Reactions of 52 soybean germplasm accessions with Rpp3 alleles to a panel of 13 Phakopsora pachyrhizi (soybean rust) isolates from the southern United States

Soybean rust (SBR) caused by Phakopsora pachyrhizi Syd. is one of the most economically important foliar diseases of soybean [Glycine max (L.) Merr.]. At least 52 soybean plant introductions (PIs) with resistance to SBR have a resistance gene at the Rpp3 locus on chromosome 6, but little is known about the degree of similarity between those alleles and the Rpp3 alleles from PI 462312 and ‘Hyuuga’, which has resistance genes at the Rpp3 and Rpp5 loci. Reactions of 52 PIs to a panel of 13 P. pachyrhizi isolates from the United States were assessed to identify accessions with reaction patterns that differed from those of PI 462312 and Hyuuga. Thirty-two PIs were similar to PI 462312 and eight more similar to Hyuuga. Twelve PIs had reactions to 2011 and 2012 isolates from Florida that differed from both PI 462312 and Hyuuga. Although the majority of the 52 accessions appear to have the same Rpp3 allele as PI 462312, at least some of the 12 PIs with different reaction patterns are likely to have unique alleles. This information will be useful in breeding rust-resistant soybean cultivars, and potentially for identifying regions of the Rpp3 locus that are involved in P. pachyrhizi pathotype recognition. None of the P. pachyrhizi isolates collected after 2012 were as virulent as 2011 and 2012 isolates from north-central Florida, demonstrating that virulence shifts in P. pachyrhizi populations are not necessarily permanent.