Evaluation of Soybean Germplasm Accessions for Resistance to Phakopsora pachyrhizi Populations in the Southeastern United States, 2009–2012

Abstract

Between 2009 and 2012, 118 soybean [Glycine max (L.) Merr.] accessions from the USDA Soybean Germplasm Collection were screened for resistance to soybean rust (Phakopsora pachyrhizi) at up to five locations in the southeastern United States. In 2009, plant introductions (PIs) from maturity groups III through IX were evaluated for relative disease severity and intensity of sporulation from uredinia compared with 12 susceptible cultivars from the same range of maturity groups. Resistance evaluations were based primarily on disease severity and intensity of sporulation from rust pustules. To assess resistance at several nurseries, a rust index score was calculated from the severity and sporulation ratings. Many of the PIs were moderately to highly resistant at the 2009 locations between Alabama and South Carolina, but the P. pachyrhizi population in Bossier City, LA, was virulent on most of those accessions. The 2011 rating data from Quincy, FL, indicated an increase in the virulence of the pathogen there since 2009, and this trend was observed again in 2012. In contrast, many of the same PIs developed substantially less soybean rust in Attapulgus, GA, in 2012. Despite the comparatively greater disease that many accessions had in Louisiana in 2009 and in Quincy in 2011 and 2012, at least 78 PIs were resistant in Georgia in 2012, and 20 of those were at least moderately resistant in both Florida and Georgia that year. No accessions were immune to rust at all of the nurseries, but PI 200492 (Rpp1), PI 547875 (a backcross line with Rpp1), and PI 567102B (Rpp6) were the most resistant of the accessions with named resistance genes and were among the most resistant accessions overall. Among the most resistant accessions with unknown resistance genes, PI 416826A, PI 417125, PI 567034, and PI 567104B consistently had effective levels of resistance in different locations and years. Information about the most resistant PIs and their reactions to soybean rust infection across years and locations will be useful for the development of rust‐resistant soybean cultivars in the United States.

Asian Soybean Rust Resistance: An Overview

“…The wide variation in the type of reaction, color and intensity of sporulation observed in the field can be a difficulty factor in genotype characterization studies [53]. Although the type of lesions are widely used in detached leaves and seedlings inoculations studies, the appearance of rust lesions in adult plants is often different from that originally reported by Bromfield & Hartwig [17].…”

“…Walker et al [53], studying the field reaction of reported resistance genes in the southern United States, noted that Rpp1 gene, present in PI 200492, and Rpp6 gene, in PI 567102B, still conferred high levels of resistance. The varieties carrying Rpp2, Rpp3, Rpp4 or Rpp5 showed incomplete resistance and moderate levels of rust development when compared to susceptible controls.…”

From Select Agent to an Established Pathogen: The Response to Phakopsora pachyrhizi (Soybean Rust) in North America

“…In addition to being an important initial source for urediniospores, kudzu could also be a source of selection pressure for the pathogen population. For example, inoculum prepared from infected kudzu leaves collected in northcentral Florida in 2012 caused unusually high levels of soybean rust on a number of historically resistant soybean germplasm accessions (Walker et al 2014a). …”

“…Multiple resistance gene alleles have been mapped to the Rpp1, Rpp3, and Rpp5 loci Hossain et al 2015), and some alleles that condition a high level of resistance to fungal populations and isolates from one continent are entirely ineffective against those from another (Walker et al 2014b). The Rpp6 gene is unusual in that it has provided a high level of resistance to numerous field populations and isolates from North and South America (Li et al 2012;Miles et al 2008;Walker et al 2011Walker et al , 2014a, though some populations or isolates have been able to overcome Rpp6-based resistance (Paul et al 2013;Walker et al 2014b). In addition, Japanese 'Hyuuga' soybean was found to have two resistance genes at the Rpp3 and Rpp5 loci (Kendrick et al 2011).…”

“…Soybean cultivars with resistance to P. pachyrhizi would be a valuable component in a sustainable soybean rust management program. The development of soybean cultivars with broad resistance to P. pachyrhizi has been complicated by extensive pathogen diversity (Table 2), and the ability of the fungus to evolve genetic diversity despite the apparent lack of a sexual stage (Vittal et al 2012b) contributes to temporal and geographic differences and shifts in the pathogenicity of P. pachyrhizi populations (Akamatsu et al 2013;Paul et al 2013;Walker et al 2014a).…”