Quantitative Disease Resistance Loci towards Phytophthora sojae and Three Species of Pythium in Six Soybean Nested Association Mapping Populations

Comparison of quantitative disease resistance loci (QDRL) towards the diverse array of soilborne pathogens that affect soybean [Glycine max (L.) Merr.] is key to the incorporation of resistance in breeding programs. The water molds Phytophthora sojae (Kauffman & Gerdmann), Pythium irregulare (Buisman), Pythium ultimum var. ultimum (Trow), and Pythium ultimum var. sporangiiferum (Drechsler) contribute to soybean yield losses annually. Six Soybean Nested Association Mapping (SoyNAM) populations were evaluated for resistance to one or more of these pathogens. Four were screened with a tray test to measure lesion length after inoculation with Ph. sojae; cup assays were used to screen three, three, and two populations for resistance towards Py. irregulare, Py. ultimum var. ultimum, and Py. ultimum var. sporangiiferum, respectively. There were two to eight major or minor QDRL identified within each SoyNAM population towards one or more of these water molds for a total of 33 QDRL. The SoyNAM populations evaluated for resistance to two or more water molds had different QDRL towards each pathogen, indicating that within a source of resistance, mechanisms are potentially specific to the pathogen. Only 3 of the 33 QDRL were associated with resistance to more than one pathogen. There was a major QDRL on chromosome 3 associated with resistance to Py. ultimum var. ultimum and Py. ultimum var. sporangiiferum, and QDRL on chromosomes 13 and 17 shared a flanking marker for both Py. irregulare and Py. ultimum var. ultimum. The SoyNAM population can serve as a diverse resource to map QDRL and compare mechanisms across pathogens and isolates.