Originating in the Great Lakes region of North America, sooty bark disease is caused by the fungus Cryptostroma corticale, primarily infecting maple tree species. Though first spotted in Washington State around 1969, there remains a growing concern around the disease as fungal growth continues to accelerate during Washington’s climate change-induced longer, warmer summers. Learn more about how to identify and prevent its spread below!
How to Identify
Trees infected with sooty bark disease can be identified by bark blisters resulting from spore production in the bark periderm. Once the bark cracks and strips away, patches of dark brown to black fungal stoma and black spores are exposed. These black patches resemble soot, hence the disease’s name. The fungus may also result in lignin degradation which leads to soft rot, giving the infected wood a greenish stain. Epicormic shoots, trunk cankers, or a wilted canopy may also be signs of an infection. Sooty bark disease is often confused with the Stegonsporium pyriforme fungus, which produces smaller black spots on the bark, rather than a layer of stomata.
Sooty bark disease primarily infects sycamore maple trees, though several other species may be susceptible to infections including Japanese maple, Pacific dogwood, and horse chestnut. These trees become infected when spores are carried by wind or rain into a tree’s open wound caused by external damage and/or pruning activities. Trees weakened by drought and high temperatures are the most vulnerable to the disease, as the fungus grows most quickly in hot and dry climates.
Sooty bark disease negatively impacts natural environments, as the infection and mass dieback of maple trees weakens biodiversity. In addition, spores produced through sooty bark disease are extremely allergenic, causing hypersensitivity pneumonitis in people with prolonged tree exposure. Those working with infected trees should wear proper PPE to avoid adverse health affects caused by spore inhalation.
Unfortunately, once a tree becomes infected with sooty bark disease it cannot be cured. To prevent further spread, individuals should have infected trees properly removed. If you see a tree with symptoms of sooty bark disease, contact your local arborist who will perform a laboratory analysis to confirm the presence of C. corticale. Once confirmed, the tree can be removed in damp weather to avoid aerosolizing large numbers of spores and either buried locally or burned in a waste incinerator. Additionally, community members can join the iNaturalist project to share observations of infected trees, helping advance knowledge about the disease distribution and susceptible hosts. More information can be found here.