Evaluation of soybean cultivars for resistance to Sclerotinia stem rot in field environments

Sclerotinia stem rot, caused by the fungal pathogen Sclerotinia sclerotioru m (Lib,) de Bary, has increased in importance for soybean [Glycine max (L,) Merr.] in recent years. The objectives of this research were to evaluate s oybean genotypes for resistance to sclerotinia stem rot over several field locations and compare these disease ratings with Seed yields, other agronom ic traits, and parentage. Eighteen soybean genotypes were evaluated for res istance to sclerotinia stem rot and agronomic traits in six Michigan enviro nments over 3 yr, The average disease severity index (DSI) of the 18 genoty pes at a single environment ranged from 2.1 to 49.9. Mean DSI values over l ocations differed (P < 0.05) among the genotypes. The genotypes 'NKS19-90', 'Asgrow A2506','Colfax', and 'Corsoy 79' had the greatest resistance to scl erotinia stem rot. The DSI ratings of the 18 genotypes were significantly c orrelated between environments for only five of 15 pairs of environments. O ver the six environments, a greater DSI of genotypes was significantly corr elated with reduced seed yield and more plant lodging. Disease severity ind ex was not significantly correlated with plant height at maturity, date of flowering, or date of maturity over the environments. Genotypes with parent age tracing to Williams' or 'Asgrow A3127' had a greater average DSI than g enotypes with no parentage tracing to these cultivars, This suggests that t he use of these cultivars as parents mag have caused greater disease suscep tibility in contemporary cultivars.