Bigleaf maples (Acer macrophyllum) are an important ecosystem component in the forests of western Washington, providing shade, food, habitat, and structural diversity in riparian and upland areas. Over the past several years, concerned landowners, the general public, and forest land managers have contacted forest health specialists from the University of Washington, the Washington Department of Natural Resources (DNR) and the US Forest Service (USFS) about what they perceive to be increased levels of bigleaf maple decline and dieback. Symptoms frequently reported include yellow flagging of large branches, small leaf size, and partial or entire crown dieback.
Verticillium albo-atrum and Verticillium dahlia — causal agents of Verticillium wilt — are reported as damaging agents of bigleaf maple (Minore and Zasada 1990). V. albo-atrum and V. dahliae are soilborne fungi that invade the xylem of host trees and can cause leaf drying, leaf curling, defoliation, wilting, dieback and tree death (Sinclair et al. 1987). A project in 2011 investigated whether or not Verticillium wilt was the primary cause of bigleaf maple decline and dieback in western Washington. Sixty-one sites were surveyed across western Washington and branch samples were submitted to the Washington State University Puyallup Plant & Insect Diagnostic Lab for microscopic examination of V. albo-atrum and V. dahlia. Verticillium was not found in any of the submitted samples. Signs of other root diseases were found in a portion of the trees surveyed, (Armillaria root disease in 11 percent of the trees surveyed, Ganoderma root disease in 3 percent of tree surveyed), but these results do not suggest that either root diseases are the primary causal agents of bigleaf maple dieback in western Washington.
This year, 2014, another survey project investigating the causal agents of bigleaf maple dieback and decline was cooperatively initiated among DNR, USFS, Oregon State University (OSU), and the Oregon Department of Forestry. DNR staff visited 55 sites in western Washington with symptomatic bigleaf maple late this summer, tagging a tree at each site for long term monitoring of the symptomatic trees. In a month or so, visits will be made back to each site and the trees will be examined for microscopic root diseases by collecting soil and fine root samples and sending them to OSU for processing.
This article written and submitted by Amy Ramsey, forest pathologist, Washington State Department of Natural Resources