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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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Summary

Huanglongbing1 (HLB) or citrus greening, a disease first observed more than a hundred years ago in Asia, is the most serious threat to the citrus-growing industry worldwide due to its complexity, destructiveness, and incalcitrance to management. First detected in Florida in 2005, HLB is now widespread in the state and threatens the survival of the Florida citrus industry despite substantial allocation of research funds by Florida citrus growers and federal and state agencies.

In Florida, HLB research is overseen by the Citrus Research and Development Foundation (CRDF), a nonprofit corporation created in 2009 through the initiative of the state’s citrus industry. In 2010, CRDF began managing projects addressing the research and technology recommendations in the 2010 National Research Council (NRC) report Strategic Planning for the Florida Citrus Industry: Addressing Citrus Greening Disease. From 2010 to 2017, CRDF awarded about $124 million to 398 projects, of which nearly 90% focused on HLB. Because the research funded to date has not produced major breakthroughs in controlling HLB, CRDF contacted the Board on Agriculture and Natural Resources of the National Academies of Sciences, Engineering, and Medicine in October 2016 to request an independent review of CRDF-funded research. The overall purpose of the review was to identify ways to reconfigure HLB research to acceler-

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1 Huanglongbing (HLB), which means yellow (huang) shoot (long) disease (bing), was unanimously adopted as the official name of the disease by the International Organization of Citrus Virologists (IOCV) at the 13th Conference of IOCV in Fuzhou, China, in 1995 (http://iocv.org/huanglongbing.htm; accessed February 22, 2018).

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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ate the development of tools and strategies to abate the damage caused by HLB and prevent the collapse of the Florida citrus industry (see Chapter 1, Box 1-1, for the full statement of task).

In its review, the committee found that research supported by CRDF and other agencies has expanded knowledge of every aspect of HLB, yet there have been no breakthroughs in HLB management. The reasons for the lack of breakthroughs in HLB management, despite the investments in research, are complex. Other than research on the Asian citrus psyllid (ACP) in Florida, most of the available information on HLB prior to 2005 was based primarily on research performed outside the United States so researchers faced a steep learning curve. The disease itself is intractable for a variety of reasons related to the citrus host (its perennial nature, the lack of resistance in any citrus relative, and the difficulty of breeding to produce HLB-resistant cultivars); the pathogen (especially the inability to be cultured in the laboratory); the insect vector (and its major role in transmission); the complexity of pathogen, vector, and host interactions; the lack of a good model system; as well as the current approach to HLB research.

CURRENT KNOWLEDGE OF HUANGLONGBING PATHOGEN, VECTOR, AND HOST AND THEIR INTERACTIONS

HLB is a disease of citrus associated with bacteria that are spread by a sucking insect, the Asian citrus psyllid. The following presents the current state of understanding of the disease system and the factors that influence its occurrence, severity, and incalcitrance to effective management.

Candidatus Liberibacter asiaticus

Three bacteria are known to be associated with HLB, Candidatus Liberibacter asiaticus (CLas), Ca. Liberibacter africanus (CLaf), and Ca. Liberibacter americanus (CLam). They are phloem-restricted in planta; because they are nonculturable they cannot be characterized to the extent required for official genus status, hence their “Candidatus” status. HLB in Florida is attributed to CLas, but because Koch’s postulates cannot be tested much of what is known or hypothesized about its pathogenicity has been deduced from multiple genome sequences of the three HLB-associated bacteria.

Asian Citrus Psyllid

The HLB vector in Florida is the Asian citrus psyllid, Diaphorina citri Kuwayama. Development from egg to adult requires 2 to 7 weeks, adults live several months, and there are 9 or 10 generations per year.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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Adults fly 25 to 50 m regularly, but can fly up to 100 m toward new leaf flushes. Movement into managed citrus groves in Florida is often from adjacent abandoned groves. Trapping data indicate no consistent ACP seasonal movement patterns, necessitating continuous monitoring.

Vector–Host Interactions

ACP infests species in at least 10 genera in the family Rutaceae (which contains about 160 genera including Citrus) of variable suitability for oviposition, development, and reproduction. Multimodal sensory inputs contribute to host finding; ACP orient to leaf flush via volatile signals and assess plant suitability using gustatory and visual cues. Ovipositing females prefer the plant species on which they developed, but preferences can shift after exposure to an alternative host. Adults are attracted to colors within the reflectance spectra of rutaceous plants. Infected plants are more attractive to ACP due to HLB-induced volatiles. However, infected trees are less suitable for ACP development, and psyllids leave infected plants shortly after pathogen acquisition, promoting pathogen spread.

Host–Pathogen Interactions and Host Plant Defense Mechanisms

Within citrus sieve tubes Ca. Liberibacters exploit cellular processes for nutrient acquisition, encoding transporters and enzymes for metabolizing host-derived nutrients. CLas effector proteins are predicted to modulate host cellular functions and disable defense reactions, benefiting CLas multiplication and phloem colonization. Citrus and its relatives reprogram their transcriptomic network in response to HLB.

Pathogen–Vector Interactions

Early-instar nymphs acquire CLas more efficiently than adults and are important in transmission. Bacterial titers in the insect change over time in ways suggestive of CLas replication in nymphs but not in adults. The pathogen survives for long periods in the insect and can be vertically transmitted at low frequency.

Diagnostics and HLB Management

Sensitive, cost-effective detection of CLas infections in citrus and ACP is critical for HLB management. Currently quantitative polymerase chain reaction (qPCR), along with conventional PCR and DNA sequencing, is used by accredited laboratories for verifying CLas infection. However, the main diagnostic challenge is sampling, which is hampered by uneven CLas

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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spatial and temporal distribution in trees and insects and the long lag time between inoculation and bacterial detectability.

Citrus Health Management Areas (CHMAs) were established in Florida to promote regional coordination of insecticide applications, but their effectiveness is limited by low grower participation in some areas. Models predict that reducing ACP populations during critical times can decrease HLB spread, but even 100% CHMA participation would not likely slow HLB progression because refugia for vector and pathogen exist in abandoned groves and backyard trees. Other management methods include use of reflective mulch to repel ACP, thermotherapy for small trees in groves or greenhouses, and manipulation of citrus nutrition and irrigation to enhance tree health. Although peer-reviewed literature on these practices is scant, the underlying logic is that stressed trees are more vulnerable to HLB.

Citrus Genetics, Breeding, and Biotechnology

Many generations of breeding and selection are required to develop new citrus cultivars having specific qualities. The long juvenility period for citrus grown from seed and the need for large land areas to grow and evaluate trees add to the challenges of conventional breeding. Genetic engineering can potentially circumvent some of these obstacles, but the introduction of genes into citrus (genetic transformation) is specific to genus and species and is limited to relatively few genotypes.

HLB RESEARCH AND DEVELOPMENT EFFORTS FUNDED BY CRDF

Eradication of HLB-infected and nearby trees in Florida eliminated one-tenth of citrus production capacity by 2008 and the disease was estimated to have caused the loss of thousands of jobs and over $1 billion in grower revenue. CRDF has strived to halt or reverse industry losses by targeting research on all disease system components. A summary of research efforts supported by CRDF and other state and federal agencies is provided below. The broad, diverse research portfolio was the central focus of the committee’s review.

HLB Pathogen, Vector, and Hosts

Research on HLB Causal or Associated Bacteria

CRDF-funded projects included efforts to culture CLas in vitro and to culture Liberibacter crescens as a CLas proxy system. Genome sequencing for CLas, CLam, and CLaf was funded, as were studies to identify HLB

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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cooperative or antagonistic microbes in the citrus microbiome. Other research focused on microbial products for ACP management and a phage-suppressing protein from an ACP endosymbiont for manipulating CLas behavior. The National Institute of Food and Agriculture Specialty Crop Research Initiative (NIFA-SCRI) supported projects to culture CLas; identify CLas-secreted proteins for targeting by plant proteases; and characterize roles of genetic regulators in CLas persistence in citrus. The Citrus Research Board (CRB) funded identification of CLas-secreted proteins for developing bacterial inhibition strategies. The Foundation for Food and Agriculture and Southern Gardens Citrus are funding the development of a root-based bioassay for culturing studies and antimicrobial screening.

Research on HLB Vector: Asian Citrus Psyllid

CRDF funded projects to characterize ACP dispersal and reproduction behavior, including measuring psyllid responses to abiotic factors to optimize trapping, predicting ACP attack, and evaluating seasonality and frequency of dispersal. NIFA funds research on the proteome and transcriptome of ACP alimentary canal, midgut, and salivary glands with and without CLas and the development of methods to generate vector-incompetent transgenic ACP. NIFA supports work toward the completion of the ACP genome. CRDF and NIFA funded the construction of complementary DNA (cDNA) libraries from CLas-infected and CLas-free ACP to characterize gene expression profiles. Several federal agencies, with CRB, supported construction of transcriptomes of egg, nymph, and adult ACP. NIFA supported annotation to complement ACP genome sequencing by the National Institutes of Health National Center for Research Resources, U.S. Department of Agriculture (USDA) Agricultural Research Service, and CRB.

Research on HLB Vector–Host Interactions

CRDF funded projects to screen citrus germplasm for traits affecting ACP attractiveness, quantify impacts of citrus species and flushing on ACP traits, and identify ACP overwintering habitats and alternative hosts. Other studies were designed to understand Cleopatra mandarin and trifoliate orange resistance to ACP and CLas, respectively; to identify semiochemicals impacting psyllid–host interactions; and to examine plant structures interfering with ACP feeding. Also studied were attractants that influence ACP flight, color preferences, and the effect of host genus on ACP; signals produced by ACP; ACP responses to volatiles produced on ACP-damaged foliage or released by CLas-infected citrus; and impacts of ACP experience on recognition of host stimuli. Host plant flush availability was evaluated, as

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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were movements between managed and abandoned groves, geographic barriers, and wind direction. The effects of abiotic factors on psyllid movement, feeding, infection status, fecundity, and population density were assessed. NIFA funds research on ACP responses to CLas infection. CRB supported studies of ACP responses to volatiles produced by flushing shoots, identification of volatiles for surveillance and management, odor coding in ACP, and mitogenome analysis to identify region of origin for ACP in California.

Research on HLB Host: Citrus

CRDF supported research on citrus genetic engineering, gene discovery, and genetic mapping. No source of high-level resistance to HLB is known, but genetically engineered (GE) resistance is under development with genes from citrus and noncitrus sources. Transgenic rootstocks have been produced using juvenile tissues, and there are efforts to transform mature tissues. Several projects attempted to create transgenic citrus expressing antimicrobial proteins, disease resistance genes, anti-CLas antibodies, regeneration and transformation-associated genes, or promoters targeting specific tissues. Efforts to edit clustered regularly interspersed short palindromic repeats (or CRISPR) were also funded by CRDF. Projects on HLB tolerance/resistance gene discovery through differential expression in susceptible and tolerant genotypes were supported, as was the use of a Citrus tristeza virus (CTV) vector to deliver and express resistance transgenes in citrus. CRDF supports a citrus transformation facility (Lake Alfred, Florida), which provides services and resources for testing gene or promoter efficacy, and the Picos Farm (Fort Pierce, Florida), a site critical for long-term testing of HLB resistance and tree performance, and for developing data necessary for regulatory approval. NIFA and CRB fund projects to identify candidate resistance genes in resistant or tolerant citrus relatives for gene editing to produce non-GE HLB-resistant citrus cultivars.

Research on HLB Pathogen–Vector Interactions

CRDF-funded projects aimed to characterize and interrupt CLas movement within, and interactions with, ACP. Other efforts are to identify molecules interfering with ACP transmission, evaluate the ability of other bacteria to trigger ACP immune responses, identify ACP gut receptors for key toxins, identify quorum-sensing compounds that disrupt CLas communication and biofilms, and induce the CLas phage lytic phase within ACP. CTV delivery of ribonucleic acid interference (RNAi) to target ACP, enhancement of transgene expression in the CTV vector, and RNAi strategies that target genes linked to ACP survival or CLas transmission were investigated. The NuPsyllid project aimed to generate a GE ACP, unable

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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to transmit CLas, to displace wild ACP populations. The USDA Animal and Plant Health Inspection Service Multi-Agency Coordination (MAC) group supports projects focused on ACP management and the use of thermotherapy to reduce ACP acquisition and transmission of CLas. NIFA and CRB funded study of the genetics of ACP transmission competency and characterization of ACP proteins regulating CLas movement and survival in the insect. Projects to integrate and curate omics data on ACP, citrus, and CLas and to use these data, along with de novo generated genomic, proteomic, and metabolomics data, to discover molecules that can block pathogenesis and transmission pathways were also funded. CRB funds projects to investigate transmission efficiencies among ACP populations, identify proteins involved in ACP–CLas interactions, and characterize factors influencing transmission efficiency.

Research on HLB Host–Pathogen Interactions

CRDF funded projects aimed at identifying host genes highly upregulated in resistant versus susceptible citrus varieties; identifying and mapping HLB resistance gene(s) in Poncirus; curating genomic sequences of CLas, ACP, and citrus; and bioinformatically analyzing proteins of each species to predict their interactions. Other CRDF projects investigated host gene–pathogen effector interactions, putative host target proteins, and enzyme activity in HLB pathogenesis. NIFA-SCRI funded efforts to identify and characterize the roles of Liberibacter spp. effectors in HLB pathogenesis and to identify candidate host genes for HLB resistance or tolerance. CRB supports research to identify and characterize CLas small RNAs, messenger RNAs, and citrus targets.

HLB Management

Research on Bacterial Control

CRDF has supported research on the use of bactericidal or bacteriostatic chemicals or antibiotics, thermotherapy, biocontrol using CLas bacteriophage, nutritional and microbiome enhancements, and induction of citrus defense responses. Research foci included expression in Arabidopsis of CLas signaling and defense marker genes, and evaluation of drug-like molecules for gene induction, using a Ca. Liberibacter psyllaurous proxy system. Research also aimed to create Arabidopsis mutants lacking specific defense mechanisms to determine the basis for disease tolerance, investigate suppression of host defenses by CLas salicylate hydroxylase, and determine effects of CLas flagellin, a protein involved in host defense induction. CRDF funded the evaluation of zinc-based formulations, nonmetal antibiotics,

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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and tetracycline derivatives against CLas. Hundreds of chemical formulations were tested for activity against L. crescens. Work to enhance delivery of antibacterial compounds into the plant, to evaluate their movement in planta, and to optimize methods of field assessment were also supported, as were projects on thermotherapy, alone or with chemo- and nutrient therapy, against HLB. The use of a psyllid repressor protein as a phage cycle regulator, and CLas phage peroxidase and phages from Xanthomonas axonopodis pv. citri for HLB therapy, are being investigated. The impact of citrus microbial or phytobiome communities on tree health and HLB resistance was examined.

Research on Insect Control (Chemical and Biological)

Major research themes have included insecticide testing against all psyllid stages, integration of insecticides into integrated pest management programs, documentation of approaches for organic orchards, examination of attractant and repellent compounds, and optimization of ACP sampling protocols for justifying pesticide use. Also supported was evaluation of indigenous ACP predators and parasites, including the wasp Tamarixia radiata and entomopathogens. Chemical and biological control programs supported by CRDF were complemented with funding from the MAC program and CRB.

Research on Cultural Control

CRDF funded projects investigating effects of nutritional supplements on plant growth and development and on root and soil health, as well as use of high-density plantings, mulches, and psyllid-proof enclosures to control ACP. MAC has supported research on cultural management, such as removing abandoned groves, reducing irrigation water pH, applying thermotherapy or mulches, and intensive grove management.

Research on Diagnostics

No current CRDF-funded projects directly address CLas diagnostics, but a past project investigated optical sensing to screen seedlings for HLB resistance. CRB has funded research on metabolomics and proteomic biomarkers in CLas-infected trees and conventional detection methods. MAC funded research on canine detection and antibody-based early detection, overlapping with a NIFA-supported effort to identify CLas-secreted effector molecules. NIFA funded projects to use yeast biosensors and loop-mediated isothermal amplification (or LAMP) technology to detect CLas in ACP. Development of technologies for detecting infection prior to symptom appearance, funded by other state and federal agencies, relies upon postinfection

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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spectral reflectance changes, changes in plant-produced volatiles released, or changes in metabolic, proteomic, RNA, and microbial population profiles due to infection.

The analysis of the CRDF research portfolio indicated that CRDF has not directly funded research on economic and sociological factors associated with citrus production and HLB management and their impacts on decision making by growers, processors, and the public. These factors influence the likelihood of implementation and success of future HLB management approaches, as shown by studies, funded by other agencies, on grower participation in CHMAs as well as surveys on grower willingness to plant GE citrus and consumer willingness to purchase GE products.

NOTABLE OUTCOMES, PITFALLS, AND FUTURE DIRECTIONS

Key research findings and recommended future research are listed here to highlight areas in which the committee found that progress had been made and to point to research efforts the committee believes should be continued or initiated.

Biology and Ecology of the HLB Causal/Associated Bacteria and Its Insect Vector

Key Research Findings

  • Numerous CLas genomes worldwide were sequenced, revealing mutation patterns and potential control targets and allowing for comparative genomic studies.
  • CLas-killing bacteriophages were characterized and factors that suppress them were identified.
  • Liberibacter crescens, a culturable CLas relative, was developed as a proxy system to study pathogen–host interactions.
  • ACP biology and reproductive behavior were characterized extensively.
  • ACP genome and transcriptome annotation led to discoveries that can reveal control points at the vector–pathogen interface.
  • Emerging biotechnologies, such as RNAi, offer mechanisms to achieve new, sustainable, and environmentally friendly ACP management.
  • Elucidation of ACP seasonal activity patterns and abundance have facilitated effective targeting and timing for management efforts.

Recommendations for Future Research

  • Sequence additional CLas isolates, monitoring for changes that have altered or could alter virulence or the efficacy of control strategies.
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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  • Continue to study bacteriophage-suppressing factors and essential CLas-encoded proteins produced in or secreted by the plant to identify control targets.
  • As more CLas proteins or compounds are confirmed as essential and produced in or secreted by the plant, shift the research focus to identifying plant-compatible strategies to inactivate or recognize them.
  • Develop protocols for ACP DNA recovery from traps to facilitate evaluation of population structure, pesticide resistance, and other biological characteristics.

Interactions of HLB Pathogen, Vector, and Host

Key Research Findings

  • Genome sequencing of Ca. Liberibacter strains allowed identification of candidate effector genes that may be involved in pathogen–plant interactions.
  • Evidence was found for disease-promoting roles for at least five CLas virulence factors that are potential control targets.
  • Resistant or tolerant citrus varieties highly upregulate host genes, including some that suppress plant immunity mechanisms and cause metabolic dysfunction, in response to Ca. Liberibacter infection.
  • Nymphal stages of ACP acquire CLas quickly and are important in CLas transmission and dissemination.
  • CLas replicates in ACP and is vertically transmitted at low levels.
  • Mechanisms and pathways of circulative transmission of CLas in ACP were elucidated.
  • CLas alters citrus and ACP biology, impacting ACP dispersal and mating behavior and the attractiveness of trees to ACP.

Recommendations for Future Research

  • Characterize additional CLas effectors and identify and functionally analyze host targets, applying new knowledge to novel HLB management tools.
  • Identify and characterize new critical citrus genes or gene products that are targets of CLas.
  • Seek new resistance genes, in citrus or other species, that counteract CLas effectors.
  • Identify new molecules that interfere with the CLas life cycle, leading to titer reductions in citrus or ACP.
  • Identify new molecules that hamper ACP transmission of CLas.
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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  • Explore strategies for physical protection of trees against ACP, including repellents, mulches, and screens.

HLB Management

Key Research Findings

Bacterial Control

  • New approaches developed for bacterial control (nanoparticle or nanoemulsion formulations, addition of adjuvants, use of chemical mixtures, combining chemical treatments with thermotherapy, or triggering host plant defense mechanisms) can enhance treatment effectiveness and minimize the amount of active antibacterial chemical needed.

Vector Control

  • Vector management remains important in Florida, as repeated inoculations speed disease development and increase symptom severity.
  • Nearly all insecticides available, and many in development, were evaluated against ACP in Florida.
  • Pesticide resistance management plans were developed and are in use in some area-wide programs.
  • The benefits of indigenous ACP predators and parasites were evaluated and quantified.
  • The imported psyllid parasite Tamarixia radiata was reared and released throughout Florida; its impact has been greatest in urban settings and abandoned orchards.

Host Resistance: Breeding

  • HLB susceptibility varies with citrus cultivar and rootstock.
  • HLB-tolerant citrus rootstock and scion genotypes and HLB-resistant citrus relatives were identified, providing material for breeding and a temporary production bridge for the industry.
  • Potentially useful levels of resistance were identified in citrus relatives for incorporation into breeding programs.

Host Resistance: Genetic Engineering

  • Numerous transgenic citrus expressing genes that may confer HLB resistance were produced and are being tested.
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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  • Genomes of a number of citrus and citrus relatives were sequenced, yielding information on host responses to HLB and processes that may be manipulated for HLB management.

Cultural Control

  • Optimizing fertilization and irrigation can result in short-term tree health improvement and reduce HLB impacts on fruit yield but has no curing effect, is expensive, and is likely unsustainable.
  • Increasing citrus density in orchards can decrease the rate of HLB spread.
  • Multiple inoculations of infected trees exacerbate HLB impact on fruit yield and quality.

Diagnostics

  • Molecular and serological diagnostic technologies for CLas are ultrasensitive, but their use for epidemiological and regulatory applications are hampered by uneven pathogen distribution in the tree.
  • No single diagnostic method will be sufficient to identify recently infected trees.
  • Detection of infection prior to symptom development is possible through detection of changes in host metabolites and volatiles.

Recommendations for Future Research

  • Determine the most effective bacterial targeting strategies for transgenic plant development and support their development in citrus.
  • Explore novel chemical therapies; chemical genetics approaches can help in identifying key candidates. Look at chemicals that act as regulators or intermediates in host defense responses.
  • Implement field testing to determine actual impacts of chemical applications on disease severity and fruit production and develop predictive disease impact models to support decision making about this management approach.
  • Focus, in Florida, on managing HLB as a chronic problem in which incremental improvements in control have value; even minor improvements should be considered.
  • Evaluate the effects of new cultural management approaches (high-density plantings and nutritional supplementation) on tree infection, fruit production, and pathogen acquisition/transmission by ACP.
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
  • Explore the effectiveness of new pesticidal chemicals, particularly those providing ACP repellency or having minimal effects on biocontrol agents, on HLB incidence.
  • Investigate new parasites/predators of ACP as possible psyllid biocontrol agents; the parasite currently established in Florida is useful but provides inadequate control on its own.
  • Continue to explore the use of biomarkers associated with HLB-diseased trees as tools for early diagnosis, comparing these to volatile organic compounds.

OVERARCHING FINDINGS, CONCLUSIONS, AND RECOMMENDATIONS

To help achieve progress in finding a viable solution to HLB, the committee provided overarching findings, conclusions, and recommendations (directed at all agencies that fund HLB research unless CRDF is specifically mentioned) related to other factors that can affect the adoption of HLB management practices, tracking research progress, fostering communication among researchers, and coordination among all funders of HLB research. Detailed recommendations are presented in Chapters 3 and 4.

Finding 3.1: CRDF support for HLB research is responsive to several recommendations contained in the 2010 NRC report, particularly to 8 of the 11 near- to intermediate-term (NI) research and technology recommendations NI-1, NI-4, NI-5, NI-6, NI-7, NI-8, NI-9, and NI-10, and long-term recommendations L-1, L-2, and L-3 (see full text of the 2010 NRC recommendations in Chapter 1, Table 1-1).

Finding 3.2: Other agencies are funding efforts to address recommendations NI-2 and NI-11.

Finding 3.3: Recommendation NI-3: Establish citrus orchard test plots for evaluation of new scouting and therapeutic methods remains to be addressed.

Conclusion 3.3: Although CRDF supports Picos Farm, a secure transgenic field test site in Florida, additional sites are needed for assessment and validation of scouting and therapeutic approaches for HLB.

Recommendation 3.3: CRDF should consider establishing an infrastructure enhancement project to assess field testing needs for all citrus disease and insect research and validation activities and to design plans to meet those needs by enhancing current field test sites, establishing new field test sites, and/or developing collaborations with citrus growers to use production orchards for testing.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
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Finding 3.4: A single breakthrough discovery for managing HLB in Florida in the future is unlikely, since intensive research efforts over almost 20 years have not led to this result.

Conclusion 3.4: Finding the best combinations of control strategies suited to different environmental and growing conditions, vector and pathogen pressure, tree cultivars, and tree health would help growers in Florida and in other states where HLB is not yet a chronic problem but soon could be, especially if HLB infection is not detected quickly and if there is reluctance to remove inoculum sources.

Recommendation 3.4: Consider specific funding for the development of sets of management approaches that can be combined in different ways, optimized, and validated for use in different locations and conditions.

Finding 3.5: Plant pathologists, sociologists, and economists are using modeling to assess the complex interactions that characterize HLB; however, no CRDF funding has directly supported research on economic and sociological factors that impact decision making and behaviors of growers, processors, and the public and can influence the adoption and success of future HLB management efforts.

Conclusion 3.5: Greater investment and researcher involvement are needed to develop and apply modeling technologies for analysis and prediction of the effects of economic and sociological factors on the acceptance and application of HLB management practices.

Recommendation 3.5.1: CRDF should consider adding these research areas to its funding portfolio.

Recommendation 3.5.2: CRDF should consider creating centralized, researcher-accessible databases to support sociological and economic modeling of HLB-related research outcomes and application projections. It should support systems approaches for field testing combinations of the most promising developments in replicated field studies, emphasizing the need to collect research data sufficient to inform and support model training and applications to effectively predict cost–benefit ratios of all HLB management strategies.

Finding 3.6: Recent and growing interest in research using genetic modification to develop HLB-resistant citrus, and concerns about stakeholder acceptance of these technologies, indicate that expanded efforts in educational outreach to growers, processors, and consumers could facilitate eventual deployment of new citrus lines. Data from previous advertising strategies directed at adjusting consumer attitudes about citrus consumption have demonstrated that such communication with the public can change behaviors.

Conclusion 3.6: Further research is needed to assess the level of current stakeholder understanding of genetic modification technology, and the fac-

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×

tors that influence their willingness to purchase genetically modified (GM) foods.

Recommendation 3.6: CRDF should consider funding research to assess consumer attitudes toward genetic modification technologies for producing HLB-resistant citrus cultivars and their willingness to consume GM citrus, as well as how targeted advertising campaigns could be effective outreach strategies.

Finding 4.1: Although research supported by CRDF and other agencies has advanced our knowledge of HLB since 2010, the disease remains an intractable threat to the Florida citrus industry and has progressed from an acute to a chronic disease present throughout the state.

Finding 4.2: A number of technical obstacles have been addressed by funded projects but continue to represent significant barriers to research progress and the generation of HLB solutions.

Conclusion 4.1: Citrus growers, particularly in Florida, still need short-term solutions for the industry to remain viable while researchers continue to generate longer-term approaches for managing HLB.

Recommendation 4.1: Continue support for both basic and applied research and both short- and long-term research efforts.

Conclusion 4.2: Longer-term HLB solutions are likely to involve citrus variety improvement, derived primarily from new molecular techniques.

Recommendation 4.2: Continue to support the development and application of gene modification, including gene editing, focusing on targets mediating molecular interactions among plant, bacteria, and vector.

Conclusion 4.3: HLB research is hampered by the lack of standardized methods and parameters for measuring, evaluating, and analyzing factors including vector transmission rates, fruit yield, plant tolerance/resistance, citrus variety performance, antibacterial compound effectiveness, and diagnostic assay evaluation. Inconsistency in experimental designs, sampling methods, and field investigations limit the ability to compare findings and use previous research as a springboard for further exploration.

Recommendation 4.3: Support the development of community-accepted standards for the conduct, evaluation, and assessment of research to facilitate comparisons of research results across teams and institutions.

Finding 4.3: Novel approaches to foster communication, collaboration, and innovation among HLB researchers and representatives of funding agencies and the citrus industry may advance progress and facilitate solutions to HLB.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×

Finding 4.4: There are inconsistencies in the format, content, and frequency of CRDF-funded research progress reporting by researchers, as well as in the inclusion of specific outcomes, impacts, and products.

Conclusion 4.4: Improved reporting consistency is needed to reduce constraints in reviewing research progress and delays in applying new information to HLB solutions.

Recommendation 4.4: CRDF should develop a standardized format, procedure, and timeline for mandatory reporting of midterm project progress and final reports, to include publications and presentations, outcomes, practical applications, and impacts. CRDF should consider hiring a staff person to review and analyze HLB research findings annually.

Finding 4.5: More timely publication of research results in refereed scientific journals and trade journals would facilitate communication among the research community and between researchers and growers and support research assessment efforts.

Finding 4.6.1: Engaging with a disease that threatens the survival of an industry and requires a short-term and sustainable solution could benefit from a nonacademic research model or approach.

Finding 4.6.2: Despite the commendable efforts of multiple funding agencies to coordinate funding and encourage appropriate interstate, interagency, and interdisciplinary collaborations, decisions about research funding priorities and allocations occur largely within the domain of each agency.

Conclusion 4.6: The committee concludes that the current system of research prioritization and funding, accomplished primarily within each relevant funding agency, is not optimally efficient and has not led to the development of an overarching master plan for HLB research and its translation to management solutions.

Recommendation 4.6: CRDF should consider working, together with representatives of other agencies at the national and state levels, to create an overarching HLB research advisory panel to develop a fresh systems approach to HLB research prioritization and the strategic distribution of resources for research leading to effective HLB management.

Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 5
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 6
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 7
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 8
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 9
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 10
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 11
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 12
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 13
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 14
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 15
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 16
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 17
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 18
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 19
Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2018. A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease. Washington, DC: The National Academies Press. doi: 10.17226/25026.
×
Page 20
Next: 1 Introduction »
A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation: Fighting a Ravaging Disease Get This Book
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Huanglongbing (HLB) or citrus greening, first observed more than a hundred years ago in Asia, is the most serious disease threat to the citrus-growing industry worldwide due to its complexity, destructiveness, and incalcitrance to management. First detected in Florida in 2005, HLB is now widespread in the state and threatens the survival of the Florida citrus industry despite substantial allocation of research funds by Florida citrus growers and federal and state agencies.

As the HLB epidemic raged in 2008, Florida citrus growers began allocating funds for HLB research in hopes of finding short-, medium-, and long-term solutions. This effort created the Citrus Research and Development Foundation (CRDF), an organization with oversight responsibility for HLB research and development efforts in Florida. This report provides an independent review of the portfolio of research projects that have been or continue to be supported by the CRDF. It seeks to identify ways to retool HLB research—which, despite significantly increasing understanding of the factors involved in HLB, has produced no major breakthroughs in controlling the disease—and accelerate the development of durable tools and strategies that could help abate the damage caused by HLB and prevent the possible collapse of the Florida citrus industry.

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