Evaluation of Three Forms of Rhizoctonia solani Mediated Pathogenicity to Sugar Beet Cultivars in Greenhouse Studies

Rhizoctonia solani causes damping-off, as well as crown and root rot of sugar beet (Beta vulgaris L). This pathogen overwinters as sclerotia or melanized mycelia. Traditionally, the resistance of cultivars to R. solani is evaluated by scoring disease reactions of the crowns and roots of older seedlings, instead of evaluating at seed germination. Most studies that have evaluated cultivar resistance to R. solani have used colonized whole barley grains as artificial inocula. Colonized grains are prone to contamination with other pathogens and are often lost to rodents/birds when applied in the field. Considering those limitations, a study was undertaken (1) to develop in vitro methods to generate natural sclerotia of R. solani on a large scale, (2) to compare pathogenic potentials of R. solani sclerotia, mycelia, and colonized barley grains for optimization of damping-off assays, and (3) to evaluate resistance of selected commercial cultivars to R. solani. Of the six-culture media, amended clarified V8 (ACV8) was the most suitable culture medium to grow and produce sclerotia on a large scale and 10% PDA was the least suitable. Three testing sizes of sclerotia were found to be equally effective in causing plant losses. Sclerotia inocula were comparable with mycelial discs and colonized barley grains in causing pre-emergence damping off under aseptic in vitro conditions. Sclerotia also were equally or more effective than mycelia plug or barley grain inocula in reducing seedling emergence, inducing damping off, and increased root rot under greenhouse in vivo conditions. To conclude, sclerotia can be prepared on a feasible scale and used as natural inocula to screen response to R. solani on sugar beet.