Reactions of Soybean Germplasm Accessions to Six Isolates from the United States.

Soybean rust, caused by Syd. & P. Syd., is one of the most economically important foliar diseases of soybean. Resistant cultivars could reduce yield losses and management costs but considerable pathogenic diversity exists among populations of the fungus; thus, resistance to a range of pathotypes is essential. Seedling and detached-leaf assays were conducted to characterize the resistance of 55 soybean plant introductions (PIs) to six purified isolates of originating from the southern United States. In the greenhouse resistance assays, the differentials Hyuuga (PI 506764) and PI 471904 and accessions PI 224268, PI 567025A, PI 567039, PI 567046A, and DT 2000 (PI 635999) were resistant to all six isolates, including Florida isolates from 2011 and 2012 that were able to defeat resistance conditioned by the through genes. Twenty-six other PIs were resistant to four or five of the six isolates. In the detached-leaf assays, eight accessions developed reddish-brown reactions to all six isolates, with an average of only 0.23 to 0.55 uredinia/lesion. These included Hyuuga, DT 2000, two differentials with a resistance allele at the locus, and accessions PI 224268, PI 423960B, PI 567025A, and PI 567046A. Many of the resistant accessions have subsequently been reported to have a resistance allele at the locus, and two others have resistance genes at the or locus. This study provided new information about resistance reaction phenotypes that can be useful for understanding mechanisms of resistance, which genes and alleles could be combined to obtain broader and more durable rust resistance in soybean cultivars, and pathotype diversity among the six isolates used.