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Rhizoctonia crown and root rot of sugar beet ( Beta vulgaris L.), caused by Rhizoctonia solani , continues to be one of the important concerns for the beet industry in Minnesota and North Dakota. Use of resistant cultivars is an important strategy in the management of R. solani in combination with seed treatment and timely fungicide application during the growing season. The objective of this greenhouse study was to determine how sugar beet plants responded to increasing age in resistance to R. solani . Each of three seed companies provided three commercial cultivars with varying R. solani resistance levels: susceptible, moderately resistant, and resistant. Seed were planted at a weekly interval to create different plant age groups from seed to 10-week-old plants, with growing degree days (GDD) ranging from 0 to 1,519 thermal time (°Cd). Seed and plants were all simultaneously inoculated with R. solani AG2-2-infested barley grains. Twenty-eight days after inoculation, plants were pulled and washed, and roots were evaluated for disease severity. All cultivars were highly susceptible to R. solani when inoculated at seed to 3 weeks old (0 to 464°Cd). At 4 and 5 weeks of plant age (617 to 766°Cd), resistant cultivars started to show significant resistance to R. solani . Proportion of the affected roots with disease score ≥ 5 followed a sigmoid response, declining with increased GDD in moderately resistant and resistant cultivars, whereas it continued to decline linearly with increased GDD in susceptible cultivars. This study demonstrated that sugar beet cultivars, regardless of their assigned level of R. solani resistance, were highly susceptible to the pathogen before they reached the six- to eight-leaf stage at 4 to 5 weeks (617 to 766°Cd) after planting. Therefore, additional protection in the form of seed treatment or fungicide application may be required to protect sensitive sugar beet seed and seedlings in fields with a history of R. solani under favorable environmental conditions.