Mapping of Phytophthora sojae resistance in soybean genotypes PI 399079 and PI 408132

Numerous novel sources of resistance to Phytophthora sojae, which causes Phytophthora root and stem rot of soybean (Glycine max, [L.] Merr), have been identified, but not all loci have been mapped and few have been cloned. Two plant introductions (PIs), PI 399079 and PI 408132, were identified as sources of Rps-gene mediated resistance through inoculations with numerous isolates individually and a combination of three isolates of P. sojae. Resistance was mapped in F₇ and F₉ recombinant inbred line (RIL) populations derived from crosses of the susceptible cultivar Williams and the two PIs. Resistance was controlled by two to three genes depending on the P. sojae isolate(s). Thus, quantitative trait locus mapping was used and identified four and three quantitative disease resistance loci (QDRL) in the PI 399079 and PI 408132 RIL populations, respectively. Each locus conferred resistance to different P. sojae isolates. Two QDRL were identified on chromosome 7, one of which is novel and the second may be an allele of Rps11 previously identified in PI 594527. These results indicate that there are new alleles to known Rps genes in these PIs.