Comparison of Pathogenic Variation among Phakopsora pachyrhizi Isolates Collected from the United States and International Locations, and Identification of Soybean Genotypes Resistant to the U.S. Isolates

Abstract

A major constraint in breeding for resistance to soybean rust has been the virulence diversity in Phakopsora pachyrhizi populations. In greenhouse experiments, reactions of 18 soybean genotypes to 24 U.S. isolates from 2007 and 2008 and 4 foreign isolates were compared. Reactions of four differentials (Rpp1 to Rpp4) to these U.S. isolates were also compared with reactions to nine foreign isolates and three U.S. isolates from 2004. Principal component analysis (PCA) of the reaction types grouped the U.S. isolates into a single virulence group, whereas each of the foreign isolates had a unique virulence pattern. In another experiment, reactions of 11 differentials to the 24 U.S. isolates were compared and significant interactions (P < 0.001) were found between the isolates and host genotypes for rust severity and uredinia densities. PCA of these two measures of disease placed the 24 isolates into seven or six aggressiveness groups, respectively. In a third experiment, evaluation of 20 soybean genotypes for resistance to the previously established aggressive groups identified 10 genotypes resistant to isolates representing most of the groups. This study confirmed the pathogenic diversity in P. pachyrhizi populations and identified soybean germplasm with resistance to representative U.S. isolates that can be used in breeding.

Pathogenic Variation of South American <i>Phakopsora pachyrhizi</i> Populations Isolated from Soybeans from 2010 to 2015

“…The US isolates expressing the same pathogenicity profile as US isolates collected in 2004 when soybean rust was first found in the US (Schneider et al 2005), have not been detected in subsequent US samples (Paul et al 2015, Paul & Hartman 2009, Pham et al 2009). Therefore, it has been suggested that P. pachyrhizi isolates existing in 2004 disappeared or were suppressed due to the subsequent winter, likely resulting in unfavorable conditions for pathogen survival a resistant reaction in response to 5%-53% of the rust populations.…”

“…To our knowledge, this paper, coupled with Akamatsu et al (2013), describes the pathogenicity of the largest P. pachyrhizi collection, consisting of populations from eight cropping seasons from 2007 to 2015 in three South American countries. (Paul et al 2015, Paul & Hartman 2009). In this study, a highly virulent Argentinean population (i.e., ANE6-6) was discovered, and it could pose a serious threat since none of the Rpp-carrying differentials demonstrated a resistant reaction to the population.…”

“…The data revealed a large difference in the rust reactions of Rpp1-carrying PI 200492 and PI 587880A; PI 587880A possesses a different Rpp1 allele compared with PI 200492, Rpp1-b (Ray et al 2009, Yamanaka et al 2015a). The reactions of PI 200492 and PI 587880A to most South American rust populations were evaluated as being susceptible and resistant, respectively, whereas those to the US isolates were evaluated as being resistant and susceptible, respectively (Paul et al 2015, Twizeyimana & Hartman 2012. In contrast, the P. pachyrhizi isolates collected in Alabama in 2004 induced a susceptible reaction in Rpp1-carrying PI 200492.…”

Effectiveness of Resistance Genes to the Soybean Rust Pathogen <i>Phakopsora pachyrhizi</i>

“…PI 200492 was immune or resistant by inoculation to isolates collected in India in 1973, Australia in 1979, and Hawaii in 1994and 1989. However, this variety produced TAN reactions to isolates collected in Taiwan in 1972and 1980, and in Thailand in 2001 (Bromfield 1984, Bonde et al 2006, Pham et al 2009, Ray et al 2009, Paul et al 2015. In Japan, although the majority of soybean and kudzu (Pueraria lobata) isolates collected from 1993 to 1997 induced susceptible reactions on PI 200492 (Yamaoka et al 2002), isolates collected in 2007 to 2009 induced resistance reactions (Yamaoka et al 2014).…”

“…In Africa, PI 200492 produced TAN reactions by inoculation with isolates collected in Zimbabwe and South Africa in 2001 (Bonde et al 2006, Pham et al 2009, Ray et al 2009, Paul et al 2015. In Uganda, low disease severity was observed in field trials conducted in (Oolka et al 2008.…”