Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/21/2007
Publication Date: 1/1/2008
Citation: Miles, M.R., Morel, W., Ray, J.D., Smith, J.R., Frederick, R.D., Hartman, G.L. 2008. Adult Plant Evaluation of Soybean Accessions for Resistance to Phakopsora pachyrhizi in the Field and Greenhouse in Paraguay. Plant Disease. 92(1):96-105. Interpretive Summary: Soybean rust causes significant yield losses in soybean in areas where it occurs regularly. Five hundred thirty-four soybean lines from maturity groups III through IX, that had been selected in greenhouse seedling screens were evaluated for rust resistance in a field trial at Centro Regional de Investigación Agrícola in Capitán Miranda, Paraguay during the 2005-06 growing season. Soybean rust severities ranged from 1.0 (resistant) to 9.0 (susceptible) in each of the maturity groups. Two lines, PI 587886 and PI 594754 were immune in both the field and greenhouse evaluations; no soybean rust was found on any of the trifoliolate leaves evaluated. In each maturity group there were accessions that had low ratings on both evaluation dates. These low ratings may be due to partial resistance traits that slow down the disease. This research is important to the soybean research community, as the lines identified may be sources of resistance to soybean rust that could be incorporated into commercial cultivars.
Technical Abstract: Soybean rust [Phakopsora pachyrhizi H. Sydow & Sydow] causes significant yield losses in soybean [Glycine max L.] in areas where it is endemic. Five hundred thirty-four soybean accessions from maturity groups (MG) III through IX, previously selected as potential sources of resistance to soybean rust in greenhouse seedling screens were evaluated for rust resistance in a replicated field trial at Centro Regional de Investigación Agrícola in Capitán Miranda, Itapúa, Paraguay during the 2005-06 growing season. The accessions were blocked by maturity and planted in 1 M plots with 1 M alleys with a pair of inoculated spreader rows of a susceptible cultivar interspersed every 10 rows. Within each MG, mean soybean rust severities tended to be normally distributed, with the distribution shifted towards higher severity in the second evaluation. The observation of the resistant red-brown (RB) lesions within individual accessions was inconsistent in the field as some replications had RB lesions while others had a mixed reaction of tan lesions and RB. However, in a subsequent greenhouse inoculation, ten accessions selected for resistance based on the field results had RB lesions with low sporulation levels. PI 587886 and PI 594754 were immune in both the field and greenhouse evaluations; no soybean rust uredinia were found on any trifoliolate leaf evaluated. In each maturity group there were accessions that had low or intermediate severity, ratings less than 4 or 5 on both evaluation dates. These accessions may be potential sources of partial resistance to soybean rust.