Citation: Volk, G.M.; Gmitter,F.G., Jr.; Krueger, R.R. Conserving Citrus Diversity: From Vavilov’s Early Explorations to Genebanks around the World. Plants 2023, 12, 814. https://doi.org/10.3390/ plants12040814 Academic Editors: Igor G. Loskutov, Axel Diederichsen and Andreas W. Ebert Received: 4 January 2023 Revised: 6 February 2023 Accepted: 9 February 2023 Published: 11 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Conserving Citrus Diversity: From Vavilov’s Early Explorations to Genebanks around the World Gayle M. Volk 1, * , Frederick G. Gmitter, Jr. 2 and Robert R. Krueger 3 1 USDA-ARS National Laboratory for Genetic Resources Preservation, 1111 S. Mason St., Fort Collins, CO 80521, USA 2 Citrus Research and Education Center (CREC), Institute of Food and Agricultural Sciences (IFAS), University of Florida, Lake Alfred, FL 33850, USA 3 USDA-ARS National Germplasm Repository for Citrus and Dates, 1060 Martin Luther King Blvd., Riverside, CA 92507, USA * Correspondence: gayle.volk@usda.gov Abstract: Citrus is among the most economically important fruit crops. Its vast species diversity and global production was observed by N.I. Vavilov during his international plant explorations from the early to mid-1900s. Currently, ex situ citrus collections located around the world conserve and protect citrus genetic resources, as revealed in a survey conducted in 2021. Responses were received from 43 collections in 27 countries, of which 35 provided data regarding collection composition, management practices, and security, as well as other information. The six largest citrus collections have between 1000 and 1735 accessions. The largest accession holdings are mandarins and sweet oranges, although all citrus fruit types are maintained: mandarin, sweet orange, lemon, pummelo, grapefruit, hybrids, lime, sour orange, citron, kumquat, papeda, finger lime, and crop wild relatives. Diseases pose significant threats to collections, though some collections are maintained in a clean-plant state as a result of intensive sanitation efforts. National and regional quarantine regulations often limit the export and import of citrus plants or propagative materials, thus limiting the availability of materials at an international level. Resources, both financial and human, are necessary to ensure the long-term safety and security of citrus collections on a global scale. Future efforts to develop citrus genebanking communities will provide opportunities for improved conservation, as well as collaborations and training. Keywords: collections; ex situ conservation; diversity; global conservation strategy; plant genetic resources; survey 1. Introduction Citrus, including oranges (Citrus × sinensis (L.) Osbeck), lemons (C. × limon (L.) Osbeck), limes (C. aurantiifolia (Christman.) Swingle), pummelos (C. maxima (Burm.) Merr.), grapefruits (C. × paradisi Macfad.), mandarins (C. reticulata Blanco), and other fruits, are among the most widely grown fruit crops on a global scale. Citrus is produced in sutropical, semitropical, and tropical regions around the world, with most commercial production between 20 ◦ and 40 ◦ latitude in the Northern and Southern hemispheres. Oranges represent the largest global harvested area, global production (in tons), and value, followed by mandarins, lemons/limes, and pummelos/grapefruits [1]. According to Kruse [2], the taxonomically complex Citrus genus has between 16 and 162 described species. Although Citrus is the most widely cultivated genus within the Rutaceae, the fruits of Clausena lansium (Lour.) Skeels are consumed fresh in China and other areas; the leaves of Bergera koenigii L. are used in cooking in South Asia; the fruits of Aegle marmelos (L.) Corrêa are the basis of beverages consumed in South Asia; the fruits of Microcitrus spp. are consumed as “bush food” in Australia; and species such as Murraya paniculata (L.) Jack and Triphasia trifolia (Burm. F.) P. Wilson are grown as ornamentals [3]. Plants 2023, 12, 814. https://doi.org/10.3390/plants12040814 https://www.mdpi.com/journal/plants