Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 25, 2013
Publication Date: August 9, 2013
Citation: Strausbaugh, C.A., Eujayl, I.A., Panella, L.W. 2013. Interaction of sugar beet host resistance and rhizoctonia solani AG-2-2 IIIB strains. Plant Disease. 97:1175-1180. Interpretive Summary: Rhizoctonia root rot caused by Rhizoctonia solani can lead to yield losses of 50% or more in commercial sugar beet fields, affect sucrose losses in stored roots, and lead to difficulties in factory processing. Management of Rhizoctonia root rot with crop rotation and fungicide applications helps limit problems, but unacceptable levels of rot still frequently occur. Host resistance would be the most desirable control measure, because it tends to be more cost effective than other approaches. Observations in field studies with R. solani strains and commercial cultivars from different seed companies suggested that an interaction between strains and source of resistance might exist. To investigate the potential for a resistance by strain interaction in sugar beet, lines with potentially different sources of Rhizoctonia root rot resistance were compared versus a diverse set of nine R. solani AG-2-2 IIIB strains. When comparing these nine strains on the susceptible sugar beet line, FC901/C817, significant differences in virulence between strains were evident, but their disease responses and ranking across all disease variables were similar. When comparing these strains against FC lines representing known sources of resistance to Rhizoctonia root rot in mature sugar beet roots, the disease responses had a tendency to vary except for FC709-2, which was resistant to all strains. With the weaker sources of resistance (FC703, FC708CMS, and FC718), strain differences and interactions were evident. Hopefully the full complement of resistance genes found in FC709-2 can be incorporated into commercial sugar beet cultivars without excessive yield drag, since this resistance appeared to be affective against all R. solani strains evaluated in this study.
Technical Abstract: Rhizoctonia root rot caused by Rhizoctonia solani can cause serious economic losses in sugar beet fields. Preliminary evidence suggests there could be interactions between different strains and resistance sources. Thus, field studies were conducted to determine if nine R. solani AG-2-2 IIIB strains varied for virulence when compared with a non-inoculated 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 the same ranking (r = 1.0) regardless of disease variable. Line FC709-2 was resistant (response not different from non-inoculated 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. Since most commercial sugar beet cultivars contain low to intermediate resistance to Rhizoctonia root rot, the strain used to screen should be considered in order to maintain consistent responses between nurseries and commercial fields.