Research Project: Conservation, Management and Distribution of Citrus and Date Genetic Resources and Associated Information

Location: National Clonal Germplasm Repository for Citrus

2021 Annual Report

Objectives
The primary goals of this project plan are to acquire, maintain, preserve, evaluate, and distribute Citrus and other members of the Rutaceae, and date palms (Phoenix dactylifera) and related Phoenix spp. and to conduct research in support of these objectives. Objective 1: Efficiently and effectively acquire citrus and date genetic resources; maintain their safety, genetic integrity, health and viability; and distribute them and associated information worldwide; cryopreserve citrus genetic resources. 1.A. Expand the collection of citrus and date genetic resources, with emphasis on wild relatives and under-represented portions of the collection. Rescue potentially valuable citrus and date germplasm in cultivation areas under threat of genetic erosion. 1.B. Efficiently and effectively conserve germplasm of citrus, citrus relatives and date palm and ensure all current and future accessions are preserved by multiple methods such as field orchards, plantings in screenhouses, and through cryopreservation of seeds, pollen, and clonal tissue. 1.C. Sanitize priority genotypes of citrus and date palms for distribution to users worldwide. Objective 2: Develop more effective maintenance, back-up, pathogen-testing, and pathogen-elimination methods and apply them to priority citrus and date genetic resources. 2.A. Test citrus genetic resources for huanglongbing (HLB)-associated bacteria using recently developed early detection technologies (EDT) such as canines, serology, metabolomics, and spectral responses, and multiple qPCR primer sets. 2.B. Expand the range of pathogens assayed during the screening portion of the sanitation process for citrus and date palms and develop improved diagnostic methods to enhance the efficiency of the sanitation process. 2.C. Develop and implement SOP’s for assaying date palms for the presence of specific pathogens. 2.D. Devise and implement protocols for the cryopreservation of date palm germplasm. Objective 3: With other NPGS genebanks and Crop Germplasm Committees, develop, update, document, and implement best management practices and Crop Vulnerability Statements for citrus and date genetic resource and information management. 3.A. Update crop vulnerability statement. 3.B. Update and expand scope of operations manual. Objective 4: Develop more effective genetic resource characterization and evaluation methods, especially for citrus host-plant resistance to huanglongbing (HLB) and leprosis, and apply them to priority citrus and date genetic resources. Record and disseminate characterization and evaluation data via GRIN-Global and other data sources. 4.A. Strategically characterize and evaluate citrus and date genetic resources for priority traits such as disease, pest, biotic and abiotic stress resistance, quality factors, and other pertinent properties. 4.B. Expand the range of accession information available electronically.

Approach
Gaps in the citrus and date palm germplasm collection are identified by taxonomy, geographic origin, characterization data, and stakeholder input. New germplasm is acquired by exchange with cooperating scientists or industry personnel and by plant exploration for wild germplasm. Areas for acquisition of new citrus germplasm include Australia and Vietnam. A trip to Vietnam will be planned and contact made with Botanic Gardens in Australia that have a good representation of native Rutaceae and are open to exchange germplasm. Native Rutaceae have shown potential resistance and/or tolerance to huanglongbing (HLB) making these vital gene sources for breeding purposes. Areas for date palm acquisition are primarily in the Middle East and North Africa, however, importation of offshoots is prohibited by the Animal and Plant Heath Inspection Service (APHIS). Seeds are unrestricted and tissue culture is allowed via permit. Pathogen-tested germplasm is maintained in an APHIS approved screenhouse, un-sanitized material is separated in other protective structures, and most accessions are also maintained in a field planting. Cryopreservation as a long-term backup will continue as accessions are released from quarantine and for wild relatives, seeds and pollen. Protocols for the cryopreservation of date palm accessions will be developed and optimized. Therapy and pathogen testing will continue for new citrus accessions and to satisfy quarantine regulations for the protected collection. A pathogen-testing program for date palms will be initiated. We will test whether canines and other early detection technologies can accurately detect Liberibacter-infected trees prior to standard qPCR methods. Dogs trained to alert to HLB-infected citrus trees will return to the Citrus Variety Collection and be shown every tree in the collection. For all dog-alerted trees and adjacents, leaves will be collected and assayed by qPCR using several different primer sets including the standard APHIS primers, tested with other early detection technologies and/or the tree will be pruned for containment in an insect-proof cage. Samples will be collected from caged trees and assayed using qPCR over time. Improvements to the citrus diagnostic protocols used will be implemented based on reports and publications of other researchers to expedite diagnostics. Date palm diagnostic methods developed by others will be optimized targeting phytoplasmas, Cadang-cadang viroid, and Fusarium oxysporum. All SOP’s will be revised. The Crop Vulnerability Statement will be updated. Citrus scion and rootstock germplasm imported from Florida will be evaluated for its horticultural value and reaction to endemic diseases under California growing conditions at two locations. Accessions in the field variety collection will be re-propagated on these tolerant rootstocks. Should HLB become widespread in southern California, we would evaluate for this disease under California conditions. Whether endemic pathogens prevent, mask, or exacerbate the development of HLB will be determined within a contained research facility. All information gathered will be updated and expanded through the GRIN-Global System.

Progress Report
In support of Objective 1, contact with Vietnamese scientists continued and a plant exploration trip was planned. A proposal to the USDA Plant Exploration funds was made to support plant explorations in Vietnam in fiscal year (FY) 2022 and 2023. Also in support of Objective 1, citrus accessions were sent to the National Laboratory for Genetic Resource Preservation but not in the quantities stated in the milestones. Accessions were regenerated as needed for local maintenance in the field and protective structures. Also, in support of Objective 1, 13 accessions were tested and biologically assayed to request quarantine release. However, this year’s index was unsuccessful and will be repeated in FY 2022. Accessions were distributed to cooperators upon request. In support of Objective 2, protected clean citrus accessions were tested for the citrus disease Huanglongbing (HLB) in accordance with the requirements of the USDA-APHIS interstate movement protocol, maintaining the compliance agreement necessary to support citrus germplasm distribution activities. Activities involving advanced detection of HLB by canines continued, and field trees “positive” by canine detection were tested using the regulatorily approved quantitative polymerase chain reaction (qPCR) test. Also, in support of Objective 2, the National Clonal Germplasm Repository for Citrus and Dates (NCGRCD) laboratory is now capable of routine testing for palm phytoplasmas. However, Covid restrictions and other factors slowed progress on development or implementation of additional diagnostics for citrus and date palms. In support of Objective 3, Crop Vulnerability Statements for citrus and dates were finalized. Progress was made on the Operations Manual (OM); however, due to a shortage of personnel, the OM has not been finalized. In support of Objective 4, a field trial of new, putatively HLB-resistant rootstocks was planted. Initial evaluations will be made in FY 2022 or FY 2023, depending on plant growth and other biotic factors. In addition, database records were updated. Due to the retirement of the Research Leader, the University of California at Davis contained research facility (CRF) experiment status is unknown.

Accomplishments
1. Citrus germplasm distributions support critical Huanglongbing research. Huanglongbing (HLB) is a devastating disease of citrus for which there is currently no cure. Research into HLB biology and management is reliant upon citrus germplasm distributed by NCGRCD. In FY 2021, citrus germplasm was distributed by ARS researchers in Riverside, California, to HLB researchers in Riverside, Davis, and Albany, California; Lake Alfred, Fort Pierce, and Waiauma, Florida; Geneva, New York; and New Haven, Connecticut.

Review Publications
Salomon-Torres, R., Krueger, R., Garcia-Vazquez, J., Villa-Angulo, R., Villa-Angulo, C., Ruiz-Ortiz, N., Samaniego-Sandoval, L. 2021. Date Palm Pollen: Features, Production, Extraction and Pollination Methods. Agronomy. 11(3). Article 504. https://doi.org/10.3390/agronomy11030504.
Salomon-Torres, R., Ortiz-Uribe, N., Valdez-Salas, B., Rosas-Gonzalez, N., Garcia-Gonzalez, C., Chavez, D., Cordova-Guerreo, I., Diaz-Rubio, L., Haro-Vazquez, M., Mijangos-Montiel, J., Morales-Maza, A., Mahadevan, P., Krueger, R. 2019. Nutritional Assessment, Phytochemical Composition and Antioxidant Analysis of the Pulp and Seed of Medjool Date Grown in Me´xico. PeerJ. 7. Article e6821. https://doi.org/10.7717/peerj.6821.
Montazar, A., Krueger, R., Corwin, D.L., Pourreza, A., Little, C., Rios, S., Snyder, R.L. 2020. Determination of actual evapotranspiration and crop coeffients of California date palms using the residual of energy balance approach. Water. 12(8). Article 2253. https://www.mdpi.com/2073-4441/12/8/2253.
Krueger R.R. 2021. Date palm (Phoenix dactylifera L.) biology and utilization. In: Al-Khayri J.M., Jain S.M., Johnson D.V., editors. The Date Palm Genome, Vol. 1. Compendium of Plant Genomes. Springer, Cham. p. 3-28. https://doi.org/10.1007/978-3-030-73746-7_1_1.