Location: Northwest Irrigation and Soils Research
2020 Annual Report
Objectives
1. Develop genetic markers that will allow for marker-assisted breeding; develop superior sugar beet germplasm with priority traits, such as high sucrose and resistance to various diseases; and release improved breeding materials, including doubled haploid lines, inbred lines, and genetic mapping populations.
1.1. Develop elite germplasm with curly top, Rhizoctonia crown and root rot (RCRR), Cercospora leaf spot (CLS), and storage rot resistance, and high sucrose and low impurities. (Eujayl, Strausbaugh)
1.2. Conduct whole genome sequencing of elite germplasm lines for genetic variation analysis for RCRR resistance. (Eujayl, Strausbaugh)
1.3. Establish a large complement of single nucleotide polymorphism (SNP) markers for genotyping mapping populations and germplasm for curly top and RCRR resistance. (Eujayl, Strausbaugh)
2. Dissect disease development pathways and host-pathogen interactions, and design improved disease management strategies and screening procedures in sugar beet.
2.1. Investigate the interaction between the most common Leuconostoc van Tiegham haplotypes and the various genetic subgroups of R. solani. (Strausbaugh)
2.2. Investigate the use of RNA interference (RNAi) for the control of Beet curly top virus (BCTV). (Strausbaugh, Eujayl)
2.3. Develop additional management strategies for curly top and pest control in sugar beet. (Strausbaugh)
Approach
The proposed research is a coordinated cooperative effort between USDA-ARS, university scientists, and industry partners which will improve sucrose yield in sugar beet production. Elite sugar beet germplasm will be developed to increase sucrose content, while reducing impurities and improving disease resistance and management options for Beet curly top virus (BCTV), Rhizoctonia solani, and storage rot fungi. The first objective is non-hypothesis research focused on improving or identifying novel traits of interest, releasing elite germplasm with these traits, and sequencing lines to map and develop markers for these traits. Genetic markers will allow for marker-assisted breeding and release of superior sugar beet germplasm. Backcrossing, mass selection, and recurrent selection will be used to produce populations and lines with disease resistance, low impurities, and high sucrose content. Doubled haploid lines from this germplasm will be used to produce hybrids and segregating populations for genetic mapping. Whole genome sequencing will be conducted using PacBio technology and optical mapping. This effort will be complemented with gene expression profiling via RNA-Seq and Iso-Seq to identify differentially expressed genes caused by R. solani infection. A large complement of single nucleotide polymorphism (SNP) markers for genotyping mapping populations and germplasm for curly top and Rhizoctonia crown and root rot resistance will be developed. If additional sources of high sucrose or disease resistance are needed, additional high sucrose parental lines and plant introduction accessions will be screened. The second objective is hypothesis driven research which advances our knowledge of disease development and interactions to improve disease management strategies and screening procedures in sugar beet production. The interaction between Leuconostoc and R. solani will be investigated, since Leuconostoc haplotypes will possibility vary in their ability to create more root rot through a synergistic interaction with genetic subgroups of R. solani. Root inoculations in field studies will be conducted with bacterial isolates representing the predominant haplotypes for L. mensenteroides and L. pseudomesenteroides and R. solani isolates representative of the diversity present in anastomosis groups found in sugar beet. Five weeks after inoculation, rotted tissue will be measured and the pH associated with that tissue will be established. Isolations from the leading edge of the rot from randomly selected roots will be conducted to complete Koch’s postulates. Based on the results from the interaction studies, fungal-bacterial combinations exhibiting the synergistic interaction will be evaluated further through inhibition and enzyme assays. To improve management options for BCTV, the use of RNA interference (RNAi) and foliar insecticides will be investigated. If RNAi proves successful, RNAi will also be investigated for the control of R. solani.
Progress Report
Significant progress was made on all sub-objectives under Objective 1, which addresses the development of genetic markers and superior sugar beet germplasm. Breeding lines resistant to Beet curly top virus (BCTV) were re-sequenced to improve the resolution of genes regulating the resistance. Breeding lines were also sequenced to decipher the genes regulating post-harvest quality traits (sucrose, impurities, etc.). Sufficient seed was obtained from highly resistant germplasm selected from the 2019 Curly Top Nursery to allow additional work. Individual plants resistant to rhizomania were selected from high sucrose genetic stock (KPS25) in the 2019 Rhizomania Nursery. Gene expression profiling was implemented using doubled haploid lines identified with differentially expressed genes. These data were then used to develop selection markers and trait profiling. Doubled haploid line KDHFC709-2, resistant to Rhizoctonia root rot, was whole-genome sequenced. In a commercial sugar beet field, breeding line KEMS08 (PI683816) was confirmed to be highly resistant to Cercospora leaf spot (CLS).
Significant progress was also made on all sub-objectives under Objective 2 which addresses disease development pathways, host-pathogen interactions, and designing improved disease management strategies. Under Sub-objective 2.1, the interaction of various Leuconostoc haplotypes versus a genetically diverse range of R. solani strains was investigated. The L. mesenteroides strain L12311 (17 mm of rot) had significantly more rot than L. pseudomesenteroides strain L12487 (13 mm of rot) when combined with the R. solani fungal strains. The R. solani anastomosis group (AG), 2-2 IIIB strains (16 mm of rot) led to significantly more rot than strains associated with other anastomosis groups: 2-2 IV (11 mm of rot), 4 HG-I (5 mm of rot), and 4 HG-II (3 mm of rot). When R. solani AG-2-2 IIIB strains from three phylogenetic groups were compared, rot ranged from 15 to 17 mm and did not differ between groups. The pH for root tissue with at least 30 mm of rot was lower (4.0 to 4.2) than tissue with less than 2 mm of rot (6.2 to 6.4). Both isolations and tissue pH suggest late season sugar beet root rot is primarily associated with the bacteria Leuconostoc and secondary organisms. However, damage was minor without both R. solani AG-2-2 and Leuconostoc strains present when internal rot initiates. Investigations into the enzymes associated with this interaction were initiated. Preliminary results suggest that L. mesenteroides strain L12311 leads to rot when combined with the enzymes cellulase, pectinase, and pectin lyase, but caused little to no rot when combined with xylanase, pectate lyase, and pectin methylesterase. Leuconostoc is not known to produce cellulase, pectinase, and pectin lyase, but the preliminary data indicate that R. solani AG-2-2 IIIB strains produces these three enzymes. When R. solani AG-2-2 IIIB strains are combined with L. mesenteroides in sugar beet root tissue, considerable rot develops. Additional studies focused on these three enzymes and their interaction with L. mesenteroides will be conducted.
Under Sub-objective 2.2, primers and protocols were established to investigate the use of RNA interference (RNAi) for the control of Beet curly top virus (BCTV). An initial small experiment was conducted. More detailed experiments will be conducted by newly hired scientists who have experience using RNA interference. Under Sub-objective 2.3, different insecticide chemistries were investigated for the management of curly top and pests on sugar beet. Eight insecticide foliar treatments (representing different insecticide classes from previous investigations) were compared for their ability to manage curly top versus the Poncho Beta neonicotinoid seed treatment and a non-treated check. The non-treated check was severely infected based on curly top ratings and yield variables even though a commercial sugar beet cultivar approved for production was utilized for the study. However, when the insecticide seed treatment Poncho Beta was used with this cultivar, disease control and yield were significantly greater. All eight foliar insecticide treatments evaluated in the study provided no control of BCTV since all variables had values similar to or worse than the non-treated check. Additional evaluations with other insecticides will be needed if an alternative to neonicotinoid seed treatments for BCTV control is to be identified.
Accomplishments
Review Publications
Strausbaugh, C.A. 2020. Interaction of Rhizoctonia solani and Leuconostoc spp. causing sugar beet root rot and tissue pH changes in Idaho. Canadian Journal of Plant Pathology. 42(2):304-314. https://doi.org/10.1080/07060661.2019.1668857.
Strausbaugh, C.A. 2020. Commercial sugar beet cultivars evaluated for rhizomania resistance and storability in Idaho, 2018. Plant Disease Management Reports. 14:CF045.
Strausbaugh, C.A. 2020. Experimental sugar beet cultivars evaluated for rhizomania resistance and storability in Idaho, 2018. Plant Disease Management Reports. 14:CF044.
Strausbaugh, C.A., Dorn, K.M., Fenwick, A.L. 2020. Beet curly top resistance in USDA-ARS Ft. Collins germplasm, 2019. Plant Disease Management Reports. 14:CF043.
Eujayl, I.A., Strausbaugh, C.A. 2020. Beet curly top resistance in USDA-ARS Kimberly germplasm, 2019. Plant Disease Management Reports. 14:CF042.
Strausbaugh, C.A., Wenninger, E.J. 2020. Foliar insecticides for the control of curly top in Idaho sugar beet, 2019. Plant Disease Management Reports. 14:CF041.
Strausbaugh, C.A., Hellier, B.C. 2019. Rhizomania and storage rot resistance in USDA-ARS plant introduction lines evaluated in Idaho, 2018. Plant Disease Management Reports. 13:CF115.
Strausbaugh, C.A., Fenwick, A.L. 2019. Ft. Collins sugar beet germplasm evaluated for rhizomania and storage rot resistance in Idaho, 2018. Plant Disease Management Reports. 13:CF114.
