AimsThe primary objective of these experiments was to reduce pathogenicity and virulence of endemic soil pathogenic Streptomyces strains that cause potato common scab (CS) using nonpathogenic Streptomyces strains to suppress CS in a field situation. Methods and ResultsNonpathogenic Streptomyces strains that had shown potential for mitigating CS in greenhouse assays were used in Michigan and Pennsylvania fields known to have high CS disease pressure. Five biocontrol (BC) strains and three potato cultivars were used in 2009, and three BC strains and three cultivars were used in 2010 in each location. The effects of BC strains on CS disease incidence and severity differed between locations, years and potato cultivars. When overall means of individual BC treatments were compared with nontreated controls, CS incidence and severity were decreased by all BC strains in PA2009, PA2010 and MI2010, particularly in cultivar Yukon Gold' in MI. Biocontrol treatments also significantly shifted the proportions of superficial, raised and pitted lesion types in some cultivar/biocontrol treatment combinations. ConclusionsAll BC strains significantly reduced CS incidence and severity on Yukon Gold' in three of four trials, and one BC strain significantly improved the lesion severity profile in cultivar Atlantic'. No BC strain significantly reduced CS incidence and severity on all potato cultivars in the different years and locations. Significance and Impact of the StudySeveral nonpathogenic Streptomyces strains showed potential to reduce CS incidence and severity on two important potato-chipping cultivars in the field. These results can be further applied to reduce CS disease severity in potatoes.