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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: Breeding citrus for HLB resistance at the USDA/ARS U.S. Horticultural Research Laboratory, Ft. Pierce, Florida

Authors
item Stover, Ed
item Bowman, Kim
item McCollum, Thomas
item Niedz, Randall
item Shatters, Robert
item Hall, David

Submitted to: Workshop Proceedings
Publication Type: Proceedings
Publication Acceptance Date: July 15, 2010
Publication Date: September 1, 2010
Citation: Stover, E., Bowman, K., McCollum, T., Niedz, R., Shatters, R.G. Jr., Hall, D. 2010. Breeding citrus for HLB resistance at the USDA/ARS U.S. Horticultural Research Laboratory, Ft. Pierce, Florida. In: Proceedings of the 2nd International North American Plant Protection Organization (NAPPO) Conference on HLB, July 20, 2010, Merida, Mexico. Available: http://www.nappo.org/en/?sv=&category=Panel%20Docuents&title=Symposiums%20and%20Presentations

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. The emergence of huanglongbing (HLB) in the US has made development of HLB resistance citrus one of our priority breeding objectives. Strategies being used include developing GEOs (genetically engineered organisms) targeting the bacterium which causes HLB. Tests of HLB-resistance in tGEO citrus are currently underway. While all tested commercial citrus is reported to be susceptible to HLB, some varieties are slower to develop severe symptoms. In recent grove surveys where most sweet orange and ‘Minneola’ trees have HLB symptoms and high pathogen levels, fewer trees of ‘Temple’ display HLB symptoms or levels of pathogen associated with HLB disease. Numerous experiments are now underway at the USHRL, including replicated trials of cultivars with high exposure to HLB, assessment of HLB in breeding populations exposed to high disease pressure, and controlled psyllid challenges. Since varieties showing HLB tolerance or their parents are used frequently within our breeding program, we hope to identify many citrus scion types with greater HLB tolerance for near-term use. Continued observation that trifoliate orange and its hybrids are among the most resistant material to HLB and Asian citrus psyllid (ACP) offers the more promise of identifying hybrids with even greater resistance. Slower development of HLB could be due to a number of factors: attractiveness of trees to ACP, pathogen establishment at ACP feeding, pathogen growth following ACP inoculation, movement of pathogen throughout the plant, and development of plant responses observed as HLB symptoms. Reduction or slowing of any of these steps may slow disease development and spread, but have different implications in overall management and commercial significance. Careful consideration needs to be given to the value and implications of such tolerance.

Technical Abstract: 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. The emergence of huanglongbing (HLB) in the US has propelled the development of HLB resistance to the forefront of our breeding objectives. Transgenic strategies offer the potential to develop truly resistant citrus even when such a phenotype has yet to be clearly identified in citrus and associated to a specific citrus gene. Strategies being employed in the lab include genetic transformation to create trees producing antimicrobial peptides and other transgenes targeting Candidatus Liberibacter asiaticus (CLas) gene products, virulence mechanisms, or induce host plant resistance mechanisms. Both constitutive and phloem-specific promoters are being utilized. Tests of HLB-resistance in transgenic lines are currently underway. While all tested commercial citrus scion cultivars are reported to be susceptible to HLB, evidence suggests some cultivars are slower to develop severe symptoms. In recent grove surveys where most sweet orange and ‘Minneola’ trees have HLB symptoms and high CLas titers, fewer trees of ‘Temple’ display HLB symptoms or CLas associated with HLB disease. These observations have precipitated numerous experiments at the USHRL, including replicated trials of cultivars with high exposure to CLas, assessment of CLas/HLB in breeding populations exposed to high disease pressure, and controlled psyllid challenges. Since varieties showing lower CLas or their parents are used frequently within our breeding program, we hope to identify many citrus scion types with greater HLB tolerance for near-term deployment. Continued observation that Poncirus trifoliata and its hybrids are among the most resistant material to HLB and Asian citrus psyllid (ACP) offers the more distant promise of identifying advanced intergeneric hybrids with even greater resistance, and suitable fruit quality. Theoretically, slower development of HLB/CLas could be due to alteration in a number of components: attractiveness of trees to ACP, CLas establishment at ACP feeding, CLas proliferation following ACP inoculation, systemic movement of CLas with subsequent further proliferation, and development of plant responses observed as HLB symptoms. Reduction or slowing of any of these steps may slow disease development and spread, but have different implications in overall management and commercial significance. Careful consideration needs to be given to the value and implications of such tolerance.

Last Modified: 9/2/2014
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