Resistance to Phakopsora pachyrhizi in Soybean PI 587905 Maps to the Rpp1 Locus and Exhibits Variable Dominance Associated with Plant Ontogeny

Soybean rust, caused by Phakopsora pachyrhizi Sydow, results in significant soybean (Glycine max (L.) Merr.) yield losses worldwide. The purpose of this research was to confirm the inheritance, gene action, and genomic location of resistance to P. pachyrhizi in soybean accession PI 587905. Two independent populations (F-2-derived F-3 lines and F-2 plants) were inoculated with P. pachyrhizi in a greenhouse and assayed in a field, respectively, in southeastern Paraguay. Both the field F-2 and the greenhouse F-2-inferred segregation ratios fit the 1:2:1 ratio expected for segregation of a single gene (P > 0.05). Unlike previous studies, gene action differed according to ontogeny. Among seedlings in the greenhouse, gene action was dominant, but in the field assay among adult plants, the gene exhibited incomplete dominance. Molecular markers were used to map the resistance gene in PI 587905 to the Rpp1 locus. This information should be useful to plant breeders for developing and selecting resistant cultivars.