Eleven woody landscape plants commonly grown in the upper Midwestern United States were inoculated with up to three unnamed Phytophthora taxa (Phytophthora taxon Pgchlamydo, a Phytophthora alni-like isolate [Phytophthora MN14d], and Phytophthora sp. MN1) to explore their host ranges. In addition, P. cactorum, P. citricola, P. citrophthora, P. hedraiandra, and P. nicotianae were used to inoculate plants to further investigate the susceptibilities of plant genera previously found associated with these pathogens, to explore the susceptibility of important landscape plants (i.e., oak) to common ornamental Phytophthora spp., and to prove Koch's postulates. Koch's postulates were completed on fragrant sumac with P. citricola and P. nicotianae and on common lilac with P. citrophthora. A nonwound or wound inoculation technique were used to determine host susceptibility. Phytophthora sp. MN1 caused symptoms on American cranberrybush, bur and red oak, common lilac, fragrant sumac, Norway maple, and 'P.J.M.' rhododendron. The newly described organism P. hedraiandra caused disease on American cranberrybush, common lilac, red oak, and 'Snowdrift' crabapple. Fragrant sumac and common lilac generally were the most susceptible hosts to all Phytophthora spp. This study demonstrated that many ornamental Phytophthora pathogens have larger potential host ranges than previously known. The biology and ecology of P. hedraiandra and Phytophthora sp. MN1 must be further investigated, and methods for rapid identification should be developed.