Response of Ancestral Soybean Lines and Commercial Cultivars to Rhizoctonia Root and Hypocotyl Rot

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Abstract
Rhizoctonia root and hypocotyl rot is a common disease of soybean caused by Rhizoctonia solani. There are no commercial cultivars marketed as resistant to Rhizoctonia root and hypocotyl rot, and only a few sources of partial resistance to this disease have been reported. Ninety ancestral soybean lines, maturity groups (MGs) 000 to X, and 700 commercial cultivars, MGs II to IV, were evaluated for resistance to R. solani under greenhouse conditions. Most of the ancestral lines and cultivars evaluated were susceptible; however, 21 of the ancestral lines and 20 of the commercial cultivars were partially resistant. Of the 21 ancestral lines, CNS, Mandarin (Ottawa), and Jackson are in the pedigree of cultivars previously reported as being partially resistant to R. solani. In an additional study, dry root weights of 21 soybean cultivars were evaluated after inoculation with R. solani. Variation in dry root weight occurred among cultivars, but there was not a significant (P = 0.05) correlation between dr... Rhizoctonia root and hypocotyl root, caused by Rhizoctonia solani, is a common and widespread soilborne disease of soybean. Only a few sources of resistance to R. solani have been reported, and there is little information on the level of resistance present in commercial soybean cultivars. The objective of this study was to evaluate ancestral and commercial soybean cultivars for resistance to R. solani, for possible sources of resistance for soybean breeding programs. Most of the cultivars were susceptible; however, 21 of 90 ancestral lines and 19 of 700 commercial cultivars were determined to be partially resistant. In an additional study, the dry root weight of inoculated and noninoculated plants from 21 soybean cultivars was determined. While there was considerable variation among cultivars, there was not a significant correlation between dry root weight and disease severity. This indicates that low levels of disease on a soybean plant with a small root system may cause yield reduction, while moderate l...