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The population structure and ecological roles of the indigenous pathogen Phellinus weirii, cause of laminated root rot in conifer forests of western North America, are examined. This pathogen kills trees in slowly expanding mortality centers, creating gaps in the forest canopy. It is widespread, locally abundant, and very long-lived. It is among the most important disturbance agents in the long intervals between stand-replacing events such as wildfire or harvest in these ecosystems and shapes the structure and composition of both wild and managed forests. Trees are infected and killed regardless of individual vigor. Management of public lands is changing dramatically, with renewed emphasis on natural forest structures and processes but pathogens, especially root rot fungi, remain a significant challenge to “ecosystem management.”