Phytophthora cactorum, P. citricola I, and P. plurivora cause bleeding cankers on mature European beech (Fagus sylvatica) trees in the northeastern United States. Inoculation experiments were conducted to compare the aggressiveness of the three Phytophthora spp. on stems, leaf disks, and roots of European beech and common lilac (Syringa vulgaris) seedlings. Isolates were obtained from bleeding cankers on European beech from five cities in New York (Albany, Ithaca, Oyster Bay, Plainview, and Rochester) and from a bleeding canker on sugar maple in Ithaca, NY. Stems were inoculated with colonized agar plugs, leaf disks with a zoospore suspension, and roots via infested soil at three inoculum levels. All organs of inoculated beech and lilac developed disease except for lilac roots inoculated with zoospores of P. cactorum. Disease incidence, severity, and plant survival were dependent on isolate and were also influenced by the tissue inoculated and host. Isolates of P. cactorum were the least aggressive and caused less necrosis than isolates of P. citricola I and P. plurivora. Results emphasize the utility of stem and root inoculation for evaluation of this canker disease and underscore critical differences in species aggressiveness.