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Abstract
The United States Department of Agriculture's National Clonal Germplasm Repository for Citrus is currently establishing a virus-free clonal germplasm collection of 830 accessions of citrus and related Aurantioideae genera at its Riverside, California, facility with the objective of reducing genetic vulnerability of the crop and providing a source of virus-free clonal germplasm to researchers worldwide. Emphasis of the collection centers on the acquisition and ex-situ preservation, maintenance, evaluation and distribution of genera, species and cultivars representing the widest possible genetic diversity with the citrus subfamily. The Repository plan includes a comprehensive program of genetic characterization, pathogen detection and elimination, pest management, propagation in a controlled and protected environment, and documentation. All accessions are evaluated for trueness-to-type using leaf isozymes and characterized for 58 descriptors. Research is expanding into RFLPs for the development of an usable genomic map. The pathogen detection program evaluates accession for 9 virus and viroid and 2 bacterial diseases. Infected accessions receive shoot-tip micrografting and thermotherapy to eliminate pathogens. All accessions are maintained as potted trees in aphid-proof screen- or greenhouses. Citrus seed, pollen and buds are being investigated for extended preservation under lowtemperature and cryostorage. Research investigations into in vitro culturing of explants have begun. A complete computerbased record of each accession is maintained on site and in the USDA-ARS Germplasm Resources Information Network (GRIN).