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Sudden oak death is a new disease affecting tanoak (Lithocarpus densiflora) and oaks (Quercus spp) in California and Oregon, caused by the recently described pathogen Phytophthora ramorum. It has reached epidemic proportions in several counties in central California, leading to the death of tens of thousands of trees. In addition to oaks and tanoak, P ramorum has been found in nearly all woody plant species in mixed evergreen and redwood forests from central California to southern Oregon. Plant species that are not killed appear to serve as a reservoir for the pathogen. The high susceptibility of tanoak to infection and death suggests that P ramorum is an exotic pathogen, but its origins, and most details of its biology and ecology, remain unknown. Our limited knowledge only compounds our concern over the long‐term implications of this epidemic for the ecology of coastal forests.