Abstract
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.
Highlights
The Soybean Nested Association Mapping (SoyNAM) Project developed and made available to the soybean research community a large nested association mapping (NAM) population composed of 40 recombinant inbred line (RIL) populations derived from crosses between one common, high-yielding hub parent (IA3023) and exotic germplasm, plant introductions, and high-yielding breeding lines with the goal of identifying quantitative trait loci (QTL) associated with yield and other desirable traits (Grant et al, 2009; Diers, 2014; Xavier et al, 2017a, 2017b, 2018; Diers et al, 2018; www.soybase.org/ SoyNAM)
These six NAM populations all responded with a wide range of resistant to highly susceptible phenotypic responses after inoculation to one or more of the water molds in a tray test towards P. sojae, and cup assays for Py. irregulare, Py. ultimum var. ultimum, and Py. ultimum var. sporangiiferum (Tables 2–5)
Co-localization of quantitative disease resistance loci (QDRL) and similar mechanisms for multiple disease resistance would be useful in developing cultivars with broad-spectrum resistance, especially if these were incorporated into the germplasm base
Summary
Two QDRL conferring resistance to Py. irregulare were identified on chromosomes 14 and 19-2 in an RIL population derived from a cross of two moderately susceptible cultivars, Conrad and Sloan (Stasko et al, 2016). Phytophthora sojae populations have continued to adapt to the Rps genes that have been deployed in cultivars across the north-central region (Nelson et al, 2008; Dorrance et al, 2016), and there is a greater focus on using partial resistance, which is conferred by QDRL (Dorrance et al, 2003, 2009; Wang et al, 2010, 2012a, 2012b; Stasko et al, 2016).
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