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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2 Current Knowledge on Huanglongbing (HLB) and the Interactions of the Pathogen, Vector, and Host
2 Current Knowledge on Huanglongbing (HLB) and the Interactions of the Pathogen, Vector, and Host
Pages 33-66
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From page 33... ...
HLB: THE CAUSAL (OR ASSOCIATED) BACTERIA Candidatus Liberibacter asiaticus Candidatus Liberibacter asiaticus (CLas)
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From page 34... ...
Sequence anno ation t indicated that these features are reflective of the loss of a number of functions during the microbe's adaptation to its intracellular parasitic lifestyle within the host plant and insect vector. Several genes related to DNA and excision repair are lacking, and the species has only a small subset of sigma factors (bacterial transcription initiation-related proteins that enable specific binding of RNA polymerase to gene promoters)
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Historically, the most susceptible are sweet oranges (Citrus sinensis L
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, Diaphorina citri Kuwayama ( emiptera: H Liviidae) , originally from India or other parts of Asia,4 has been recognized as an invader in North America and Latin America for several decades.
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. Adult ACPs are oviparous and eggs are very small, elongate, and thicker at the base and tapered toward the distal end (Martin et al., 2012)
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The adults can jump or leap greater distances when disturbed and may even fly short distances. Adults are usually found in aggregations on lower leaf surfaces with their heads often pressed against the abaxial (lower)
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The lack of consistency in adult ACP movement patterns underlies the need for vigilance in monitoring and surveillance of these mobile pests. Microbial Associates and Interactions with the Asian Citrus Psyllid The microbiome of ACP has recently been investigated by analysis of adults, nymphs, eggs, and cell cultures.
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. Ancient Roman literature describes citrus culture, and people of the Mediterranean region have continued to cultivate citrus trees, particularly mandarins, sweet orange, and lemons, producing fruits for local consumption and for export (Webber, 1967)
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. Citrus fruits are less tolerant to freezing conditions than citrus trees or foliage.
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. For more information on Florida citrus pests and diseases, see the 2017–2018 Florida Citrus Production Guide (Rogers et al., 2017)
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. Genetics, Breeding, and Biotechnology The genetic complexity of the genus Citrus results in high levels of h eterozygosity, and consequently many generations of breeding and selection are required to develop new cultivars having the combinations of genes that produce the desired fruit and juice quality, productivity, and stress and disease resistance traits that are required for the commercial fresh or processing markets.
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. Commercially important cultivars that can be transformed include "Pineapple," "Hamlin" and "Valencia" oranges, "Rio Red" grapefruit, "Eureka" lemon, and "Sucarri" sweet orange (Hu et al., 2016)
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Ca. Liberibacter genomes carry genes encoding transporters and enzymes for metabolism of nucleotides, carbohydrates, amino acids, and lipids derived from host cells (Duan et al., 2009; Lin et al., 2011; Wulff et al., 2014)
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. Plant defense mechanisms are typically suppressed by pathogen viru lence proteins, and CLas appears to suppress defense responses in cit rus (Nwugo et al., 2013a)
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. Certain genes reflecting the transcriptomic responses of host plants to Ca.
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Subsequently the bacteria can replicate and systemically infect the insect, ultimately moving to the salivary tissues from which they can be expelled with salivary secretions into other host plants (Ammar et al., 2011; Kruse et al., 2017)
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. Not all genera reported as host plants are equally suitable for ovi osition, development, and adult feeding and reproduction, however.
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arizonica, and Helietta parvifolia all support ACP growth and reproduction (Sétamou et al., 2016) and may serve as potential host plants.
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However, infected trees are less suitable than uninfected plants as hosts for development of ACP, and the tendency of psyllids to leave infected plants shortly after pathogen acquisition may promote the spread of the pathogen. DIAGNOSTICS Accurate, sensitive, and cost-effective means of detecting CLas infections in both citrus trees and ACP are of paramount importance to the management of HLB.
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Infected tree removal and vector control were the primary eradication strategies early on in Florida, prior to the realization that significant numbers of trees were infected and serving as sources of inoculum while still asymptomatic. Currently, the incidence of HLB in Florida citrus has reached or is approaching 100% and removal of i noculum on individual groves is no longer an option.
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. Timely and effective disposition of abandoned citrus groves and infected backyard trees in urban areas near commercial citrus production areas has not been a practice in Florida, and these pathogen sources continue to add to the disease pressure on all Florida citrus (Tiwari et al., 2010)
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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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2017. Physiologic, anatomic, and gene expression changes in Citrus sunki, Poncirus trifoliata, and their hybrids after "Candidatus Liberibacter asiaticus" infection.
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2009. Asian Citrus Psyllid Diaphorina citri Kuwayama (Hemiptera: Psyllidae)
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2016. Transcriptome analysis of sweet orange trees infected with "Candidatus Liberibacter asiaticus" and two strains of Citrus tristeza virus.
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2004. Distribution of the Asian citrus psyllid, Diaphorina citri Kuwayama (Rhynchota: Psyllidae)
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2011. Dispersal of adult Asian citrus psyllid, Diaphorina citri Kuwayama (Homoptera: Psyllidae)
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2017. Combining ‘omics and microscopy to visual ize interactions between the Asian citrus psyllid vector and the huanglongbing pathogen Candidatus Liberibacter asiaticus in the insect gut.
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2010. Transmission parameters for Candidatus Liberibacter asiaticus by Asian citrus psyllid (Hemiptera: Psyllidae)
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2017. Molecular mechanisms behind the ac cumulation of ATP and H2O2 in citrus plants in response to "Candidatus Liberibacter asiaticus" infection.
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2016. Quality matters: Influences of citrus flush physicochemical characteristics on population dynamics of the Asian citrus psyllid (Hemiptera: Liviidae)
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2017. Asian Citrus Psyllid.
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2017. Reprogramming of a defense signaling pathway in rough lemon and sweet orange is a critical element of the early response to "Candidatus Liberibacter asiaticus." Horticul tural Research 4:17063.
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in response to Asian citrus psyllid-vectored huanglongbing infection. International Journal of Molecular Sci ences 17(7)
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