Evaluation location: Colorado, United States
Thirty sea beet (Beta vulgaris subsp. maritima (L.) Arcang accessions from the Beta collection of the USDA- ARS National Plant Germplasm System were screened for resistance to Rhizoctonia root and crown rot, (RRCR) by the USDA-ARS at the Colorado State University ARDEC Research Facility. The Rhizoctonia screening nursery used two highly resistant germplasm, one resistant germplasm and one susceptible germplasm 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 was planted to Grazex BMR 737 (a sorghum/sudan grass hybrid) in 2013, Mycogen 2Y479 maize in 2014, fallowed in 2015, and hard red winter wheat in 2016. In 2017, the field was not fertilized due to available nitrogen and was bedded on 16 May with initial irrigation on 17 May. 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 11 Jul(8 to 12 leaf growth stage) at a rate of 8.5 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 6 Sept with a single row lifter (pulled and cleaned by hand), and each root was rated for RRCR on a scale of 0 (no damage) to 7 (dead plant with root completely rotted). 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. Because the analysis of variance does not group the entries into discrete classes, Dunnett’s one-tailed t-test (p = 0.05) was used to compare entries to the resistant control (FC703), the highly resistant control (FC709-2) and the most susceptible Plant Introduction (PI 546516).
Spring of 2017 in Fort Collins, CO was hot and dry. The crop was planted and irrigated with overhead linear irrigation. The amount of moisture from planting to harvest was 21.06 cm of rainfall and 27.31 cm of irrigation water. Temperatures were warm through harvest, disease progression was excellent and at harvest we had severe levels of RRCR in the nursery. The disease pressure was uniform throughout the test. There were significant differences among entries for DI in this test and separation between resistant and susceptible check entries with a range of DI from 3.6 to 7. One entry, PI 535827, was not significantly different (p = 0.05) from the highly resistant control (FC 705/1) and potentially represents a novel source of resistance to RRCR. The superior performance of PI 535827 is especially apparent when the percent harvestable (%0-3) is compared with that of other entries. PI 546516, was the most susceptible in the test, and there were 13 entries not significantly different than it. There were 16 PI’s that were significantly more resistant than the worst performing line but significantly different in performance from the resistant control (FC703). 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.