Role of fungal diseases in decline of Pacific madrone
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There is concern over the health of Pacific madrone (Arbutus menziesii), a tree native to California, Oregon, Washington, USA, and British Columbia, Canada. Declining trees have been reported in western Washington and British Columbia during the past 30 years. The fungus Nattrassia mangiferae causes cankers and shoot blight and is associated with declining madrones. Fusicoccum aesculi causes branch dieback but is a secondary pathogen that attacks stressed trees. We found both fungi on declining madrone trees in King County, Washington. More canker fungi were isolated within the first centimeter from the canker margin, and occurred more often on large diameter wood. N. mangiferae was found in 90% of the samples from the margin of madrone cankers and is considered the primary pathogen. Trees that were heavily infected with canker fungi had less stored starch in the root burl. Starch content in the root burl of declining trees was significantly lower than in healthy trees. Pacific madrone decline has a similar pattern to other declines involving early successional species that establish after a disturbance, where mature trees are more severely affected. Fire was the major natural disturbance agent with madrone, but disease now appears to be replacing fire as the main disturbance agent responsible for killing aboveground plant parts. Unlike fire, disease decreases starch accumulation in the root burl, so that declining trees are less able to resprout after the aboveground portion of the tree is killed by disease.