Laboratory and Field Assessment of Resistance in Soybean to Stem Rot Caused by Sclerotinia sclerotiorum

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Abstract
To evaluate resistance to stem rot, commercial soybean cultivars were grown in a greenhouse without fertilizer for 5 wk, then cut apices of excised plants with leaves and stem apices removed were inoculated with mycelial disks from cultures of Sclerotinia slcerotiorum grown for 5 days on millet seed agar. The inoculated stems were incubated on moist vermiculite in trays at 20-22.degree. C. The trays were covered with aluminum foil for 3 days, then exposed to ambient light on a laboratory bench. Lesion lengths were measured after 7 days. Commercial soybean cultivars of maturity groups 0, I, II, and III differed in their responses to stem rot with the laboratory method, but results of different experiments often were not consistent. The same cultivars also were tested in an infested field. Disease incidence ranged from 2.2 to 40% in 1984 and from 0 to 52% in 1985. The disease reactions of 13 cultivars tested in both years were significantly correlated (r = 0.72) with each other. Seed yields of 16 cultivars in 1985 were inversely correlated with stem rot incidence (r = -0.94). For every 10% increase in disease from 0 to 52%, yield was reduced by 7.8% of the maximum yield of 3,024 kg/ha (45 bu/acre). Lesion lengths in commercial cultivars assessed by the laboratory method showed varying correlations with disease incidence in the field experiments (r = -0.17-0.86). Ten cultivars of field-grown plants subjected to the laboratory assay gave reactions significantly correlated (r = 0.68) with field reactions of those cultivars.