Phenotypic reactions of 53 soybean genotypes to infection with each of 16 isolates of Phakopsora pachyrhizi

Soybean rust, caused by the fungal pathogen Phakopsora pachyrhizi, is an economically significant disease for soybean (Glycine max (L.) Merr.) production throughout the world. Genes for resistance to P. pachyrhizi (Rpp) have been identified at multiple loci in the soybean genome, but each conditions resistance to specific P. pachyrhizi isolates rather than providing broad resistance to the species. Identifying soybean cultivars with previously unidentified Rpp genes, combined with efforts to breed soybean containing multiple Rpp genes, is a key part of the ongoing strategy to manage the disease and minimize its economic impact. Fifty-three soybean genotypes, possessing known and unknown Rpp genes, were challenged with 16 P. pachyrhizi isolates from a global collection. The observed phenotypic reaction patterns provided insight into the presence of Rpp genes. This study tentatively confirms the presence of Rpp1b, or an allele, in eight soybean lines, of previously unknown resistance gene, and in another three lines produced by targeted breeding. Examples of resistance gene pyramiding were identified in lines 4014-242-341 and 4018-4-1-1-212, which may contain both Rpp2 and Rpp4 genes and line RN6-313-362, which may contain both Rpp1b and Rpp4. Three possible sources of new resistance genes may exist within GC00138-29, PI 567145 C, and PI 605833. This study provides preliminary information on potential new sources of soybean-rust-resistance that may be used to focus specific future research and inform breeding programs.