A Review of the Citrus Greening Research and Development Efforts Supported by the Citrus Research and Development Foundation Fighting a Ravaging Disease (2018) / Chapter Skim
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3 HLB Research and Development Efforts
3 HLB Research and Development Efforts
Pages 67-112
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From page 67... ...
, a nonprofit corporation that was created through the initiative of the Florida citrus industry in 2009, currently serves as the organization in Florida with oversight responsibility for HLB research and commercial product development and delivery. This chapter examines the HLB research areas that have received or continue to receive support from CRDF1 and discusses whether these research efforts are in line with the recommendations in the NRC 2010 report (notable outcomes from HLB research and pitfalls are discussed in Chapter 4 of this report)
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From page 68... ...
These factors were not mentioned specifically in the 2010 report, but the committee deems them to be critical to the success of any future HLB management efforts and, indeed, to the health of the citrus industry in Florida. HLB PATHOGEN, VECTOR, AND HOSTS HLB Causal or Associated Bacteria CRDF-Funded Research CRDF has funded 26 projects, totaling $8.6 million, with at least one major objective related to understanding the biology of Candidatus L iberibacter asiaticus (CLas)
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From page 69... ...
. These efforts are in line with the 2010 recommendations NI-5: Support development of HLB model systems and NI-6: Exploit the CLas genome sequence for new strategies of HLB mitigation.
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From page 70... ...
toxin against the Asian citrus psyllid (ACP)
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From page 71... ...
Finally, K Mandadi at Texas A&M was awarded the Foundation for Food and Agriculture New Innovator Award in 2017, receiving $600,000 from the Foundation for Food and Agriculture Research and Southern Gardens Citrus to develop a root-based bioassay for culturing studies and antimicrobial screening.6 HLB Vector: Asian Citrus Psyllid CRDF-Funded Research CRDF has previously funded 15 projects, totaling $5.9 million, that have at least one major objective related to understanding the biology and ecology of ACP.
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From page 72... ...
NIFA funding supported research on ACP behavioral responses to the presence of CLas in trees. CLas infection leads to changes in the host's odor, which influences attraction of noninoculative vectors and repellency of inoculative individuals.
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From page 73... ...
Transcriptome assemblies and other sequence data are available at websites of the International Asian Citrus Psyllid Genome Consortium11 and the National Center for Biotechnology Information.12 Bioinformatic and annotation resources for genomes of ACP and other HLB components have been assembled in a NIFA-funded comprehensive, publicly available resource, CitrusGreening Solutions.13 Investigations of insect gustation (act or sensation of tasting) have examined the function of ACP sugar receptors and identified compounds that enhance or inhibit their activity, testing whether the presence of these compounds alters feeding behavior.
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From page 74... ...
While varying levels of tolerance to HLB have been noted in a variety of rootstock and fruiting cultivars, there appear to be no sources of high-level resistance that are viable in the sense that commercially acceptable fruiting citrus cultivars could be produced in time to sustain the Florida citrus industry. Currently, the most promising approach is the development of high-level resistance through genetic engineering using genes from citrus-related (sexually compatible)
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From page 75... ...
Citrus Tristeza Virus Gene Delivery. While the development of new highly HLB-resistant citrus cultivars through conventional breeding or genetic engineering would revitalize and stabilize the citrus industry, both approaches will take time, which a suffering industry can ill afford.
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From page 76... ...
that aims to identify candidate HLB resistance genes in resistant or tolerant citrus relatives and utilize these for gene editing producing non-genetically modified HLB-resistant citrus cultivars is funded by NIFA (#2015-7001623027)
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From page 77... ...
Project #581/581-1 was focused on identifying overwintering habitats and alternative host plants of ACP and to assess the suitability of alternative hosts for oviposition and develop 15 hemical substances emitted by an insect or plant that have an adaptive benefit, such as C a stimulus for oviposition, to another species.
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From page 78... ...
trifoliata in order to characterize phytochemicals that influence ACP behavior and physiology. The project's three major objectives were to identify host plant-produced volatile chemicals and leaf/plant metabolites that are attractive or repellent to adult ACP; to test preference and performance of ACP adults on susceptible and resistant host plants; and to evaluate ACP behavior in response to odorant and tastant blends.
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From page 79... ...
CRDF funded projects aimed at characterizing psyllid responses to abiotic environmental factors with the ultimate goal of acquiring data useful in optimizing trapping and predicting risks of psyllid attack. A completed project examining abiotic factors was Project #214, which aimed to evaluate seasonality and frequency of psyllid dispersal.
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From page 80... ...
. Research Funded by Other Agencies Of 16 NIFA SCRI grants supporting HLB research, evidently no funding was directed specifically toward elucidating the interaction between ACP and its host plants.
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From page 81... ...
. Overlaps and Duplications With respect to characterizing interactions between ACP and its host plants, there was little overlap in objectives between projects funded by CRDF and other funding agencies.
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From page 82... ...
A majority of the transmission-related projects are aimed at identifying or developing molecules that could have direct or indirect effects on the ability of ACP to transmit CLas. Projects include studies on the effects of thermotherapy (see also the section HLB Management, Bacterial Control, page 86)
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From page 83... ...
Research Funded by Other Agencies The USDA APHIS MAC program has many projects focused on the management of ACP, but because few of them integrate CLas transmission with vector control it is difficult to assess what ACP management strategies will have any meaningful effect on reducing HLB incidence.
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From page 84... ...
The past projects were focused on determining the physiological response of host plants to HLB infection (#002 and 045) , identifying host resistance or tolerance genes to HLB (#523 and 536)
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From page 85... ...
and its derivatives, messengers of the host defense system, leading to weakened resistance in host plants (Li et al., 2017)
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From page 86... ...
Development of approaches directed at reducing or eliminating the pathogen are hampered by the phloem-restricted microbial habitat and the uncultivable nature of CLas, but researchers are exploring a variety of avenues to target the pathogen as a means of disease management. CRDF-Funded Research CRDF has supported a number of past research projects studying HLB management by targeting the bacterial pathogen; many, but not all, of these have resulted in publications in peer-reviewed journals and have served as a foundation for current research efforts, as noted below.
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From page 87... ...
, several tetracycline derivatives that lack activity against human bacterial pathogens were tested against strains of Liberibacter. 16 Currently the citrus canker pathogen is referred to as Xanthomonas axonopodis pv.
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From page 88... ...
. A key point in the evaluation of antimicrobials is that their use may be enhanced when used in combination with other HLB management strategies in an integrated pest management (IPM)
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From page 89... ...
Most nutritional treatments for HLB management have focused on enhancement of tree health with the goals of extended productive lifespan and maintenance of fruit production and quality in HLB-infected trees (see also the section Cultural Control, page 91)
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From page 90... ...
Considerable funding has also been expended for psyllid suppression using biocontrol strategies, including eight projects funded by CRDF for approximately $1.9 million (including #711, 760-1, 212, and 434)
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From page 91... ...
to repel ACP and improve tree growth. Numerous CRDF-funded projects have investigated several potential
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From page 92... ...
These efforts are in line with the 2010 recommendation NI-8: Support small-scale studies on the feasibility of alternative horticultural systems suited to endemic HLB. Research Funded by Other Agencies The USDA APHIS MAC program has contributed in excess of $8 million to research on numerous management topics, including removal of abandoned groves, reducing pH of irrigation water, thermotherapy, integrated management, mulch, and intensive grove management.
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From page 93... ...
. Canine detection is funded by the USDA, ARS, and USDA APHIS MAC program (Project #14-8130-0474-CA)
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From page 94... ...
However, when HLB arrived in Florida the severe impacts of citrus canker -- both the disease and the management strategies -- were still fresh in the minds of growers and other industry personnel. Many growers had suffered financially from canker and had neither the funds nor the will to invest heavily in managing HLB.
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From page 95... ...
was an attempt to encourage neighboring citrus growers to share and coordinate HLB management actions, especially those targeting the psyllid vector, to more effectively manage pathogen dissemination and limit disease spread (Muraro, 2012; UF IFAS, 2018)
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From page 96... ...
Indeed, many growers are moving to new strategies having very different final objectives. New Directions of HLB Management Recent trends in HLB management in Florida have shifted from concerted efforts to stop the disease to actions intended to support and prolong the health and fruit production of HLB-infected trees (Roka et al., 2009; Muraro, 2012; Zansler, 2017)
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From page 97... ...
However, many Florida citrus growers have adopted some form of this strategy, and many believe that these approaches are working (Muraro, 2012) , anecdotally reporting higher yield and more robust tree growth.
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From page 98... ...
Modeling should be further developed as a systems approach to HLB management, in which it can be used in a myriad of applications, for example, to evaluate risk, to predict how best to incorporate HLB-tolerant groves in relation to susceptible neighboring fields, to provide guidance on how to deploy the CTV vector to express AMPs and RNAi and potential benefits of doing so, to help assess replant strategies, to compare the effectiveness of different single and combinatorial HLB management practices, and to inform economic decision making. Genetically Modified HLB-Resistant Citrus Most of the strategies discussed here for managing HLB in Florida are considered by growers, processors, and consumers to be stop-gap measures to preserve as much as possible of the state's citrus industry while awaiting the development and release of HLB-resistant cultivars that still retain desirable characteristics such as yield, flavor, size, and shelf life.
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, and (4) the rationale for using genetic modification technology ("saving an industry" was partial justification but did not completely overcome their objections)
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From page 100... ...
effects for possible suppression of ACP • NI-10: Develop in vitro culture techniques for CLas to facilitate experimental manipulation of the bacterium for insights into gene function CRDF also funded research efforts that address three of the four 2010 NRC long-term research and technology recommendations: L-1: Support the development of transgenic HLB-resistant and ACP-resistant citrus; L-2: Support development and testing of bactericides, therapeutics, or SAR activators; and L-3: Support analysis of ACP behavior, ACP–plant interactions, and ecology to enhance the knowledge base available for new ACP management strategies. Finding 3.2: Other agencies are funding efforts to address Recommendation NI-11: Sequence, assemble, and annotate the ACP genome to provide a basis for new approaches to ACP management as well as NI-2: Support searches for biomarkers that may be exploited to detect CLas-infected citrus, although there are CRDF-funded projects that could provide useful data leading to improved HLB detection.
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From page 101... ...
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.
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From page 102... ...
2016. Acquisi tion, replication and inoculation of Candidatus Liberibacter asiaticus following various acquisition periods on huanglongbing-infected citrus by nymphs and adults of the Asian citrus psyllid.
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From page 103... ...
2014a. Odorants for surveillance and control of the Asian citrus psyllid (Diaphorina citri)
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From page 104... ...
2013. Silencing abnormal wing disc gene of the Asian citrus psyllid, Diaphorina citri disrupts adult wing develop ment and increases nymph mortality.
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From page 105... ...
2012. Asian citrus psyllid, Diaphorina citri, vector of citrus huanglongbing disease.
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From page 106... ...
2015. Seasonal movement patterns and long-range dispersal of Asian citrus psyllid in Florida citrus.
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From page 107... ...
2013. Chemical and behav ioral analysis of the cuticular hydrocarbons from Asian citrus psyllid, Diaphorina citri.
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From page 108... ...
2010. Transmission parameters for Candidatus Liberibacter asiaticus by Asian citrus psyllid (Hemiptera: Psyllidae)
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From page 109... ...
2014. A Chemical Genomics Approach to Identify Targets for Control of Asian Citrus Psyllid and HLB, Project No.
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From page 110... ...
activity levels in Candidatus Liberibacter asiaticus infected and uninfected Asian citrus psyllid (Hemiptera: Psyllidae)
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From page 111... ...
2014. Effective antibiotics against "Candidatus Liberibacter asiaticus" in HLB-affected citrus plants identified via the graft-based evaluation.
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