Location: Soil Management and Sugarbeet ResearchTitle: Evaluation of USDA-ARS Beta vulgaris pre-breeding germplasm for resistance to Rhizoctonia crown and root rot, 2021
Submitted to: Plant Disease Management Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2022
Publication Date: 8/9/2022
Citation: Dorn, K.M., Fenwick, A.L., Metz, N.J., Yeater, K.M., Nielson, A.L., Floyd, B.A., Sowder, B.M. 2022. Evaluation of USDA-ARS Beta vulgaris pre-breeding germplasm for resistance to Rhizoctonia crown and root rot, 2021. Plant Disease Management Reports. 16. Article eV162.
Interpretive Summary: Below ground fungal pathogens present an ongoing challenge to sugar beet production. For example, the fungus Rhizoctonia solani causes severe disease in sugar beet fields resulting in above and below ground rotting. There are limited chemical controls for this fungus that last season long, so farmers rely heavily on genetic resistance. USDA-ARS scientists in Fort Collins, Colorado screened pre-breeding lines for resistance to Rhizoctonia to identify potentially new sources of genetic resistance. The researchers identified that 19 lines that performed as well as the highly resistant check line, plus additional lines with low levels of resistance that could represent new minor forms of resistance against Rhizoctonia.
Technical Abstract: Twenty-six sugar beet (Beta vulgaris subsp. vulgaris) USDA-ARS breeding lines from the Fort Collins Sugar Beet Genetics Lab 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 one highly resistant germplasm (FC705/1) 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 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 Jul (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. A total of 4.8 cm of rainfall fell from between planting and harvest, and an additional 26.6 cm of water was applied via overhead linear irrigation. The average daytime high was 30 degrees C the average nighttime low was 11.7 degrees C. During the first 14 days following inoculation, there were 10 consecutive days with temperatures exceeding 29.4 degrees C leading to rapid disease development with severe and uniform levels of RCRR in the nursery. 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). Average disease severity per plot was determined to create a disease index (DI) for each entry. The root ratings were rank transformed prior to analysis with the mixed linear models. The Dunnett’s one-tailed t-test (P = 0.05) option in the LSMEANS statement was used to compare entries to the resistant check FC705/1 (19851032H) and susceptible check 20151020, based on the rank of DI. At time of rating, there were severe levels of RCRR throughout the entire test, and there was good phenotypic separation between resistant and susceptible checks. A total of 19 entries were not statistically different than the resistant check FC705/1, which had the lowest DI in the trial. Similarly, 9 entries plus the resistant check FC705/1 showed statistically different levels of resistance compared to the susceptible check (20151020). Of these highest performing lines, 5 were RCRR selections from FC201, with the rest having FC700-series parents in their pedigrees. A number of entries with low levels of resistance could represent novel sources of minor RCRR resistance genes from crop wild relatives (20151019, 20131006, 20121054), but none of these were statistically different than the susceptible check.