Soybean Germplasm Accession Seedling Reactions to Soybean Rust Isolates from Georgia

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.