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This dissertation presents a body of work collected from the very beginning of the outbreak of the introduced forest pathogen Phytophthora ramorum. The first data were collected the same year that P. ramorum was identified as the causal pathogen of the disease in California. The research, aimed at gathering the scientific knowledge necessary for an understanding and informed response to a new problem, placed an emphasis on bridging evolutionary ecological theory and management practice. First, my colleagues and I outline two quantitative polymerase chain reaction (qPCR) diagnostic assays for P. ramorum and their use as part of a widespread survey screening California plants for the pathogen. In Chapter 3, my colleagues and I report the first surveys of variation in susceptibility to P. ramorum on detached leaves of field-collected tanoak trees. We found low but non-zero heritability of susceptibility, and that the structure of phenotypic variation in this measure of susceptibility echoed that of neutral genetic markers. Finally, I describe the establishment of a greenhouse common garden population of tanoak seedlings concurrently with a field test garden in an infested natural area. Using these study populations, we calculated the heritability of susceptibility in detached leaves with much greater precision, and expanded the assay techniques and types of host resistance we measured. In so doing we discovered highly variable responses of seedling families to intact-seedling tip inoculations, the majority of which variation was due to genetic factors. With the qPCR detection techniques described in Chapter 2, we confirmed seedlings' infection within the first 7 months of the field test garden's establishment, with disease incidence rising over the course of the study. We found that both P. ramorum infection status and family origin have significant effects on tanoak survival in natural disease conditions. Together with the laboratory assays, our findings show that tanoak does possess some forms of genetic resistance to P. ramorum. The gardens we established provide the infrastructure necessary to determine if this resistance affects survival under natural conditions, to further investigate its inheritance and action, and to test the effect of varying environment and host genotype on its expression.