Bigleaf maple decline continues to be a scientific mystery

Forest pathologists in Washington have been investigating the decline of bigleaf maple (Acer macrophyllum) in the Pacific Northwest since it emerged in 2010. Recently, Dave Shaw, OSU Extension forest health specialist, has noticed the maple's decline in Oregon has intensified and hypothesized that it may be attributed to something other than drought, winter weather, canker diseases, squirrel damage or other common culprits. Bigleaf maple decline has been reported throughout the tree’s entire geographic range, including in California, Oregon, Washington and British Columbia.

Currently, Shaw and others report that there is no known cause associated with this decline. Scientists in Washington have yet to find a conclusive answer as to what’s causing the decline of this beloved riparian species.

There are theories out there, one of which is that it may be caused by a leafhopper. Water stress from drought may also be playing a role. One study by University of Washington scientists found that it was associated with increased human development, proximity to roadways, as well as hotter and drier summers.

While nothing is conclusive at this point, Shaw is asking that we all keep an eye out for symptoms. Symptoms include partial or complete crown dieback, discoloration and reduced size of leaves with heavy seed crops, crown thinning and tree death.

Additional symptoms may include leaves with yellow edges and red-to-brown tips, typical of leaf scorch. These are all signs that something is awry with the tree, which could include drought stress, root disturbance, chemical imbalance or pathogens.

So, what can we do about this? The Pacific Northwest Plant Disease Management Handbook suggests that landowners “maintain trees as best as you can with minimal care and infrequent deep waterings during the summer months while forest pathologists scratch their heads.”

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