Possible Role of Mycorrhizas in Resistance to Decline in Arbutus menziesii

A complex set of factors is causing decline of Pacific madrone (Arbutus menziesii) in the Puget Sound region; yet, some trees remain healthy. It is unclear why they do. We are investigating the hypothesis that symbiotic associations between the roots of madrone and soil-dwelling fungi make the trees less susceptible to disease. The effect could occur either directly or by providing physical links to nearby Douglas-fir (Pseudotsuga menziesii) trees through which carbon compounds are transferred to the mycorrhizal systems and, perhaps ultimately, to the madrones. In this initial study, our objectives are to determine whether: 1) madrones growing near Douglas-firs are healthier than madrones not growing near Douglas-firs; 2) healthy madrones have more and/or better developed mycorrhizal associations than declining madrones; and, 3) madrones associate with the same mycorrhizal fungi as nearby Douglas-firs. In the Puget Sound region, many Pacific madrone (Arbutus menziesii) trees are dying or in poor health. Usually health of the the affected trees declines over an extended period before they die, commonly from root-rot fungi such as Armillaria, heart-rot fungi, and wood-boring insects. Environmental stresses from a variety of causes such as over-watering, drought, high or low soil fertility and growing in exposed situations cause the trees to have low resistance to infections by fungi such as Nattrassia mangiferae (the cause of Arbutus canker) and Fusicoccum aesculi (the cause of branch dieback). Infection by these fungi reduces tree resistance and allows the root-rot fungi, heart-rot fungi and insects to become established. In general, reduced resistance seems to be associated with reduced capacity for photosynthesis (Elliott 1999). Despite the widespread occurrence of decline,