Mapping Quantitative Trait Loci for Partial Resistance to Phytophthora sojae in a Soybean Interspecific Cross

Phytophthora root and stem rot of soybean, caused by Phytophthora sojae Kaufmann & Gerdemann, is a serious limitation to soybean [Glycine max (L.) Merr.] production in the USA. Partial resistance or field resistance to P sojae in soybean is effective against multiple races of the pathogen and is a form of incomplete resistance. An interspecific recombinant inbred line (RIL) population consisting of 298 individuals derived from the cross of V71-370 by PI407162 was inoculated with the P. sojae isolate C2S1 using the slant board technique in three separate experiments (designated as 2005, 2006a, and 2006b). In each replication, seven day old seedlings from each RIL were inoculated and lesion lengths were recorded 7 d later to assess partial resistance with three replications for the 2005 and 2006a experiments and one replication for the 2006b screening. Interval mapping located a lesion length QTL on each of the molecular linkage groups (MLGs)-J (chrom. 16),-I (chrom. 20) and-G (chrom. 18) in all three experiments. The lesion length QTL on MLG-J accounted for 32, 42, and 22% of the phenotypic variation in the 2005, 2006a and 2006b experiments, respectively. Mapped QTL locations in the current study provide breeders with new sources of P. sojae resistance and suggest that new sources may be identified in soybean germplasm.