Characterization of Major QTL Associated with Partial Resistance to Phytophthora Stem and Root Rot in Soybean.

Phytophthora stem and root rot, caused by the soilborne oomycete Phytophthora sojae (Kaufmann and Gerdemann), is a serious disease of soybean (Glycine max (L.) Merr.).  Host resistance is a common disease management practice and race-specific, dominant resistance genes (Rps genes) have been incorporated into numerous commercial varieties.  However, the widespread deployment of Rps genes has led to a shift in physiological races of P. sojae.  Partial resistance is quantitatively inherited and effective against a wider range of races of P. sojae, resulting in a reduced level of root rot.  Due to its broad spectrum nature and the development of low levels of disease, partial resistance places less selection pressure on P. sojae populations and is theoretically more robust than Rps gene mediated resistance.  Recently a major quantitative trait loci (QTL) was identified on chromosome 18 (8-16 cM), explaining 10-45% of phenotypic variance.  A QTL of large effect is an uncommon occurrence in P. sojae – soybean interactions and it remains unknown if it represents a unique resistance mechanism.   Near isogenic lines (NILs) were developed from three recombinant inbred lines (RILs), derived from crosses between either OX20-8  and PI 427105B  or OX20-8 and PI 427106, identified as segregating for the QTL of interest.  A total of 51 homozygous lines were selected from the three RILs based on flanking SSR marker genotypes.  These 51 lines were phenotyped for partial resistance using two different disease assays, the tray and layer test, in addition to field evaluations in Defiance, OH.  A significant difference in root rot levels between NILs with the susceptible allele and NILs with the resistant allele was found only in the layer test, possibly indicating the mechanism in which this specific QTL is operating under.  This major QTL represents a novel source of resistance to Phytophthora stem and root rot and the development of NILs will enable additional phenotypic and genetic characterization of this QTL.