Unprecedented weather has led to the emergence of sooty bark disease in B.C.’s maple trees.
March 2025
By Madison VanCamp for Simply Science
Sooty bark disease (SBD) is coming out of the woodwork in force since its first sighting in British Columbia back in 2022. This disease, caused by the fungus Cryptostroma corticale, first appeared on European species — Norway and sycamore maple trees — planted in the Victoria and Vancouver areas. At first, SBD seemed to be confined to just these ornamental species, but by 2024 it was discovered in a native species: the bigleaf maple, Canada’s largest maple species. “This species is one of the most iconic deciduous trees that we have on the west coast,” says Nicolas Feau, a research scientist with the Pacific Forestry Centre (PFC).
The fungus that causes SBD can kill a tree in as little as three years. Here, infected trees display typical mottled trunks. (Right) A microscopic view of cryptostroma corticale.
Understanding Sooty Bark Disease
The fungus that causes SBD is an invisible ticking time bomb, one that can lie undetected for years. “It’s already in the trees — potentially for years, if not decades — before you see the disease hit,” explains Joey Tanney, a research scientist with the Pacific Forestry Centre. The fungus stays dormant inside the tree until a perfect storm of heat and drought stress triggers its fatal symptoms. And in recent years, B.C. has seen plenty of these triggers.
First came the Pacific heat dome in 2021, an unprecedented heat wave that pushed temperatures to record highs. Then, in the summer of 2022, the Victoria area experienced a stretch of 100 days without any measurable rain. The combined stresses of heat and drought act as a switch for this fungus; so suddenly, what had been quietly growing inside of the tree began to kill it.
Then the disease progresses rapidly, and a healthy-looking tree can die within months.
Trees grown in a lab setting provide clues to what environmental conditions trigger sooty bark disease.
The spread to big leaf maples
The first bigleaf maple discovered to have fallen victim to SBD in 2024 was found right on the property of the Pacific Forestry Centre in Victoria. “Right behind the Pacific Forestry Centre there’s a creek, we were walking along,” recalls Joey, “and that’s where we first found it on bigleaf maple.” Since then, nearly 100 bigleaf maples have been confirmed dead as a result of SBD, and there are even more sightings in sycamore and Norway maples each season.
The rapid rate of spread is troubling. “It happened very suddenly,” says Nicolas. “Two to three years ago, working in the same area, we didn’t find any infected trees.”
Is this fungus native or new?
One of the biggest mysteries about SBD is whether the fungus causing it is native to B.C. or an introduced species. There’s no historical record of it in the region, but that doesn’t mean it wasn’t there before.
“If it was introduced, it could have arrived 50 years ago or more,” says Joey. “And we wouldn’t have known until disease symptoms started to manifest with these unprecedented heat waves.” On the other hand, it’s also possible that the fungus is native and hasn’t historically caused enough disease to be noticed — until now. Climate change may have tipped the scales.
While this fungus hasn’t been known in the past to actually cause SBD in Canada, it was first described back in 1889 near London, Ontario and is endemic to the Great Lakes Region. In that area, it’s a little-known species that decomposes dead sugar maple trees. While it’s not known to cause SBD in trees there, Joey and Nicolas are asking the obvious and worrisome question: “If under enough stress, could it?”
(Left) Nicolas Feau at work in his lab. (right) You can barely see Joey Tanney standing next to an infected maple tree. Both NRCan research scientists at the Pacific Forestry Centre are working together to understand SBD.
Climate change may have tipped the scales
To examine this possibility further, Joey and Nicolas are ramping up surveys to study how widespread SBD is among bigleaf maple trees in B.C. By coring trees and analyzing their DNA, they hope to uncover the extent of asymptomatic infections and prepare for potential outbreaks.
They’re also focusing their attention on sugar maple trees. Through experimental testing, they hope to find out if stressing sugar maple trees infected with the fungus can trigger the disease. By better understanding what environmental conditions trigger SBD in sugar maples, researchers can anticipate and prepare for a potentially new disease in this emblematic and economically important Canadian tree species.
Rising to the challenge
As SBD is emerging dramatically in British Columbia in recent years, Joey and Nicolas are meeting this challenge with curiosity and hope. They’ve developed diagnostic tests that can quickly confirm if a fungus is SBD, and not one of its many lookalikes. These tests give many interested parties — scientists, arborists, municipality workers and park officials, to name a few — the ability to respond and manage outbreaks more effectively and safely.
And there’s also the human health aspects to the problem. The fungal spores that cause sooty bark disease pose potential health risks to those who are exposed to them, since billions of these allergenic spores can be released from a single symptomatic tree. Exposure can lead to a rare lung disease called hypersensitivity pneumonitis, also known as “farmer’s lung.” For people who must remove and dispose of dead trees, the diagnostic tests developed at the Pacific Forest Centre provide clear answers both to what protective measures are needed to ensure human health is protected and which infected trees should be properly disposed of to avoid infection. For the researchers themselves, these tests also provide a way to identify in advance asymptomatic infections in trees so they can get a head start in assessing whether environmental stresses are likely to cause sooty bark disease to develop in B.C.’s forests.
The team also plans to engage with local communities in the Victoria area through community science. They’ve set up an iNaturalist project in which community members can upload photos of suspected SBD cases. Joey and Nicolas are then able to use the photos to identify cases, request samples or even visit sites to test for infections.
Overall, the team is optimistic about how technology and collaboration can improve our understanding of this disease. “We have brand-new technology and great community-driven science — all these new tools at our disposal,” says Joey.
Discover more:
Sooty bark disease - Natural Resources Canada
Sooty bark disease - NRCan fact sheet