant portion of municipal canopy covers, and they’re important culturally. It was just made the official tree in Toronto.”

Logan adds that oaks are one of the most diverse species of tree in Canada, so this fungus, left unaddressed, could have devastating impacts on local ecology. He cautions that oak wilt “100 per cent has the chance to be as dangerous as the Emerald Ash Borer” that decimated ash tree populations.

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