Novel Sources of Partial Resistance against Phytophthora sojae in Soybean PI 399036

With the rapid emergence of novel pathogen pathotypes, durable sources of resistance against Phytophthora sojae in soybean [Glycine max (L.) Merrill] are needed to manage Phytopthora stem and root rot (PRR). This study used an inoculum source composed of multiple isolates of the pathogen to characterize partial resistance (PR) to P. sojae in plant introduction (PI) 399036. Two recombinant inbred line (RIL) F-5:7 populations developed by crossing PI 399036 (high PR), with two germplasm lines (low PR), AR2 (AX20925) and AR3 (AX20931), were screened. Corrected dry weights (CDW-root and shoot) and percentage root rot data were collected. Inclusive composite interval mapping identified one putative quantitative trait locus (QTL) on chromosome 2 by all disease assessment parameters in the AX20925 population that explained 6 to 14% of the total phenotypic variation and five putative minor QTL on 5 chromosomes (3, 6, 12, 15, and 19) that individually explained 5 to 9% of the phenotypic variation. In the AX20931 population, two QTL, mapped to chromosome 9 and 17, were commonly identified by CDW while the QTL identified on chromosome 9 accounted for 21 to 30% of the total phenotypic variation. Additional QTL identified by CDW on chromosomes 9, 13, 14, and 15 individually accounted for 5 to 8% of the phenotypic variation. PI 399036 contributed the favorable alleles for the majority of the identified QTL. This study demonstrates the usefulness of PI 399036 as a novel source for breeding for resistance to PRR and that an inoculum mixture of P. sojae isolates with varying pathotypes can be used to identify putative QTL for PR to P. sojae using the rice (Oryza sativa) method.