Novel QTL Associated with Rhizoctonia solani Kühn Resistance Identified in Two Table Beet x Sugar Beet F _2:3 Populations Using a New Table Beet Reference Genome

The necrotrophic fungus Rhizoctonia solani Kuhn is a major concern for table beet (Beta vulgaris L. ssp. vulgaris) producers across the United States causing upward of 75% losses in severe instances. Thus far, there have been minimal efforts to incorporate host resistance to R. solani in table beet germplasm. To investigate the genetic control of R. solani resistance in table beet, we developed two mapping populations. Parents of the two populations were a Rhizoctonia-susceptible table beet inbred W357A and a resistant sugar beet germplasm FC709-2 (sugar beet resistance population, SBRP) and a Rhizoctonia-resistant table beet inbred W364B and a susceptible sugar beet inbred FC901/C817 (table beet resistance population, TBRP). In Spring 2020, F-2:3 families were evaluated for response to artificial inoculation with R. solani AG 2-2 IIIB isolate R1 in replicated greenhouse experiments. This work also represents the first use of the W357B table beet reference genome, utilized here to align genotyping-by-sequencing reads to identify polymorphic markers. Using interval linkage mapping, we identified one quantitative trait locus (QTL) in each of the two populations, each accounting for 30% of the phenotypic variation. The QTL in both the SBRP and TBRP were found on chromosome 2 and contained several putative resistance genes in annotations of the Beta vulgaris and Arabidopsis thaliana genomes. This is the first report of a QTL on chromosome 2 for resistance to R. solani in B. vulgaris ssp. vulgaris and the first identification of QTL for disease resistance in table beet. The newly developed table beet reference genome and markers identified in this study may be of value for marker-assisted selection in breeding for resistance to R. solani in both sugar beet and table beet breeding programs.