Interaction of Sugar Beet Host Resistance and Rhizoctonia solani AG-2-2 IIIB Strains

Strausbaugh, C. A., Eujayl, I. A., and Panella, L. W. 2013. Interaction of sugar beet host resistance and Rhizoctonia solani AG-2-2 IIIB strains. Plant Dis. 97:1175-1180. 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. Rhizoctonia crown and root rot caused by Rhizoctonia solani Kuhn can lead to yield losses of 50% or more in commercial sugar beet (Beta vulgaris L.) fields, affect sucrose losses in stored roots, and lead to difficulties in factory processing (6,17,33,36,39). Because R. solani strains form a species complex, strains have been further classified into subgroups known as anastomosis groups (AGs) and intraspecific groups (ISGs) (8,11,30). In Idaho, R. solani AG-2-2 IIIB is the primary AG and ISG associated with the most damaging rot in mature sugar beet roots (33). Recent Idaho studies have shown that the fungus is primarily limited to damaging the outer 3 to 5% of the root mass, while subsequent bacterial rot led by Leuconostoc mesenteroides subsp. dextranicum (Bei