Reaction of Soybean Cultivars to Sclerotinia Stem Rot in Field, Greenhouse, and Laboratory Evaluations

Abstract

programs. Soybean cultivars have been evaluated for resistance to sclerotinia stem rot under field conditions Sclerotinia stem rot of soybean [Glycine max (L.) Merr.], caused and some with partial resistance to the disease have by the fungal pathogen Sclerotinia sclerotiorum (Lib.) de Bary, recently has increased in importance in the northern U.S. soybean pro- been identified (Grau et al., 1982; Boland and Hall, duction area. The objective of our study was to determine the effec- 1987; Nelson et al., 1991; Kim et al., 1999). Although tiveness of three different inoculation techniques in predicting the researchers have been successful in identifying partial field reactions of cultivars to sclerotinia stem rot. Eighteen soybean resistance using field evaluations, these evaluations are cultivars were field tested in six Michigan environments from 1994 to difficult because of the need for a cool, wet environment 1996 and tested in the greenhouse or laboratory with three inoculation for disease development and the high spatial variability methods. The cultivars were inoculated by placing infested oat (Avena of disease foci across fields. For these reasons, researchsativa L.) seed or mycelial plugs on cotyledons or by placing mycelial ers would benefit from having a controlled-environment plugs on detached leaves. There were significant (P , 0.05) differences screening method that accurately predicts the reaction in resistance to sclerotinia stem rot among cultivars at all but one of soybean germplasm in field environments. field environment and for all inoculation methods. The disease severity ratings based on the inoculations were significantly correlated with Both physiological resistance and escape mechanisms the field results, with the exception of one method. Disease severity contribute to differences in the reaction of cultivars to ratings for the three inoculation methods were significantly correlated sclerotinia stem rot in field trials. Escape mechanisms with only two exceptions. Cultivars such as Novartis S19-90 and Corsoy include early flowering and maturity, less lodging, and 79 consistently had the lowest disease severity ratings in the field tests an upright, open canopy. One or more of these mechaand for the inoculation methods. Similarly, a number of cultivars were nisms have been shown to be significantly associated rated as susceptible in all tests. Ratings for cultivars with intermediate with reduced levels of sclerotinia stem rot in several reactions were not consistent across tests. The inoculation methods studies (Boland and Hall, 1987; Nelson et al., 1991; Kim tested can provide some useful information on the resistance of soy