IDENTIFICATION OF RESISTANT GERMPLASM, MARKER-TRAIT ASSOCIATIONS, AND QUANTITATIVE TRAIT LOCI UNDERLYING SOYBEAN RESISTANCE TO FUSARIUM GRAMINEARUM AND PYTHIUM IRREGULARE

This study aims to expand the resources available for breeding soybean (Glycine max (L.) Merr.) cultivars with improved resistance to two of the most prevalent and virulent seedling pathogens, Fusarium graminearum Schwabe (teleomorph Gibberella zeae (Schwein.) Petch) and Pythium irregulare Buisman (syn. Globisporangium irregulare Uzuhashi, Tojo & Kakish.). To address the need for resistant germplasm, we screened a diverse panel of 208 soybean accessions from the USDA Germplasm Resources Information Network’s core collection for resistance to both pathogens. Disease resistance was quantified by comparing the root weights of pathogen-inoculated plants against mock-inoculated controls. Through a linear mixed model analysis, we identified eight accessions with significantly higher resistance to F. graminearum and ten accessions with enhanced resistance to P. irregulare. To uncover the genetic basis of the observed resistance, a genome-wide association study (GWAS) was conducted. This analysis identified five significant marker-trait associations (MTAs) for F. graminearum resistance located on chromosomes Gm02, Gm03, Gm06, Gm07, and Gm13, each explaining 3.0–4.8% of the phenotypic variance. Similarly, two significant MTAs associated with resistance to P. irregulare were mapped to chromosomes Gm10 and Gm15, accounting for 9.3% and 17.2% of the phenotypic variance, respectively. Building on these findings, mapping populations derived from selected resistant accessions were developed for quantitative trait loci (QTL) mapping. A population of 271 F_2:3 plants from a bi-parental cross of resistant line PI 438500 x susceptible line PI 548631 was evaluated for resistance to F. graminearum. This mapping effort identified a novel QTL for resistance located on chromosome 16. Additionally, a population of 244 F_2:3 plants from a bi-parental cross of resistant line PI 548360 x susceptible line PI 548631 was evaluated for resistance to P. irregulare. A separate QTL for P. irregulare resistance was located on chromosome 15. This study documents novel sources of resistant germplasm, genetic markers, and QTLs for use in breeding programs to improve soybean resistance to seedling diseases.