Evaluation of Beta vulgaris USDA-ARS germplasm releases for rhizoctonia crown and root rot resistance, 2020

Authors: Dorn, Kevin; Fenwick, Ann; Yeater, Kathleen; Fall, Amy; Floyd, Bradley; Metz, Nicholas; Sowder, Bradley

Submitted to: Plant Disease Management Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/19/2021
Publication Date: 8/16/2021
Citation: Dorn, K.M., Fenwick, A.L., Yeater, K.M., Nielson, A.L., Floyd, B.A., Metz, N.J., Sowder, B.M. 2021. Evaluation of Beta vulgaris USDA-ARS germplasm releases for rhizoctonia crown and root rot resistance, 2020. Plant Disease Management Reports. 15. Article eV166.

Interpretive Summary: Thirty-two sugar beet (Beta vulgaris subsp. vulgaris) 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 nursery included one resistant germplasm (FC703) and one susceptible germplasm (20151020) 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). In 2017, the field was planted to dry bean, followed by hard red winter wheat in 2018, and corn in 2019. In 2020, the field was not fertilized due to available nitrogen and was bedded on 23 April. Nortron and Phosphorus (50lb/acre) was applied on 18 May and the field was planted on 21 May with an initial overhead irrigation of 1.25cm on 23 May. The field was watered weekly to assist with germination and seedling emergence. On 24 May, a hard rain (1.6cm) perpetuated the formation of crust on the soil. Between 7 June and 8 June, a storm with driving rain (3.4 cm) occurred and concluded with pea size hail and low nighttime temperatures (3.9'C), causing crusting and seedling loss. Subsequent stand counts indicated 5 entries were lost (less than 8 plants survived total over 5 reps). There were 14 additional entries that had one rep with less than 6 beets per plot. The field plot alleys were cut on 16 June and hand weeded. Hand thinning and weeding occurred multiple times between 23 June and 8 July. The field was inoculated with dry, ground hull less barley grain infested Rhizoctonia solani isolate R-9 (AG2-2 IIIB). The inoculum was applied to the crown of the plants on 15 July (8 to 12 leaf growth stage) at a rate of 6.5 g m-1 of row using a Gandy® electrically driven applicator. The field was cultivated immediately following inoculation to place soil onto the plant crowns. From planting to harvest, there was 4.8 cm of rainfall, and 26.6 cm of water applied via overhead linear irrigation. During the entire test, the average daytime high was 30'C, and the average nighttime low was 11.7' C. During the first 14 days following inoculation, there 10 consecutive days with temperatures exceeding 29.4'C, leading to rapid disease development with severe and uniform levels of RCRR in the nursery.

Technical Abstract: Roots were defoliated and harvested Aug 17 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) (Plant Dis. Rep. 63:518-522). Average disease severity per plot was determined to create a disease index (DI) for each entry. Analysis of variance (GLIMMIX procedure) was performed in SAS (Version 9.4, SAS/STAT 14.3) on ranks of DI by replication (as determined via the RANK procedure). For categorical observations, 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. The Rank transformed response and the arcsine square root data transformations are appropriate for these response patterns, and the studentized residuals as observed in GLIMMIX (data not shown) exhibited normal distributions. The Dunnett’s two-tailed t-test (P = 0.05) option in the LSMEANS statement was used to compare entries to the resistant control FC703 (20041005) and the susceptible check (20151020), based on the Rank of DI. There were significant differences among entries for DI in this test. With the severe disease pressure in this nursery, there were only 7 entries that were significantly more resistant than the susceptible check 20151020 and 14 entries that were not significantly different than the resistant check FC703 (20041005). Each entry in this test was chosen because it was a part of a whole genome resequencing project. These entries will be used to develop genetic stocks with high phenotypic homogeneity for future genomics-guided resistance gene mapping. These results will be accessible to interested parties through the USDA-ARS, NPGS GRIN database http://www.ars-grin.gov.