Location: Crop Germplasm Research
2021 Annual Report
Objectives
Objective 1: Efficiently and effectively acquire Carya genetic resources; maintain their safety, genetic integrity, health and viability; and distribute them and associated information worldwide.
Sub-objective 1A: Acquire high priority Carya genetic resources, characterize them using molecular profiling methods, integrate them into secure, temporary maintenance systems, and distribute associated information and appropriate propagules worldwide.
Sub-objective 1B: Revise diagnostic methods for detecting Xylella fastidiosa in Carya.
Sub-objective 1C: Screen ex situ and in situ Carya populations for Xylella fastidiosa.
Sub-objective 1D: Confirm efficacy of remediation methods to eliminate Xylella fastidiosa from nuts and/or scions of Carya.
Objective 2: Develop more effective genetic resource conservation, evaluation, and characterization methods and apply them to priority Carya genetic resources. Record and disseminate evaluation and characterization data via GRIN-Global and other data sources.
Objective 3: With other NPGS genebanks and Crop Germplasm Committees, develop, update, document, and implement best management practices for Carya genetic resource and information management.
Approach
This project represents one of the eight National Clonal Germplasm Repositories in the National Plant Germplasm System. It shares the common mission of all clonal repositories to collect, maintain, evaluate, document, and distribute valuable plant genetic resources. It is unique in its focus on the Carya genus, which includes the most valuable nut tree native to North America, the pecan [Carya illinoinensis (Wangenh.) K. Koch]. This project is a sister project of the Pecan Breeding Program (3091-21000-039-00D) and furnishes diverse Carya germplasm for use as parents within that program, as well as to other domestic and international breeding programs. Other North American species of Carya do not provide economic incentive for commercial nut production, but are valuable components of their ecosystems and are represented for use in breeding, with increasing interest on endemic and microbiotic associations. All diploid Carya, including commercially valuable Asian species, hybridize with pecan. Some Carya species exist only as fragmented populations in threatened habitats. Strategies for the appropriate characterization and utilization of those species are addressed in this project. Molecular genetic tools have been developed by the precursor project. These tools will continue to be cooperatively developed, refined, and applied by the current project in order to understand genetic diversity and gene flow in native populations, to verify identity of individual cultivars, to confirm parentage in controlled crosses, and to facilitate management of the Collection and its knowledgeable deployment.
Progress Report
In Fiscal Year (FY) 2021, significant progress was made in conservation and improvement of the National Collection. Under Objective 1, young leaves from our Mexican provenance orchard were subjected to genome re-sequencing. The DNA sequencing has been completed and the analysis is in progress. Due to a critical scintific research position vacancy and ARS international travel restrictions associated with the COVID pandemic, the proposed species collections and plantings were not accomplished. Project work developed and evaluated thermal treatments using microwave irradiation and microwave absorbers [sterile deionized water (dH2O) and carbon nanotubes (CNTs)] as novel disinfectant methods for remediating Xylella fastidiosa, which causes pecan bacterial leaf scorch disease, in pecan scions (graftwood). The treatments did not compromise plant health, and the microwave irradiation was effective in reducing the presence of X. fastidiosa in treated scions. The conventional hot-water treatment method against X. fastidiosa was also confirmed to be effective in significantly reducing the presence of the bacterium in infected scions. Work under Objective 2 developed important new information on the genetic make-up of pecan; the data obtained were transferred to the scientific and user communities through written and oral forums, and preserved in appropriate databases. Objective 3 work continued to develop improved procedures for pecan resource and information management; improved and more efficient processes were developed for management of orchard resources to assure ongoing viability and sustainability of critical pecan germplasm types with limited human resources. Overall, project work in FY 2021 resulted in full protection of the pecan germplasm repository genetic resources under the challenging COVID pandemic environment. No significant germplasm resources were lost or compromised, and advancements were made in developing improved protocols for repository management.
Accomplishments
1. Microwave irradiation for the phytosanitation of Xylella fastidiosa in pecan graftwood. Pecan bacterial leaf scorch disease caused by Xylella fastidiosa is widespread in U.S. pecan production areas and can be transmitted from scion to rootstock via grafting. Efficient approaches to disease control are needed. ARS researchers at College Station, Texas, in collaboration with university colleagues, developed a thermal treatment using microwave irradiation and microwave absorbers to kill or deactivate X. fastidiosa in pecan scions. These techniques provide researchers and industry personnel with additional options for treating infected pecan graftwood, which will help to reduce the spread of the leaf scorch disease. Microwave irradiation uses less energy and time when compared to the hot-water treatment method developed for treating Xylella-infected pecan graftwood, and it promotes long-term sustainability. On a broader scale, this accomplishment provides a foundation for the development of novel phytosanitary approaches using microwave irradiation and microwave absorbers for the control of vascular pathogens in other crop species.
Review Publications
Bentley, N., Grauke, L.J., Ruhlman, E., Klein, R.R., Kubenka, K.A., Wang, X., Klein, P. 2020. Linkage mapping and QTL analysis of pecan (Carya illinoinensis) full-siblings using genotyping-by-sequencing. Tree Genetics and Genomes. 16. Article 83. https://doi.org/10.1007/s11295-020-01476-6.
Hilton, A., Jeong, M., Hsu, J., Cao, F., Choi, W., Wang, X., Yu, C., Jo, Y. 2021. Thermal treatment using microwave radiation for the phytosanitation of Xylella fastidiosa in pecan graftwood. PLoS ONE. 16(1). Article e0244758. https://doi.org/10.1371/journal.pone.0244758.
