Soybean [Glycine max (L.) Merr] is one of the most important commodities in the US and worldwide. Phytophthora stem and root rot, caused by Phytophthora sojae, account for one of the largest yield-limiting factors in the US. Qualitative host resistance, conferred by race-specific, single, Rps- (Resistance to P. sojae-) genes/alleles have been incorporated in soybean breeding programs and cultivars to manage this disease. To date, over 30 Rps-genes have been reported, and on-going research has identified up to 46 novels Rps-genes sources. With pathogen populations constantly evolving, characterization of P. sojae races or pathotypes is critical for more informed introgression and deployment of Rps-genes. Differentials, near-isogenic lines differing only for the presence of an Rps-gene, are used to assess pathotypes diversity in a pathogen population. To characterize the virulence of P. sojae populations against these novel Rps-genes, my research objective is to develop soybean differentials for each Rps-gene through backcrossing of the exotic sources into a cultivated background (cv. "Williams"). Molecular markers will be developed to select genetic regions associated with Rps-genes as well as the "Williams" genetic background. In addition to enabling characterization of the virulence of P. sojae populations, this work will facilitate development of cultivars with these novel Rps-genes through (1) identification of the most effective Rps-genes, (2) differentials serving as an improved breeding source of novel Rps-genes, and (3) application of developed markers in marker assistant selection. Our overall goal is to generate germplasm that can be utilized by soybean breeders and pathologists throughout the US.