The sudden oak death pathogen, Phytophthora ramorum, is present in southwestern Oregon, and while an eradication effort is underway, the potential impact of the polyphagous pathogen on surrounding vegetation is unknown. Plant communities in the area are substantially different from those affected in California, although tanoak (Lithocarpus densiflorus), evergreen huckleberry (Vaccinium ovatum) and Pacific rhododendron (Rhododendron macrophyllum) are hosts found in both areas. Other species are likely to be susceptible to the pathogen. Detached leaf and whole plant assays were used to test species commonly associated with tanoak, as well as three rare or endemic ericaceous species of the western Siskiyou Mountains and their associated taxa. Leaves and plants were challenged with zoospore suspensions that were capable of generating symptoms in the known hosts tanoak and evergreen huckleberry. Most (78%) of the previously unchallenged species developed necrotic lesions in detached leaf assays although severity (% leaf area necrotic) was variable. All three of the ericaceous species of conservation concern: Arctostaphylos hispidula, Kalmiopsis leachiana, and Leucothoe davisiae were susceptible in detached leaf assays. Factors important in determining whether or not these species will become infected in their native habitat are discussed. / Graduation date: 2004