Soybean Germplasm Resistant to Pythium irregulare and Molecular Mapping of Resistance Quantitative Trait Loci derived from the Soybean Accession PI 424354

Pythium irregulare Buisman is an important seed and seedling pathogen of many hosts including soybean [Glycine max (L.) Merr.] and has been increasingly linked to reports of stand losses. The objectives of this research were to identify sources of resistance to P. irregulare, characterize the type of resistance identified, and genetically map the resistance. Sixty-five soybean genotypes were evaluated for resistance to two isolates of P. irregulare, and approximately one-third had moderate to high levels of resistance. The PI 424354 had the highest levels of resistance to P. irregulare, and resistance was mapped in two three-way cross populations: 'OHS 303' (moderately susceptible) x ('Williams' (moderately susceptible) x PI 424354) and 'Dennison' (moderately susceptible) x (Williams x PI 424354). Resistance quantitative trait loci (QTL) with alleles from PI 424354 were mapped on chromosomes 1 and 6 in OHS 303 x (Williams x PI 424354) using composite interval mapping (CIM) for root weights and root rot scores. Resistance QTL from PI 424354 were also mapped on chromosomes 8 and 11 with CIM for root weight data in the Dennison x (Williams x PI 424354) population and chromosome 13 using root rot scores. Each QTL explained 7.9 to 17.8% of the phenotypic variation. These are the first reported putative QTL for resistance to P. irregulare in soybean.