RESUMEN
Rhizoctonia crown and root rot caused by Rhizoctonia solani can cause serious economic losses in sugar beet fields. Preliminary evidence suggests that there could be interactions between different strains and resistance sources. Thus, field studies were conducted to determine whether nine R. solani AG-2-2 IIIB strains varied for virulence when compared with a noninoculated check and interacted with five sugar beet lines (four resistant lines and a susceptible check). The studies were arranged in a randomized complete block design with six replications. Roots were evaluated for surface rot and internal fungal and bacterial rot in September. All strains were virulent on the susceptible check, FC901/C817, and had a similar ranking (r = 0.80 to 0.97; P = 0.0096 to <0.0001) regardless of disease variable. Line FC709-2 was resistant (response not different from noninoculated check, P ≥ 0.1042) to all strains, while the strain responses resulted in weak interactions with less-resistant lines in 14 of 19 variable-year combinations. Because most commercial sugar beet cultivars contain low to intermediate resistance to Rhizoctonia crown and root rot, the strain used to screen should be considered in order to maintain consistent responses between nurseries and commercial fields.