Evaluation of Rhizoctonia root and crown rot resistance in USDA-ARS Fort Collins germplasm releases, 2020

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

Submitted to: Plant Disease Management Reports
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/6/2022
Publication Date: 8/9/2022
Citation: Metz, N.J., Fenwick, A.L., Yeater, K.M., Nielson, A.L., Floyd, B.A., Sowder, B.M., Dorn, K.M. 2022. Evaluation of Rhizoctonia root and crown rot resistance in USDA-ARS Fort Collins germplasm releases, 2020. Plant Disease Management Reports. 16. Article eV173.

Interpretive Summary: Fungal diseases present an ongoing threat to sugar beet production in the United States. Pathogens like Rhizoctonia can greatly reduce yields, and there are few chemical control options that last for a while growing season. USDA-ARS scientists in Fort Collins, Colorado tapped into the over 50 year long history of breeding efforts for improving genetic resistance to Rhizoctonia. The researchers screened 50 total sugar beet lines for resistance, and discovered 22 lines with statistically higher levels of resistance. These resistant lines will be used to develop new pre-breeding lines with improved resistance, as well as to identify the underlying genes that impart resistance.

Technical Abstract: Forty-eight 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) in Fort Collins, Colorado. The Rhizoctonia screening nursery used one highly resistant line (FC709-2) and one susceptible germplasm (20151020) as controls. The nursery was planted in a row/column design with 5 replications per entry in one-row plots (76 cm row spacing) 3.7 m long. Plots were planted on 21 May with 1.25cm of irrigation applied on 23 May. The field was watered weekly to assist with germination and seedling emergence. 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 15 Jul (8 to 12 leaf growth stage) at a rate of 6.5 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 defoliated and harvested Aug 18 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 was performed in SAS on DI. Dunnett’s one-tailed t-test (P = 0.05) was used to compare entries to the most resistant (19921022) and susceptible (20151020) lines tested. The entries tested here represented over 50 years of pre-breeding releases from the USDA-ARS Fort Collins program with improved resistance to RCRR. Twenty-two entries were not statistically different than the most resistant entry tested, FC702/7. Interestingly, 21 entries were statistically similar to the highly susceptible check 20151020, which had the highest DI in this test. Of these 21 lines not statistically different than the susceptible check, 16 were originally released with notable enrichment of RCRR resistance. Lines demonstrating the highest levels of resistance will be used for genetic mapping experiments to identify causal resistance genes, as well as developing new pre-breeding lines with more homogeneous phenotypic responses to RCRR.