Quantitative trait loci for partial resistance to Phytophthora sojae in soybean

Partial resistance to Phytophthora sojae Kauffmann and Gerdemann in soybean [Glycine max (L.) Merr.] is expressed as a reduced level of root rot and is effective against all populations of the pathogen. The objective of this study was to identify simple sequence repeat (SSR) markers associated with putative quantitative trait loci (QTLs) for partial resistance to P. sojae in the soybean 'Conrad'. Three recombinant inbred soybean populations, Conrad X 'Sloan', Conrad X 'Harosoy', and Conrad X 'Williams', were evaluated for root lesion growth rate in growth chamber experiments following inoculation with P. sojae and with SSR markers to identify putative QTLs. The three populations segregated for root lesion growth rate after root inoculations. Two putative QTLs donated by Conrad were identified in all three populations and were positioned on soybean molecular linkage groups (MLGs) F and D1b+W. The QTL on MLG F explained 32.4, 35.0, and 21.4% of the genotypic variation for Conrad X Sloan, Conrad X Harosoy, and Conrad X Williams populations, respectively. The QTL on MLG D1b+W explained 10.6, 15.9, and 20.7% of the genotypic variation for the same three populations, respectively. The QTL on MLG F appears to be of more value based on the percentage of genotypic variation explained. Because the results indicate that QTLs for partial resistance to P. sojae map to different regions in soybean compared with the known Rps genes poses a challenge to soybean breeders. Marker-assisted selection may expedite the process of combining both Rps genes with partial resistance into high-yielding cultivars.