Why are leaves on several maple trees in Toronto turning white?

Stunning reds, oranges and yellows are popping up across Toronto as fall colours settle in, but it’s white leaves in several neighbourhoods that have many taking notice.

“We are getting a lot of calls about what we’re seeing,” Joel Harrison-Off, a forest health care inspector with the City of Toronto, said on Wednesday.

CityNews saw multiple Norway and Crimson King maple trees at Humewood and Sir Winston Churchill parks, as well as in Nordheimer Ravine in the Bathurst Street and St. Clair Avenue West area with what Harrison-Off described as powdery mildew.

“It’s a white fungus, and basically that fungus uses that leaf surface quite well to spread out,” Harrison-Off explained.

“Powdery mildew is such a visual thing. A lot of people are concerned about the health of their trees.”

He said the unsightly fungus can cause leaves to curl up and fall early. However, Harrison-Off said people shouldn’t be worried right now and that there isn’t a risk to human health.

“In general, especially on Norway Maples, it’s not going to be a long-term health issue to the trees. These trees typically recover. Next year, you’re gonna see a new flush of leaves on these trees,” he said.

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So what’s behind the spread of powdery mildew in Toronto?

“We had perfect weather conditions for it to really blow up and show up really well on trees, but it’s right across Toronto, right across southern Ontario,” Harrison-Off said.

He said excessive humidity combined with bunches of leaves where air can’t circulate easily help powdery mildew develop. Harrison-Off added fungus spores can spread through the wind.

Another fungus found on several Toronto maple trees is tar spot.

“You’re going to see it as a dark patch right in the middle of the leaf. Again, not something that’s going to hurt the overall health of the tree,” Harrison-Off said.

Meanwhile, if a tree on your property has powdery mildew, Harrison-Off said there are a couple of actions you can take but noted there isn’t a corrective fix.

“Rake up those leaves and hopefully get rid of some of those spores, but also (do) a general pruning to thin out that tree. Get some airflow in there (and) that can help reduce some of the humidity build-up,” he said.