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

Sclerotinia stem rot, caused by the fungal pathogen Sclerotinia sclerotiorum (Lib,) de Bary, has increased in importance for soybean [Glycine max (L,) Merr.] in recent years. The objectives of this research were to evaluate soybean genotypes for resistance to sclerotinia stem rot over several field locations and compare these disease ratings with Seed yields, other agronomic traits, and parentage. Eighteen soybean genotypes were evaluated for resistance to sclerotinia stem rot and agronomic traits in six Michigan environments over 3 yr, The average disease severity index (DSI) of the 18 genotypes at a single environment ranged from 2.1 to 49.9. Mean DSI values over locations differed (P < 0.05) among the genotypes. The genotypes 'NKS19-90','Asgrow A2506','Colfax', and 'Corsoy 79' had the greatest resistance to sclerotinia stem rot. The DSI ratings of the 18 genotypes were significantly correlated between environments for only five of 15 pairs of environments. Over the six environments, a greater DSI of genotypes was significantly correlated with reduced seed yield and more plant lodging. Disease severity index was not significantly correlated with plant height at maturity, date of flowering, or date of maturity over the environments. Genotypes with parentage tracing to Williams' or 'Asgrow A3127' had a greater average DSI than genotypes with no parentage tracing to these cultivars, This suggests that the use of these cultivars as parents mag have caused greater disease susceptibility in contemporary cultivars.