Root rot of sugar beet (Beta vulgaris L.), caused by Rhizoctonia solani anastomosis group AG 2-2 IIIB is responsible for significant crop losses in North Dakota and Minnesota, USA. Understanding the association between plant age and inoculum density with disease severity of sugar beet cultivars is a prerequisite to properly screen for varietal resistance. Therefore, investigations were conducted to determine the responses of 4-, 6-, and 8-week-old plants in seven commercial sugar beet cultivars to inoculum densities of one, two, and three grains of R. solani-colonized barley in a greenhouse and with three corresponding levels of colonized barley, mycelial plugs, and sclerotia in field experiments. Under greenhouse conditions, disease severity was greatest before plants reached six weeks of age (P=0.05). There was a positive linear relationship between the density of the inoculum and disease severity. All seven cultivars were equally susceptible (P>0.05) to R. solani. Interactions between cultivars and plant age and between plant age and intensity of inoculum were not significant (P>0.05). Field experiments showed that the density of inoculums was significant (P<0.001), and the disease severity was highest in plants inoculated with three colonized barley seeds per plant compared to doses of other inoculum types.