How a sweet-smelling fungus is threatening mighty oak trees

Arborists in southwestern Ontario are gearing up for the potential arrival of oak wilt, a fungal organism that can kill trees so quickly, it's been compared to the Emerald Ash Borer.

Southwestern Ontario cities preparing for the fungal infection that can quickly kill the mightiest oaks

Oak wilt disease was discovered in Belle Isle in 2016, that's within a few football field lengths from the Canadian border. (Michigan Department of Natural Resources)

It's not an insect, but a fruity-smelling fungus with the potential for tree devastation that some are comparing to the Emerald Ash Borer. 

It's called oak wilt and though there have been no confirmed cases in Canada, arborists here are gearing up for its potential arrival in southern Ontario. 

Jill-Anne Spence is London's urban forestry manager. She says the city is ramping up its efforts to warn the public about the disease through a public awareness campaign and increased training for city staff.

"It kills the tree rather quickly," said Spence.

Oak wilt kills by blocking an infected tree's vascular system, depriving it of water and nutrients until it dies. 

It can be spread through the roots of infected trees or by beetles moving from infected trees to healthy ones.

An outbreak in 2016 on Belle Isle, Michigan — that's about 600 metres from downtown Windsor — means it's close, and could easily cross the border into Canada. London's location along the Highway 401 corridor could make the Forest City a key front against the fungus should that border-hopping happen.

A report about the oak wilt threat is coming to the city of London's planning committee Tuesday.

Oak wilt can infect red and white oak species but red oaks are particularly susceptible. Oaks make up only about two per cent of London's forest canopy (roughly 140,000 trees) but because they're so large and long-living, the loss of even one from a backyard or city boulevard is often keenly felt, Spence said. 

"There are fewer oaks in London but they tend to be larger trees," she said. "A lot of wildlife and industry relies on them." 

Though there is no cure for oak wilt, there are ways to detect if a tree is infected.

In cases where the fungus is advanced, patches of white, grey and black can appear on the tree. The fungus emits a smell similar to Juicy Fruit gum. 

Another indication of possible infection: Brown leaves that drop as early as July.

Spence said the city is asking landowners not to transport firewood over long distances because it can spread the fungus. 

The city will also release material about how to spot oak wilt and what do to if an infection is suspected.

"We want to educate and outreach to folks in our community so they can be aware of it and identify it," said Spence. "One of the ways to help in limiting its spread is through early detection so it can be managed as quickly as possible." 


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