Population structure and genetic diversity of <i>Phakopsora pachyrhizi</i> in the Southeastern United States

Phakopsora pachyrhizi, the causal agent of soybean rust disease (SBR) on Glycine max (soybean), is considered one of the most globally devastating diseases of soybeans and is a particular problem in Brazil, China, Sub-Saharan Africa, and the southern United States. To better understand genetic diversity and epidemiological history of SBR in the United States, 49 P. pachyrhizi isolates collected from soybean fields in four Southeastern states (Alabama, Florida, Georgia, and Louisiana) from the 2008 to 2017 growing seasons were genotyped through restriction site-associated genotype by sequencing (GBS). Rarefaction analysis identified 54 informative SNPs among the P. pachyrhizi isolates. We found no evidence suggesting sexual or parasexual recombination, and measurements of genetic diversity were low to moderately low. Multiple different statistical approaches, including neighbor-joining trees, K-means hierarchical clustering, discriminant analysis of principal components, and principal coordinates analysis (PCoA) all identified two groups of P. pachyrhizi genotypes that associated with geographic location. One group was composed of isolates from south Georgia, and the other with isolates from Alabama, Florida, Georgia (excluding south Georgia), and Louisiana. Our results suggest that two genetically related but distinct genotypes were introduced to the continental United States in a two-phase introduction and overwinter in South Georgia and Florida. The first introduction of one genotype likely occurred in South Georgia in 2004 followed by a later introduction of a second genotype. One genotype remained in South Georgia while the other genotype became established through the Southeastern United States. Future studies are necessary to determine whether SBR in Brazil, China, or Sub-Saharan Africa shows similar patterns of genotype distribution and history or if the United States situation is unique.