Exploring Tolerance to Root Infection in a Retrospective Soybean Breeding Population.

Root infection can manifest a spectrum of agronomic maladies, from seedling death to nutrient deficiencies and yield loss.  While many disease resistance loci have been identified for soybean (Glycine max) pathogens, these loci may be overcome quickly by evolving virulence within pathogen populations.  Therefore, we aimed to identify loci responsible for intermediate levels of resistance and/or tolerance to root infection, traits that may contribute to durable resistance and increased agronomic performance.  To do so, we evaluated a set of 279 soybean lines that represent the historical breeding germplasm for the University of Minnesota’s early maturity (MG I, 0, and 00) soybean breeding program.  These lines have been genotyped with the Illumina BARCSoy 6K SNP platform and include plant introductions, advanced and elite breeding lines, and historical and recent cultivars.  To assess tolerance to root infection, the lines were phenotyped in greenhouse screens for partial resistance to virulent Phytophthera sojae isolates from Minnesota, and in greenhouse screens for tolerance to soybean cyst nematode (SCN, Heterodera glycines) race 4. In addition to standard root rot disease ratings for P. sojae, the screenings included dry weight measures of paired infected and uninfected plant roots.  Similarly, the SCN experiments measured plant and nodule weights of paired infected and uninfected plant roots, and SCN egg density.  These traits were used as indicators of tolerance to infection, and marker-trait associations were identified using an association mapping approach.