This article is an update to the one published by Tree Link in October of 2014: Bigleaf Maple Dieback in Western Washington?

DNR’s Forest Health staff have received many calls and emails from concerned residents about the health of bigleaf maples this year. These contacts started in June and have become more numerous throughout the summer.

The question starts, “what is going on with bigleaf maples?” followed closely by, “the trees look like they are dying.”

Forest Pathologists at DNR and elsewhere have been investigating crown and branch dieback of bigleaf maples since 2011, and the truth is, we don’t know exactly what’s going on.

The good news is that we have a much better idea of what isn’t going on. Initial surveys sampled for Verticillium wilt, a fungal pathogen frequently associated with wilting maples, yet we failed to detect the fungus in any of our sampled trees. Next, we surveyed for Armillaria root disease, which was found in 11% of the samples. However, this was not enough evidence to identify Armillaria as the culprit.

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Symptomatic bigleaf maple with thinning crown next to another that died in 2015. Photo by DNR

In 2014, DNR received funding from the USDA Forest Service to establish permanent research plots across western Washington and western Oregon where bigleaf maple symptoms have been observed. In partnership with pathologists from the Oregon Department of Forestry, Oregon State University and the USDA Forest Service, samples of soils, fine roots and leaves were collected from these plots and sent to the forest pathology lab at Oregon State University for analysis. Preliminary results include Pythium, very little Phytophthora and some Armillaria. At this time however, none of these pathogens were found with enough consistency to suggest that any of them are the causal agent.

Root pathogens have been the focus of our investigations because the dieback pattern being observed is consistent with signs of root disease: thin crowns, smaller leaves in the upper canopy, heavy seeds crops, little-to-no epicormicing branching (new growth from the main tree stem) and dieback of entire branches.

This summer we have been revisiting the sample plots, noting significant changes in canopy condition and whether or not mortality has occurred. We are also collecting stem core samples and examining them for signs or symptoms of root diseases or other damage agents. Bigleaf maple mortality is occurring in a small portion of our permanent plots and nearly all of our stem cores have signs of heart decay.

We will continue to survey plots and send samples to Oregon State University in hopes of discovering the true cause of the dieback.

A link to the report from the 2011 survey can be found here: http://www.fs.fed.us/foresthealth/fhm/posters/posters12/Omdahl_maple_poster.pdf .

If you have any questions or comments about this project, please contact Amy Ramsey, amy.ramsey@dnr.wa.gov.