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    ABSTRACT: Sudden oak death (SOD) has been shown to be caused by a new species of Phytophthora, P. ramorum. A basic understanding of the genetics of P. ramorum is critical to any management strategy. We have initiated a number of studies to examine species concepts, population biology and mating behavior of the pathogen. Based on a number of morphological features (e.g., a combination of deciduous sporangia and chlamydospores), the P. ramorum does not match any of the currently described species of Phytophthora. Sequences of the internal transcribed spacer (ITS) of nuclear ribosomal DNA were identical for isolates from Lithocarpus densiflorus, Quercus spp., Rhododendron sp., Vaccinium ovatum, Umbellularia californica, and Aesculus californica. The sequences were also identical to a recently described species from Europe, P. ramorum. Based on ITS sequences, the closest species to P. ramorum is P. lateralis; ITS sequences between the two species differ by 12 nucleotides. We are now examining the population structure of P. ramorum using amplified fragment length polymorphisms (AFLPs) to determine variability within pathogen populations. This information will provide insights into whether P. ramorum is an exotic pathogen and whether sexual recombination is taking place in California populations. Finally, the use of genetic data allows for the development of species specific diagnostic probes. PCR primers based on the ITS region have been used to facilitate the rapid identification of the pathogen from plant tissue.