Skip to main content

HLB Tolerant Hybrids as New Citrus Commercial Cultivars

HLB Tolerant Hybrids as New Citrus Commercial Cultivars




Citrus is the most economically important fruit tree crop in Florida, contributing substantially to employment, and providing cultural context for many generations of growers. It had an estimated on-tree fruit value of $1.5 billion for 2006-07 prior to HLB and declined to $0.82 billion for 2015-16 (USDA, 2017). The value-added processed orange and grapefruit juice brings the industry value to $8-9 billion. Florida citrus is mainly a juice industry for which the main product is orange (Citrus x sinensis (L) Osbeck) juice with 90% of the oranges being juiced. Sixty percent of grapefruit (C. paradisi MacFadyen) grown in the U.S. is produced in Florida, with about half going to the fresh market and half processed for juice. Unfortunately, citrus greening or huanglongbing (HLB) is devastating the Florida citrus industry and is a serious threat to other citrus-growing states, with HLB-finds accelerating in CA and TX. The disease is also a threat to other citrus growers worldwide, such as Brazil and China.

HLB is caused by a phloem-restricted bacterium, Candidatus Liberibacter asiaticus (CLas) that is spread by the insect vector Diaphorina citri (Asian citrus pysllid or ACP). Sweet orange cultivars, which are actually interspecific, complex hybrids, range widely in traits such as time of ripening, sugar/acid levels, aroma profiles and overall fruit/juice quality. Individual cultivars arose through mutations selected over generations (possibly millennia) of clonal propagation so they are almost genetically identical (Novelli et al., 2006). Sweet orange cultivars are the most widely planted tree fruits in the world (FAOSTAT, 2017) and dominate the Florida citrus processing industry and CA fresh fruit industry. Unfortunately, sweet orange is highly susceptible to HLB (Stover et al., 2011, 2016).

Largely due to the advent of HLB, Florida orange production has declined by over 70% in the last 10 years. If this trend is not reversed, processing plants will not have enough fruit to maintain profitability and will shut down. The loss of this valuable infrastructure cannot be easily recovered. Several major processing plants have closed, leaving 6 major plants open but operating well below capacity. There may not be enough fruit to justify operations for all of them next season. Packing houses have likewise declined from 106 to 26 in the last 16 years (hurricanes and disease). Therefore, it is vital to put HLB tolerant citrus trees into growers’ hands to maintain and ultimately increase yields. For this to happen our objective is to identify HLB-tolerant scion genotypes that will be acceptable for fresh fruit and juice varieties (either alone or in blends for juice) and permit profitable citrus production and maintaining infrastructure.

Florida law requires that orange juice, the main citrus product, be comprised of at least 90% sweet orange juice. Given the poor growth, cropping, and reduced fruit quality common in HLB-affected (CLas-infected) sweet orange, the Florida juice industry may want to reconsider this restriction. Transitioning to different cultivars with greater HLB-tolerance and ultimately resistance is critical to a sustainable Florida citrus juice industry, especially if orange-like and grapefruit-like juice, already demanded by consumers, can be maintained. Classification of some tolerant hybrids as “sweet orange” based on their volatile and perhaps non-volatile chemical profiles may be one avenue to get HLB-tolerant germplasm in the field for use as juice oranges, as was done for ‘Ambersweet’ in 1989 (Stover et al., 2016; Hearn, 1989).

This project is focused on identifying HLB-tolerant scion cultivars that can be quickly mobilized for use by the US citrus industry. We will 1) identify HLB-resistant/tolerant hybrids with good flavor quality (including orange-like and grapefruit-like); 2) Identify DNA markers associated with tolerance to expedite screening and provide confidence in selecting scion genotypes considered promising and explore tolerance-related responses to pathogen effectors/pathogen-associated molecular patterns (PAMPs), for use as a screening tool; 3) develop a transgenic strategy to enhance cultivars resistance responses; 4) characterize fruit and juice quality of HLB-tolerant scions, for use either as stand-alone varieties, hybrid classification as “orange” and/or development of juice combinations to supplement the current processing stream; and 5) communicate these results to facilitate industry implementation of HLB-tolerant scion cultivars.


  1. Develop markers to quickly identify hybrids with mandarin-derived and Poncirus-derived HLB tolerance.
  2. Utilization of HLB-tolerant citrus selection/cultivars in resynthesizing “orange-like” and “grapefruit-like” fresh fruits and juice products
  3. Education and outreach of the project theme, research progress and capabilities.


Elizabeth Baldwin Research Horticulturist US Horticultural Research Laboratory
2001 S Rock Rd
Ft Pierce, FL 34945
phone 772-462-5951
fax 772-462-5986
Program Director (PD)
Ed Stover Research Horticulturist US Horticultural Research Laboratory
2001 S Rock Rd
Ft Pierce, FL 34945
phone 772-462-5951
fax 772-462-5986
Co-PD, team lead for objective 1
Mikeal Roose Professor of Genetics University of California at Riverside (UCR) Department of Botany and Plant Sciences
University of California
Riverside, CA 92521
Co-PD, team lead for objective 1
Jinhe Bai Research Chemist US Horticultural Research Laboratory
2001 S Rock Rd
Ft Pierce, FL 34945
phone 772-462-5880
fax 772-462-5986
Co-PD, team lead for objective 2
Rhuanito Ferrarezi Extension Citriculturist University of Florida
Indian River Research and Education Center (UF)
2199 South Rock Road
Ft.Pierce Fl. 34945
Co-PD, team lead for objective 3
Fred Gmitter Plant Breeder and Professor University of Florida
Citrus Research and Education Center (UF)
700 Experiment Station Rd.
Lake Alfred, FL 33850
Co-PI, objective 1, breeding program provides materials for all objectives
Goutam Gupta Senior Research Scientist New Mexico Consortium (NMC) 100 Entrada Drive
Los Alamos, NM 87544
phone 505-412-6547
Co-PI, objective 1
Anne Plotto Research Plant Physiologist US Horticultural Research Laboratory
2001 S Rock Rd
Ft Pierce, FL 34945
phone 772-462-5844
fax 772-462-5986
Co-PI, objective 2, sensory studies
John Manthey Research Chemist US Horticultural Research Laboratory
2001 S Rock Rd
Ft Pierce, FL 34945
phone 772-462-5930
fax 772-462-5986
Co-PI, objective 2, analysis of limonoids and flavonoids
Yu Wang Flavor Chemist University of Florida
Citrus Research and Education Center (UF)
700 Experiment Station Rd.
Lake Alfred, FL 33850
Co-PI, objective 2, fruit/juice chemical and sensory quality


Richard Bennet - President, Bennet Farms Inc., Exeter, California.
Mr. Bennet is a veteran citrus grower, and has been chairman of the California New Variety Committee, Research Priority Committee, and president of California Citrus Research Board (CRB). CRB administers the California Citrus Research Program, a grower-funded and grower-directed program established in 1968 under the California Marketing Act as the mechanism enabling the state’s citrus producers to sponsor and support needed research. As president of CRB, Mr. Bennet and the leadership team made “controlling HLB” the number one goal. His involvement is useful for providing stakeholder input on the project.

Peter Chaires - Executive Director, New Varieties Development & Management Corp. (NVDMC), and of the Florida Citrus Packers, Maitland, Florida.
Mr. Chaires found edNVDMC in 2005, and the company provides financial and strategic direction to Florida citrus breeding efforts and facilitates the commercialization and introduction to industry. He also serves as Executive President of Florida Citrus Packers Association representing fresh citrus packing and shipping operations. He will provide input to the research and extension projects, as well as to help assess and evaluate the quality and potential outcomes and impacts.

James Gorden - President, California Citrus Research Board, Visalia, California and leading citrus grower.
Mr. Gorden’s primary areas of professional involvement have been integrated pest management and citriculture, and he is a recognized international expert in those fields. For more than 35 years, he was a partner in Pest Management Associates, which was the first independent citrus pest management consulting company providing service to the San Joaquin Valley. Mr. Gorden also has been involved in other leadership positions within the citrus industry and associations. He has served on the California Citrus Pest and Disease Prevention Committee since its inception and continues to remain heavily involved for the last eight years with the California Citrus Nursery BoardMr. Gorden is well aware of his fellow citrus growers’ desire for tools to help mitigate the threat posed by HLB disease to their liveihoods. He will provide stakeholder input to the research project.

Dale Murden - President, Texas Citrus Mutual, Mission, Texas and citrus grower.
Mr. Murden is also serving as executive director of the Texas Citrus and Disease Corp. and USDA Agriculture Trade Advisory Committee for Fruits and Vegetables (ATAC). He previously was president of Texas A&M Citrus Center Advisory Board. His background and involvement in the state’s citrus industry set a strong foundation to lead the citrus industry in this challenging time with citrus greening disease. He is a citrus grower and has been in the Texas citrus industry since graduating high school in 1980 when he started as an integrated pest management scout for Texas A&M University. Mr. Murden will provide input to the research and extension projects as well as to evaluate quality and potential outcomes and impact.

Etienne Rabe - VP Agronomy, Wonderful Citrus, Delano, California.
Dr. Rabe is the Chairman of California Citrus Research Board (CRB) and a well-known and respected citrus professional throughout the global citrus industry. He has also been serving in various citrus states and Federal committees involved with HLB-related aspects. Dr. Rabe has a strong scientific background and can evaluate the potential needs and outcomes from an industry point of view, but also the validity of the scientific approaches for this proposed project. Dr. Rabe got his MS and PhD from the University of Pretoria, and eventually became a professor of Horticulture and at the University of Stellenbosch and moved to California to join Sun Pacific Farming.

Gregory Nelson - Greg Nelson is the President of Bernard Egan & Company, a family owned business founded in 1914.
The Company has grown to include over 14,000 acres of citrus in Florida and is a leading shipper of quality, fresh citrus in North America and overseas. Greg has been with the Company for the past thirty-two years and has been President since 1993. Because the Company is a vertically integrated company, he is involved with growing and packing operations as well as marketing. He is a Director of Florida Citrus Mutual, Florida Citrus Administrative Committee, the Indian River Citrus League among other citrus organizations.

Mike Irey - Director, Research and Business Development, Southern Gardens Citrus, Clewiston, Florida.
Southern Gardens Citrus is a grower/processor operation. Mr. Irey is a Plant Pathologist, and has represented the citrus growers and processors in joint efforts of the government-research-industry to battle HLB. He has held various positions including, Director of Plant Pathology/Molecular Biology, Director of Research and most recently Director of Research and Business Development. He was a research scientist with USDA/ARS for several years and has extensive experience with disease diagnosis, disease management and the development of transgenic crops. Since 2006, he has been the science lead in two projects to develop genetically engineered citrus resistant to HLB and to develop a genetically engineered biopesticide to control HLB. His group also runs one of two labs that are available to industry growers and researchers for the diagnosis of HLB, running about 30,000 assays per year. He can bridge the gap between science and industry.

Nick Kretchman - Coca-Cola Juice Research & Development Center, Coca-Cola, Apopka, Florida.
Nick Kretchman has been working in the Citrus industry in different capacities over the last 16 years. He has a B.S. in Food Science from the University of Florida. Nick started his career at Tropicana Products in Bradenton, Florida, where he supported Tropicana Pure Premium, and later worked on International Tropicana, supporting product launches in countries including India, Saudi Arabia, Lebanon, and the Philippines. Nick joined Coca-Cola in 2007, and has supported the Simply Orange and Minute Maid juices brands since that time. Nick lead the company’s research on the impacts of HLB on the quality of juice, oil, and pulp across all citrus types (Orange, Grapefruit, Lemon, and Lime) across many growing regions around the globe. Nick’s focus remains on maintaining quality and supply continuity for Coca-Cola’s citrus juice brands. Nick and his wife Nicole have a son (Benjamin) and a daughter (Daniela). Nick enjoys youth sports, and is on the board of directors for Windermere Little League, where his son plays baseball.

Sequences in which activites are to be performed

Tasks (See detailed timeline for personnel role, metrics and evaluation plan)
Year 1
Year 2
Year 3
Year 4
Objective 1: Develop markers to quickly identify hybrids with mandarin-derived and Poncirus-derived HLB tolerance  
Measurement of tree growth, health index, bacterial titer and disease symptom rating        
Genotyping-by-sequencing analysis of parents and hybrids        
Genotyping using dense SNP markers in segregating F1, F2 and other hybrids        
RNA-seq to correlate with identified markers for disease resistance-related genes        
Test range of hybrids using DNA and RNA markers        
Compile information on most promising selections for outreach        
Data analysis, project conclusions, and publications        
Objective 2: Accelerate screening of HLB tolerant materials focusing on interactions between citrus genes and identified CLas genes, to enhance citrus immune responsiveness to CLas  
Identify highly expressed CLas effectors and citrus response genes        
Correlate CLas effector response with DNA tolerance markers in citrus populations        
In silico design of citrus FLS2 proteins with greater CLas affinity        
Transform citrus with improved FLS2        
Challenge FLS2 transgenics and assess CLas levels and defense responses        
Data analysis, project conclusions, and publications        
Objective 3: Utilization of HLB-tolerant citrus selection/cultivars in resynthesizing “orange-like” and “grapefruit-like” fresh fruits and juice products  
Identify HLB tolerant/resistant varieties        
Collect fruit quality data on HLB tolerant/resistant varieties        
Analyze volatile data to identify stand-alone processing types and blends like OJ and GFJ        
Taste-test juice and juice blends from identified tolerant/resistant selections        
Compile information on most promising selections for outreach        
Data analysis, project conclusions, and publications        
Objective 4: Education and outreach of the project theme, research progress and capabilities  
Create and maintain website containing interactive resources        
Share information at field days, workshops, and conferences        
Extension publications        
Citrus scion selection guide for Apple and Android users        

Logic Model Chart

Participating Organizations



USDA. Florida Citrus Statistics 2015-16. In SERVICES, F. D. O. A. A. C., Ed. Tallahassee, Florida, 2017.

Novelli, V. M.; Cristofani, M.; Souza, A. A.; Machado, M. A. Development and characterization of polymorphic microsatellite markers for the sweet orange (Citrus sinensis L. Osbeck). Genet. Mol. Biol. 2006, 29, 90-96.

FAOSTAT. In Nations, F. a. A. O. o. t. U., Ed. 2017.

Stover, E.; McCollum, G. Incidence and Severity of Huanglongbing and Candidatus Liberibacter asiaticus Titer among Field-infected Citrus Cultivars. Hortscience 2011, 46, 1344-1348.

Stover, E.; Inch, S.; Richardson, M. L.; Hall, D. G. Conventional Citrus of Some Scion/Rootstock Combinations Show Field Tolerance under High Huanglongbing Disease Pressure. Hortscience 2016, 51, 127-132.

Hearn, C. J. Yield and Fruit Quality of Ambersweet Orange Hybrid on Different Rootstocks. Proc. Fla. State. Hort. Soc. 1989, 75-78.