Evaluation location: Colorado, United States
Thirty 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 (RCRR) at the Colorado State University Agricultural Research, Development, and Education Center in Fort Collins, Colorado. The Rhizoctonia screening nursery used two highly resistant germplasm (FC705/1 and FC709-2), one resistant germplasm (FC703) and one susceptible germplasm (FC901/C817) as controls. The nursery was planted in a completely randomized design with five replications 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 plot was planted to corn in 2015, pinto beans in 2016, and hard red winter wheat in 2017. The test was planted on 31 May, with an initial irrigation on 01 Jun. 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 12 Jul (8 to 12 leaf growth stage) at a rate of 8.5 g m-1 of row using a Gandy® electrically driven applicator. Following inoculation, the field was cultivated afterwards to place soil onto the plant crowns. The amount of moisture from planting to harvest was 7.72 cm of rainfall and 40.64 cm of irrigation water applied via overhead linear irrigation. Temperatures were warm through harvest, with average daytime highs of 85F and average nighttime lows of 55F. Disease progression was excellent and at harvest we had severe levels of RCRR in the nursery. The disease pressure was uniform throughout the test.
Roots were harvested 26 Aug with a single row lifter (pulled and cleaned by hand), and each root was rated for RCRR on a scale of 0 (no damage) to 7 (dead plant with root completely rotted) (Plant Dis. Rep. 63:518-522). Average disease severity per plot was determined to create a disease index (DI) for each entry and DI was treated as a continuous variable. Analysis of variance (PROC MIXED) was performed in SAS (Ver. 9.2) on DI. Data are also represented as the percentage of sugar beet roots in classes 0 through 1, considered as healthy and in classes 0 through 3, considered harvestable. Data in classes 0-1 and 0-3 were transformed using arcsine square root to normalize the data for analyses %0-1 and %0-3. Dunnett’s one-tailed t-test (P = 0.05) was used to compare entries to the highly resistant control FC705/1 (19851032H) and the most susceptible line tested (PI504179). There were significant differences among entries for DI. The highest performing entries were the highly resistant checks FC705/1 and FC709-2, and the resistant check FC703. The next four most resistant entries, PI590664, PI232887, NSL31344, and PI408965 were not significantly different from the highly resistant check FC705/1. There were 14 entries significantly more resistant than the worst performing entry, PI 504179. These accessions will be retested and, if resistance is confirmed, incorporated into the USDA-ARS RRCR resistance breeding program at Fort Collins, CO to enhance sugar beet germplasm. These results will be accessible to interested parties through the USDA-ARS, NPGS GRIN database http://www.ars-grin.gov.