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United States Department of Agriculture

Agricultural Research Service

Research Project: GENETIC IMPROVEMENT OF CITRUS

Location: Subtropical Insects and Horticulture Research

Title: Citrus scion breeding at the USDA/ARS U.S. Horticultural Research Laboratory

Authors
item Stover, Ed
item McCollum, Thomas
item Niedz, Randall
item Bowman, Kim

Submitted to: International Society of Citriculture Proceedings
Publication Type: Proceedings
Publication Acceptance Date: June 30, 2008
Publication Date: September 1, 2008
Citation: Stover, E.W., McCollum, T.G., Niedz, R.P., Bowman, K.D. 2008. Citrus scion breeding at the USDA/ARS U.S. Horticultural Research Laboratory. In: Proceedings of the International Society of Citriculture. 1:50-54.

Interpretive Summary: Citrus breeding has been conducted by the USDA since 1893. 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 originally resulted from crosses intended to produce cold-hardy scions. The first recorded tangelos resulted when USDA hybridized ‘Duncan’ grapefruit with pollen from ‘Dancy’ tangerine. 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 mutants of ‘Fallglo’ and ‘Pineapple’ sweet oranges have been created. 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 U.S. has compelled the development of HLB resistance to the forefront of our breeding objectives. Strategies using biotechnology offer the greatest potential for rapid development of resistant citrus, and have become a major emphasis in our current work. Tests of HLB-resistance in genetically enhanced (GE) lines from biotechnology are currently underway. When GE citrus provides sustainable production in the presence of HLB, ongoing plant improvement will still require selection from among non-GE progenies before further enhancement through biotechnology. Our program must evolve to permit field evaluation removed from the threat of HLB and/or develop techniques to eliminate the HLB pathogen before ultimate release to the industry.

Technical Abstract: 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.

Last Modified: 9/20/2014
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