Evaluation location: Colorado, United States
Twenty-nine beet (Beta vulgaris subsp. vulgaris and Beta vulgaris subsp. maritima (L.) Arcang) accessions from the Beta collection of the USDA-Agricultural Research Service National Plant Germplasm System were screened for resistance to Rhizoctonia crown and root rot, 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 2014 Rhizoctonia screening nursery was a randomized complete-block 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 hard red winter wheat in 2012, and Grazex BMR 737 (a sorghum/sudangrass hybrid) in 2013. In 2014, the field was fertilized (60 lbs N acre-1 and 30 lbs P2O5 acre-1) and bedded on the 22 May. Sugar beet seed was planted on 29 May to moisture and irrigated as needed with an overhead linear irrigation system. The herbicide Betamix (2 oz acre-1; 8% phenmedipham, 8% desmedipham [v/v] and 84% inert ingredients) was applied on 12 and 19 Jun. The field was hand weeded and thinned on 28 Jun and 19 Jul. 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 21 Jul (at the 8-12 leaf growth stage) at a rate of 6.01 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 16 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 (DI) was determined with the DI treated as a continuous variable for each replicate of each entry. Analyses of variance (PROC GLIMMIX) 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 control (FC709-2) and the most susceptible plant introduction accession (PI 590719) for DI.
At harvest there was moderate Rhizoctonia root rot and other diseases were not evident. There were significant differences among entries for all three variables (PROC GLIMMIX). The DI was 1.5 in the highly resistant control and 7.0 in the most susceptible entry. Two entries, PI 560340 and PI 560339, for which DI<3.3 in the table, were not significantly different from the highly resistant control (Dunnett’s one tailed t-test for DI, p = 0.05). All entries below Ames 2652 (DI>4.4) in the table were not significantly different (Dunnett’s one tailed t-test for DI, p = 0.05) from the most susceptible entry (DI = 7.0). The eight entries with DI > 4.5 and > 3.2 had moderate resistance. The two entries with the highest resistance (PI 560339, PI 560340) were released from the USDA-ARS Salinas breeding program. PI 560339 was released for resistance to virus yellows, Erwinia (reclassified as Pectobacterium betavasculorum), powdery mildew (Erysiphe polygoni), and improvement for percent sucrose. PI 560340 was released for resistance to rhizomania and Rhizoctonia crown and root rot. All accessions significantly more resistant than PI 590719 will be retested and, if the resistance is confirmed, entered into the USDA-ARS Rhizoctonia root rot-resistance breeding program at Fort Collins, CO to develop sugar beet germplasm with increased resistance to Rhizoctonia root rot. These results will be accessible to interested parties through the USDA-ARS, NPGS GRIN database (http://www.ars-grin.gov/npgs/index.html).