Phytophthora alni is a species complex of pathogenic oomycetes (water molds) that can cause lethal disease in alder trees, Alnus spp. One variant, P. alni subsp. alni, is widespread across Europe, devastating stands of alder since the 1990s. One less lethal member of the species complex, P. alni subsp. uniformis, has already been found in the wild in Alaska and Oregon, but not in Washington State. Recently, it has been detected in potted alders in nurseries in Pierce County. It is currently unknown whether any member of the P. alni species complex is in the wild in Washington, either naturally or through introduction from nursery plants. To begin efforts to detect Phytophthora alni in Washington State, a risk assessment map will be created using Geographic Information System (GIS) techniques. The spatial analysis will involve examining the environmental factors that increase infection susceptibility, such as slope, soil grain size, and flooding, and correlating with the distribution of alders. Important questions that could be revealed are whether high risk areas are upstream, where infection could spread, or downstream, and whether high risk areas correlate with urban or agricultural land use. The risk assessment will provide a starting point for choosing sampling sites, which is the next step in detecting the existence of P. alni. Additionally, the final analysis will inform forest management practices, as the highest risk areas could be inspected for symptomatic alders and mitigation measures could be enacted if any are found. The assessment will also have implications for restoration sites, where native trees such as alder are planted from nursery stock.