Soybean Germplasm Accession Seedling Reactions to Soybean Rust Isolates from Georgia

Abstract

Soybean rust (SBR), caused by Phakopsora pachyrhizi Syd. and P. Syd., is a threat to soybean [Glycine max (L.) Merr.], production in regions of the world where winters are not cold enough to completely eliminate the many hosts of the fungus, so resistant soybean cultivars would be useful in managing this disease. Resistant germplasm accessions have been identified, and resistance (Rpp) genes have been mapped to six independent loci, but the high degree of virulence variability among P. pachyrhizi populations and isolates means that the level of resistance to different populations and isolates can vary considerably. Greenhouse assays were conducted from 2008 until 2013 in which seedlings of soybean plant introductions were challenged with bulk P. pachyrhizi isolates collected in the state of Georgia. Accessions were included in the tests either because they carried known Rpp genes or because they had appeared resistant to SBR in previous field assays. The resistance was assessed on the basis of infection type, visible urediniospore development, and sometimes lesion density. With a few exceptions, most of the accessions that appeared resistant as adult plants in the field also had resistance reactions as seedlings, and these reactions were generally similar or identical in the different years that the greenhouse assays were conducted. This study demonstrates that greenhouse seedling reactions should usually be reliable indicators of adult plant resistance to related P. pachyrhizi isolates or populations, and confirms the resistance of more than 100 soybean germplasm accessions to fungal isolates and field populations from Georgia.

Molecular Characterization of Resistance to Soybean Rust (Phakopsora pachyrhizi Syd. & Syd.) in Soybean Cultivar DT 2000 (PI 635999)

“…These results were in agreement with an earlier study at the FDWSRU, in which DT 2000 was challenged with ten different rust isolates [29]. In field assays conducted in the southeastern USA between 2006 and 2013, DT 2000 was highly resistant in some year-location environments, but less resistant in others [19,32]. The objectives of the present study were to identify and characterize genomic locations or gene(s) associated with resistance to SBR in advanced inbred lines derived from DT 2000 under field conditions in Ha Noi, Vietnam, and Quincy, Florida, USA.…”

“…Walker et al [19,32] evaluated the resistance of DT 2000 to field populations of P . pachyrhizi in the southern United States in 2007 (three locations), 2008 (five locations), 2009 (five locations), 2011 (one location) and 2012 (two locations).…”

“…While the original Rpp1 gene from PI 200492 has provided high levels of resistance to U.S. P . pachyrhizi populations in multiple years and locations, other resistance alleles like Rpp1-b that were effective against foreign isolates failed to protect plants from SBR in the southern United States [19]. In addition to Rpp resistance genes identified in cultivated soybean, resistance has also been found in the wild perennial species Glycine tomentella [20].…”

“…pachyrhizi isolates at the USDA-ARS Foreign Disease-Weed Science Research Unit (FDWSRU), at Fort Detrick, MD, and identified 805 accessions with low severity and/or RB lesions. Subsets of these accessions have subsequently been evaluated under field conditions in Nigeria, Paraguay, Vietnam and the United States [19,28–32]. In Vietnam, Pham et al [30] reported that cultivar DT 2000, a local resistant accession, consistently developed RB lesions and had a low area under disease progress curve (AUDPC).…”

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

“…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).…”