Comparison of Quantitative Trait Loci in Two Soybean Germplasm Conferring Partial Resistance to P. Sojae Using Two Different P. Sojae Isolates.

Root rot caused by Phytophthora sojae is one of the most destructive diseases of soybean in northern United States. Partial resistance provides an effective defense towards a broad spectrum of P. sojae races and is known as a more durable resistance than single gene-mediated resistance. It has been reported that many small-effect genes are involved in expressing partial resistance in the previous studies. The objectives of this study were i) to identify quantitative trait loci (QTL) for partial resistance to two P. sojae isolates in two different recombinant inbred populations sharing one common susceptible parent, and ii) to compare QTL between populations and isolates to elucidate the complex network or interaction of QTL dependent on germplasm or isolates . Two F_7:8 recombinant inbred populations derived from a cross of OX20-8 (susceptible parent) × PI 427106 and PI 427105B (resistant parents) were used. The genetic map of each population was constructed with genotyping of over 200 single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) markers. Partial resistance was evaluated on root rot score and disease-affected phenotypes - plant weight, fresh root weight, and dry root weight- via a layer test with P. sojae isolates 1.S.1.1 and race 30. The layer test was set up in a Styrofoam cup which was filled with fine vermiculite and a 14-day-old P. sojae culture grown on a lima bean agar placed in the middle of vermiculite. Fifteen seeds per line were planted in a cup and a root rot score was taken and related phenotypes were measured two weeks after planting. The phenotypic and genotypic data were used for QTL identification.