Evaluation location: Colorado, United States
Thirty beet accessions of either cultivated beet or sea beet (Beta vulgaris subsp. vulgaris or Beta vulgaris subsp. maritima (L.) Arcang) from the Beta collection of the USDA-Agricultural Research Service National Plant Germplasm System were screened for resistance to Rhizoctonia crown and root rot (Rcrr) at the Colorado State University ARDEC facility in Fort Collins, CO. There were two highly resistant germplasms, one resistant germplasm, and one susceptible germplasm used as controls. The 2015 Rhizoctonia screening nursery was a completely randomized design with five replicates in one-row plots (76 cm row spacing) 3.7 m long. The soil is a Fort Collins loam (0 to 1% slope, pH 7.2). The field had been planted to Channel 197-14VT3 maize in 2012, Grazex BMR 737 (a sorghum/sudan grass hybrid) in 2013, and hard red winter wheat in 2014. In 2015, the field was not fertilized due to available nitrogen and was bedded on 2 Jun. Sugar beet seed was planted on 9 Jun to moisture. The herbicide Betamix® (60 mL acre-1; 8% phenmedipham, 8% desmedipham [v/v] and 84% inert ingredients) was applied on 22 Jun. The field was hand weeded and thinned on 3 Jul and 9 Aug. An inoculum of dry, ground, hulless-barley grain, infested with Rhizoctonia solani isolate R-9 (AG-2-2), was applied to the crown of the plants on 28 Jul (at the 8-12 leaf growth stage) at a rate of 7.0 g m-1 of row. A Gandy® electrically driven applicator was used to apply the inoculum and the field was cultivated afterwards to place soil onto the plant crowns. Roots were harvested on 8 Sep with a single row lifter (pulled and cleaned by hand), and each root was rated for rot on a scale of 0 (no disease) to 7 (dead plant, leaves necrotic with root completely rotted) (Plant Dis. Rep. 63:518–522). Average disease severity per plot was determined with the Disease Index (DI) treated as a continuous variable for each replicate of each entry. Analyses of variance (PROC GLM) were performed on disease indices, percent of healthy roots (classes 0 and 1 combined) and percent of the roots in classes 0 through 3 (harvestable roots). Data in classes 0-1 and 0-3 were transformed using arcsine square root to normalize the data for analyses (AP 0-1 and AP 0-3, respectively). Additionally, an analysis of variance (PROC MIXED) was performed on DI and Dunnett’s one-tailed t-test (P = 0.05) was used to compare all entries to the highly resistant controls (FC709-2 and FC705/1) and the most susceptible plant introduction accession (PI 518325) for DI.