Response of ancestral soybean lines and commercial cultivars to Rhizoctonia root and hypocotyl rot

Rhizoctonia root and hypocotyl rot is a common disease of soybean caused by Rhizoctonia solani. There are no commercial cultivars marketed as resistan t to Rhizoctonia root and hypocotyl rot, and only a few sources of partial resistance to this disease have been reported. Ninety ancestral soybean lin es, 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; how ever, 21 of the ancestral lines and 20 of the commercial cultivars were par tially resistant. Of the 21 ancestral lines, CNS, Mandarin (Ottawa), and Ja ckson are in the pedigree of cultivars previously reported as being partial ly resistant to R. solani. In an additional study, dry root weights of 21 s oybean cultivars were evaluated after inoculation with R. solani. Variation in dry root weight occurred among cultivars, but there was not a significa nt (P = 0.05) correlation between dry root weight and disease severity.