Genetic Resistance to Soybean Rust in PI567099A is at or Near the Rpp3 Locus

Our objective was to map the soybean rust (SBR) resistance genes(s) in PI 567099A. A population segregating for SBR resistance was evaluated in 2008 and 2009 in Paraguay. In both seasons, F2:3 families were grown in a field naturally infested with SBR. F2:3 families were rated as resistant, segregating, or susceptible on the basis of the lesion type present, and this classification was used to infer the F2-phenotype. Molecular markers flanking five SBR-resistance genes were applied to the F2 population, and markers flanking Rpp3 were significantly associated with the observed resistance (P < 0.0001) in both years. The phenotype of 24-F1 plants evaluated in the 2008 season indicated that the resistance was recessive. This is the first report of recessive resistance at or near the Rpp3 locus. Knowledge of the location and nature of resistance in PI 567099A will allow its more efficient utilization as an SBR-resistance source in breeding programs.