Identification of Quantitative Trait Loci Conditioning Partial Resistance to Phytophthora sojae in Soybean PI 407861A

Improving resistance for Phytophthora root and stem rot is an important goal in soybean [Glycine max (L.) Merr.] breeding. The objective of this study was to identify quantitative trait loci (QTLs) conferring partial resistance to Phytophthora sojae in an OX20-8 x PI 407861A cross, in which PI 407861A was used as the source of partial resistance. One hundred fifty-seven F-7-derived recombinant inbred lines (RILs) were evaluated for partial resistance to P. sojae isolate OH25 using a tray test. Composite interval mapping identified three QTLs on chromosomes 8, 13, and 15 by a genomewide logarithm of odds (LOD) threshold and six QTLs on chromosomes 3, 4, 10, and 18 by chromosomewide LOD threshold. The phenotypic variance explained by each QTL ranged from 2.4 to 8.6%, with a total of 41.6%. Quantitative trait loci on chromosomes 3 and 8 are novel QTLs first reported for partial resistance to P. sojae in this study. All nine QTLs were localized near known resistance gene rich regions or those previously reported for resistance to other soil-borne pathogens. Genomewide distribution of the QTLs in this population was distinct from the QTLs identified from previous studies with 'Conrad' but many were in common with those identified with another South Korean soybean accession. This indicates that the underlying genes for partial resistance in these exotic accessions may be distinct from genes controlling variation for partial resistance observed in Conrad. Plant Introduction 407861A is a promising genetic source of alleles that can be used to increase the genetic diversity and enhance levels of partial resistance to P. sojae in North American soybean cultivars.