Submitted to: International Citrus Congress Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: June 30, 2008
Publication Date: September 1, 2010
Citation: Stover, E.W., Mccollum, T.G., Niedz, R.P., Bowman, K.D. 2010. Citrus scion breeding at the USDA/ARS U.S. Horticultural Research Laboratory. International Citrus Congress Proceedings. p. 50-54.
Citrus breeding has been conducted by the USDA since 1893 when Walter Swingle made the first crosses at the USDA Subtropical Laboratory in Eustis, Florida. The initial objectives included improved disease-resistance, cold hardiness, and easy peeling fruit, which are still important breeding objectives today. Several standard industry rootstock categories (citranges and citrumellos) originally resulted from the Poncirus x Citrus crosses intended to produce cold-hardy scions. The first recorded tangelos resulted when Swingle hybridized ‘Duncan’ grapefruit (Citrus paradisi) with pollen from ‘Dancy’ tangerine (Citrus reticulata). Two released tangelo selections, ‘Orlando’ and ‘Minneola’, continue to be important in commerce. Other notable USDA citrus releases include: the mandarin hybrids ‘Osceola’, ‘Lee’, ‘Nova’, ‘Robinson’, ‘Page’, ‘Sunburst’ and ‘Fallglo’; ‘Sunstar’, ‘Midsweet’, and ‘Gardner’ sweet oranges; and ‘Flame’ grapefruit. More recently, seedless ‘Fallglo’ and ‘Pineapple’ sweet oranges have been created via irradiation and along with a few mandarin hybrids will be released over the next 18 months. Many thousands of additional scion hybrids are in various stages of evaluation, and although the USHRL is in Florida, material is evaluated for potential in other US states as well. The emergence of huanglongbing (HLB) in the US has compelled the development of HLB resistance to the forefront of our breeding objectives. Transgenic strategies offer the greatest potential for rapid development of resistant citrus, and plant transformation using antimicrobial peptides has become a major emphasis in our current work. Both constitutive and phloem-specific promoters are being utilized, and other transgenes will be used as opportunities are identified to target Liberibacter gene products and virulence mechanisms. Tests of HLB-resistance in transgenic lines are currently underway. When GM citrus provides sustainable production in the presence of HLB, ongoing plant improvement will still require selection from among non-GM progenies before investing in transgene insertion. Our program must evolve to permit field evaluation removed from the threat of HLB and/or develop techniques to eliminate Liberibacter from field-tested progeny for phytosanitary propagation, transformation, and ultimate release to the industry.