Reaction of soybean cultivars to sclerotinia stem rot in field, greenhouse, and laboratory evaluations

Sclerotinia stem rot of soybean [Glycine max (L.) Merr.], caused by the fungal pathogen Sclerotinia sclerotiorum (Lib.) de Bary, recently has increased in importance in the northern U.S. soybean production area. The objective of our study was to determine the effectiveness of three different inoculation techniques in predicting the field reactions of cultivars to sclerotinia stem rot. Eighteen soybean cultivars were field tested in six Michigan environments from 1994 to 1996 and tested in the greenhouse or laboratory with three inoculation methods. The cultivars were inoculated by placing infested oat (Avena sativa L.) seed or mycelial plugs on cotyledons or by placing mycelial plugs on detached leaves. There were significant (P < 0.05) differences in resistance to sclerotinia stem rot among cultivars at all but one field environment and for all inoculation methods. The disease severity ratings based on the inoculations were significantly correlated with the field results, with the exception of one method. Disease severity ratings for the three inoculation methods were significantly correlated with only two exceptions. Cultivars such as Novartis S19-90 and Corsoy 79 consistently had the lowest disease severity ratings in the field tests and for the inoculation methods. Similarly, a number of cultivars were rated as susceptible in all tests. Ratings for cultivars with intermediate reactions were not consistent across tests. The inoculation methods tested can provide some useful information on the resistance of soybean genotypes to sclerotinia stem rot. However, resistance identified by inoculation methods should be confirmed with field tests, since these methods can misclassify the resistance of some cultivars.