Asiatic Citrus Canker Research Articles

Diversity of copper resistant Xanthomonas citri subsp. citri strains, the causal agent of Asiatic citrus canker, in Iran

Diversity of copper resistant Xanthomonas citri subsp. citri strains, the causal agent of Asiatic citrus canker, in Iran

Focus Issue Articles on Diagnostic Assay Development and Validation: The Science of Getting It Right

Focus Issue Articles on Diagnostic Assay Development and Validation: The Science of Getting It Right

Genome-Informed Multiplex Conventional PCR for Identification and Differentiation of Xanthomonas citri pv. citri Subpathotypes, the Causal Agents of Asiatic Citrus Canker

Asiatic citrus canker, one of the most important diseases of citrus, is caused by Xanthomonas citri pv. citri. It has high economic impact and can spread easily, and the disease is difficult to manage; it is a quarantine organism in many citrus-producing countries. X. citri pv. citri has been separated into three subpathotypes (A, A*, Aw) that differ in host range and geographical distribution, thus creating a need to differentiate subpathotypes for surveillance and disease management. Availability of useful diagnostic tools is the cornerstone of successful surveillance, quarantine, and eradication measures. In this study, a multiplex conventional PCR (cPCR) assay was developed for detection and subpathotype determination of X. citri pv. citri. Assay specificity was assessed by four different labs on a total of 146 X. citri pv. citri and 58 other Xanthomonas isolates. The assay demonstrated high analytical sensitivity, specificity, and selectivity. False negatives were observed with A Lineage 2 strains, and potential false positives were observed for X. citri pv. bilvae. Combined with a simple extraction protocol, the assay has been deployed successfully at the Plant Protection and Quarantine Plant Pathogen Confirmatory Diagnostics Laboratory. This assay has proven useful for differentiating Asiatic citrus canker subpathotypes from symptomatic citrus tissue. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Re-occurrence of Asiatic citrus canker in the Makkah Province of Saudi Arabia and characterization of the causal agent Xanthomonas citri pv. citri isolated from Mexican lime

Re-occurrence of Asiatic citrus canker in the Makkah Province of Saudi Arabia and characterization of the causal agent Xanthomonas citri pv. citri isolated from Mexican lime

Molecular epidemiology of the citrus bacterial pathogen Xanthomonas citri pv. citri from the Arabian Peninsula reveals a complex structure of specialist and generalist strains.

Molecular epidemiology studies are essential to refine our understanding of migrations of phytopathogenic bacteria, the major determining factor in their emergence, and to understand the factors that shape their population structure. Microsatellite and minisatellite typing are useful techniques for deciphering the population structure of Xanthomonas citri pv. citri, the causal agent of Asiatic citrus canker. This paper presents a molecular epidemiology study, which has improved our understanding of the history of the pathogen's introductions into the Arabian Peninsula, since it was first reported in the 1980s. An unexpectedly high genetic diversity of the pathogen was revealed. The four distinct genetic lineages within X. citri pv. citri, which have been reported throughout the world, were identified in the Arabian Peninsula, most likely as the result of multiple introductions. No copper‐resistant X. citri pv. citri strains were identified. The pathogen's population structure on Mexican lime (their shared host species) was closely examined in two countries, Saudi Arabia and Yemen. We highlighted the marked prevalence of specialist pathotype A* strains in both countries, which suggests that specialist strains of X. citri pv. citri may perform better than generalist strains when they occur concomitantly in this environment. Subclade 4.2 was the prevailing lineage identified. Several analyses (genetic structure deciphered by discriminant analysis of principal components, RST‐based genetic differentiation, geographic structure) congruently suggested the role of human activities in the pathogen's spread. We discuss the implications of these results on the management of Asiatic citrus canker in the region.

Comparative Genomic Analysis Uncovered Evolution of Pathogenicity Factors, Horizontal Gene Transfer Events, and Heavy Metal Resistance Traits in Citrus Canker Bacterium Xanthomonas citri subsp. citri.

Background: Worldwide citrus production is severely threatened by Asiatic citrus canker which is caused by the proteobacterium Xanthomonas citri subsp. citri. Foliar sprays of copper-based bactericides are frequently used to control plant bacterial diseases. Despite the sequencing of many X. citri strains, the genome diversity and distribution of genes responsible for metal resistance in X. citri subsp. citri strains from orchards with different management practices in Taiwan are not well understood.Results: The genomes of three X. citri subsp. citri strains including one copper-resistant strain collected from farms with different management regimes in Taiwan were sequenced by Illumina and Nanopore sequencing and assembled into complete circular chromosomes and plasmids. CRISPR spoligotyping and phylogenomic analysis indicated that the three strains were located in the same phylogenetic lineages and shared ∼3,000 core-genes with published X. citri subsp. citri strains. These strains differed mainly in the CRISPR repeats and pathogenicity-related plasmid-borne transcription activator-like effector (TALE)-encoding pthA genes. The copper-resistant strain has a unique, large copper resistance plasmid due to an unusual ∼40 kbp inverted repeat. Each repeat contains a complete set of the gene cluster responsible for copper and heavy metal resistance. Conversely, the copper sensitive strains carry no metal resistance genes in the plasmid. Through comparative analysis, the origin and evolution of the metal resistance clusters was resolved.Conclusion: Chromosomes remained constant among three strains collected in Taiwan, but plasmids likely played an important role in maintaining pathogenicity and developing bacterial fitness in the field. The evolution of pathogenicity factors and horizontal gene transfer events were observed in the three strains. These data suggest that agricultural management practices could be a potential trigger for the evolution of citrus canker pathogens. The decrease in the number of CRISPR repeats and pthA genes might be the result of adaptation to a less stressful environment. The metal resistance genes in the copper resistant X. citri strain likely originated from the Mauritian strain not the local copper-resistant X. euvesicatoria strain. This study highlights the importance of plasmids as ‘vehicles’ for exchanging genetic elements between plant pathogenic bacteria and contributing to bacterial adaptation to the environment.

Incidence of Asiatic Citrus Canker on Trifoliate Orange and Its Hybrid Accessions in a Florida Field Planting

Asiatic citrus canker (ACC) foliar lesions were evaluated on progenies of 84 seed-source genotypes (“parent genotypes”) from the Citrus Variety Collection (CVC) of the University of California at Riverside (UCR) of Citrus trifoliata and hybrids between C. trifoliata and other Citrus species and hybrids. Trees were planted Aug. 2013 in a completely randomized design at the Fort Pierce U.S. Department of Agriculture (USDA) grove. Plants were assessed visually Aug. 2017, Sept. 2019, and Sept. 2020 for distinctive ACC lesion incidence and severity. Progeny were compared by parent genotypes using nonparametric analysis. Incidence of ACC [percentage of leaves displaying symptoms, verified by quantitative polymerase chain reaction (qPCR) to be associated with Xanthomonas citri pv. citri] across parent genotypes ranged from 8% to 80% (mean, 49%) of leaves affected in 2017, from 4% to 58% (mean, 29%) in 2019, and 8% to 46% (mean, 25%) in 2020. In 2017, of 49 C. trifoliata parent genotypes, only four separated from the two highest ACC-incidence statistical categories [Citrus Research Center (CRC) 3345, 3484, 3888, and 4017]. whereas 29 of the 35 C. trifoliata hybrids displayed lower ACC incidence, which separated from the two highest statistical categories. In 2019, of the C. trifoliata, only six separated from the highest ACC-incidence statistical category (CRC 3330, 3484, 3547, 3549, 3876, and 3888), whereas all 35 C. trifoliata hybrids displayed lower ACC incidence and separated from the highest statistical category, and 26 hybrids separated from 18 of the C. trifoliata. In 2020, only three C. trifoliata separated from the highest ACC-incidence statistical category (CRC 2861, 3549, and 3888) and 20 hybrids separated from 18 of the C. trifoliata. By parent genotype, ACC incidence correlated substantially between each pair of the 3 years, with r2 values of 0.39, 0.57, and 0.65. Of 34 hybrids validated, similar numbers had C. trifoliata, grapefruit (C. ×aurantium var. racemosa), and sweet orange (C. ×aurantium var. sinensis) chloroplasts. Chloroplast type affected ACC incidence and severity, but not in a consistent manner. Near-isogenic groups within C. trifoliata, as determined by DNA markers, were associated with some statistically different ACC sensitivity. Overall, hybrids of C. trifoliata with other citrus types displayed markedly reduced ACC sensitivity compared with C. trifoliata, indicating that this trait is readily overcome through breeding.

Genomic Analysis for Citrus Disease Detection

Citrus is an important group of globally produced fruit crops, holding great economic, cultural, and health value. Belonging to the Rutacaeae family, the genus Citrus includes some of the most iconic and widely appreciated variants of fruits such as the orange, lemon, lime, grapefruit, and tangerine. The spread of various diseases threatens the worldwide production of citrus fruit crops. Diseases such as Asiatic citrus canker, citrus tristeza virus, citrus leprosis, and especially citrus greening disease (also known as Huanglongbing) cause various symptoms harmful to plant growth and fruit production, inflicting tremendous economic damages. Advancements in genetic analysis technologies have offered new tools to investigate the molecular mechanisms underlining these diseases. In this review, we briefly overview the utility of genetic analysis in detection and monitoring of citrus disease-causing pathogens. We then focus our discussion on one of the most damaging citrus diseases, citrus greening disease (Huanglongbing). Genomic and gene expression analysis of citrus plants and their disease-causing microbes, along with tissue and cellular structural imaging, has demonstrated the potential allowing early pathogen detection and disease monitoring. These will eventually help to develop effective treatments to protect and promote citrus crop production.

Riboswitch theo/metE as a Transcription Regulation Tool for Xanthomonas citri subsp. citri.

Xanthomonas citri subsp. citri (X. citri) is the causal agent of Asiatic Citrus Canker (ACC), a disease that affects citrus. ACC has no cure, and growers must rely on special agricultural practices to prevent bacterial spreading. Understanding X. citri basic biology is essential to foresee potential genetic targets to control ACC. Traditionally, microbial genetics use gene deletion/disruption to investigate gene function. However, essential genes are difficult to study this way. Techniques based on small-RNAs and antisense-RNAs are powerful for gene characterization, but not yet fully explored in prokaryotes. One alternative is riboswitches, which derive from bacteria, and can control transcription/translation. Riboswitches are non-coding RNAs able to modulate gene expression in the presence of specific ligands. Here we demonstrate that the riboswitch theo/metE decreases parB expression in X. citri in a platform responsive to theophylline. By monitoring cell respiration, we showed that higher concentrations of the ligand interfered with bacterial viability. Therefore, we determined the safe dose of theophylline to be used with X. citri. Finally, in downstream investigations of parB transcription modulation, we show evidence for the fact that ParB is stable, remains functional throughout the cell cycle, and is inherited by the daughter cells upon cell division.

The Return of Asiatic Citrus Canker to Texas: Surveys and Eradication Efforts

Citrus canker caused by Xanthomonas citri subsp. citri (syn. X. axonopodis pv. citri) was first reported in South Texas during the early 1900s, prompting quarantine and eradication programs by the federal government. Its last reported incidence in Texas was from two trees from Corpus Christi in 1943. In May 2016, USDA Animal and Plant Health Inspection Service-Plant Protection and Quarantine (PPQ) confirmed the presence of the Asiatic A strain, which is by far the most widespread and severe form infecting most citrus varieties, on two sour orange trees in a park in Houston. Subsequently, infected trees were detected in a nursery in Richmond (Fort Bend Co.) in August 2016 and in dooryard trees in Pearland (Brazoria Co.) in 2018. PPQ and Texas Department of Agriculture collected a combined 1,949 positive A strain samples from Harris, Fort Bend, and Brazoria Counties during May 2016 to January 2021. These findings resulted in seven active citrus canker quarantine zones being established in the Upper Gulf Coast area. The Texas Department of Agriculture is removing positive trees, and surveys are continuing.

Periplasm-enriched fractions from Xanthomonas citri subsp. citri type A and X. fuscans subsp. aurantifolii type B present distinct proteomic profiles under in vitro pathogenicity induction.

The causative agent of Asiatic citrus canker, the Gram-negative bacterium Xanthomonas citri subsp. citri (XAC), produces more severe symptoms and attacks a larger number of citric hosts than Xanthomonas fuscans subsp. aurantifolii XauB and XauC, the causative agents of cancrosis, a milder form of the disease. Here we report a comparative proteomic analysis of periplasmic-enriched fractions of XAC and XauB in XAM-M, a pathogenicity- inducing culture medium, for identification of differential proteins. Proteins were resolved by two-dimensional electrophoresis combined with liquid chromatography-mass spectrometry. Among the 12 proteins identified from the 4 unique spots from XAC in XAM-M (p<0.05) were phosphoglucomutase (PGM), enolase, xylose isomerase (XI), transglycosylase, NAD(P)H-dependent glycerol 3-phosphate dehydrogenase, succinyl-CoA synthetase β subunit, 6-phosphogluconate dehydrogenase, and conserved hypothetical proteins XAC0901 and XAC0223; most of them were not detected as differential for XAC when both bacteria were grown in NB medium, a pathogenicity non-inducing medium. XauB showed a very different profile from XAC in XAM-M, presenting 29 unique spots containing proteins related to a great diversity of metabolic pathways. Preponderant expression of PGM and XI in XAC was validated by Western Blot analysis in the periplasmic-enriched fractions of both bacteria. This work shows remarkable differences between the periplasmic-enriched proteomes of XAC and XauB, bacteria that cause symptoms with distinct degrees of severity during citrus infection. The results suggest that some proteins identified in XAC can have an important role in XAC pathogenicity.

Canine Olfactory Detection of a Non-Systemic Phytobacterial Citrus Pathogen of International Quarantine Significance.

For millennia humans have benefitted from application of the acute canine sense of smell to hunt, track and find targets of importance. In this report, canines were evaluated for their ability to detect the severe exotic phytobacterial arboreal pathogen Xanthomonas citri pv. citri (Xcc), which is the causal agent of Asiatic citrus canker (Acc). Since Xcc causes only local lesions, infections are non-systemic, limiting the use of serological and molecular diagnostic tools for field-level detection. This necessitates reliance on human visual surveys for Acc symptoms, which is highly inefficient at low disease incidence, and thus for early detection. In simulated orchards the overall combined performance metrics for a pair of canines were 0.9856, 0.9974, 0.9257 and 0.9970, for sensitivity, specificity, precision, and accuracy, respectively, with 1–2 s/tree detection time. Detection of trace Xcc infections on commercial packinghouse fruit resulted in 0.7313, 0.9947, 0.8750, and 0.9821 for the same performance metrics across a range of cartons with 0–10% Xcc-infected fruit despite the noisy, hot and potentially distracting environment. In orchards, the sensitivity of canines increased with lesion incidence, whereas the specificity and overall accuracy was >0.99 across all incidence levels; i.e., false positive rates were uniformly low. Canines also alerted to a range of 1–12-week-old infections with equal accuracy. When trained to either Xcc-infected trees or Xcc axenic cultures, canines inherently detected the homologous and heterologous targets, suggesting they can detect Xcc directly rather than only volatiles produced by the host following infection. Canines were able to detect the Xcc scent signature at very low concentrations (10,000× less than 1 bacterial cell per sample), which implies that the scent signature is composed of bacterial cell volatile organic compound constituents or exudates that occur at concentrations many fold that of the bacterial cells. The results imply that canines can be trained as viable early detectors of Xcc and deployed across citrus orchards, packinghouses, and nurseries.

Investigating the Modes of Action of the Antimicrobial Chalcones BC1 and T9A

Xanthomonas citri subsp. citri (X. citri) is an important phytopathogen and causes Asiatic Citrus Canker (ACC). To control ACC, copper sprays are commonly used. As copper is an environmentally damaging heavy metal, new antimicrobials are needed to combat citrus canker. Here, we explored the antimicrobial activity of chalcones, specifically the methoxychalcone BC1 and the hydroxychalcone T9A, against X. citri and the model organism Bacillus subtilis. BC1 and T9A prevented growth of X. citri and B. subtilis in concentrations varying from 20 µg/mL to 40 µg/mL. BC1 and T9A decreased incorporation of radiolabeled precursors of DNA, RNA, protein, and peptidoglycan in X. citri and B. subtilis. Both compounds mildly affected respiratory activity in X. citri, but T9A strongly decreased respiratory activity in B. subtilis. In line with that finding, intracellular ATP decreased strongly in B. subtilis upon T9A treatment, whereas BC1 increased intracellular ATP. In X. citri, both compounds resulted in a decrease in intracellular ATP. Cell division seems not to be affected in X. citri, and, although in B. subtilis the formation of FtsZ-rings is affected, a FtsZ GTPase activity assay suggests that this is an indirect effect. The chalcones studied here represent a sustainable alternative to copper for the control of ACC, and further studies are ongoing to elucidate their precise modes of action.

Hexyl gallate for the control of citrus canker caused by Xanthomonas citri subsp citri.

Brazil is the biggest producer of sweet oranges and the main exporter of concentrated orange juice in the world. Among the diseases that affect citriculture, Asiatic citrus canker, caused by the bacterial pathogen Xanthomonas citri, represents one of the most significant threats. The current Brazilian legislation regulating the control of citrus canker no longer requires the eradication of affected trees in states where the incidence of the disease is high. Instead, control involves disease control measures, including periodic preventative spraying of copper compounds. The long‐term use of copper for plant disease control has raised concerns about environmental accumulation and toxicity, as well as the selective pressure it exerts leading to the emergence of copper‐resistant X. citri strains. Here, we evaluated hexyl gallate (G6) as an alternative to copper compounds for citrus plant protection. G6 was able to protect citrus nursery trees against X. citri infection. Thirty days after inoculation, the trees treated with G6 developed 0.5 lesions/cm2 leaf area compared with the 2.84 lesions/cm2 observed in the untreated control trees. Also, G6 did not interfere with germination and root development of tomato, lettuce, and arugula, which is consistent with our previous data showing that G6 is safe for tissue culture cell lines. Membrane permeability tests showed that the primary target of G6 is the bacterial outer membrane. Finally, we could not isolate spontaneous X. citri mutants resistant to G6 nor induce resistance to G6 after long‐term exposures to increasing concentrations of the compound, which suggests that G6 may have multiple cellular targets. This study demonstrated that G6 is a promising candidate for the development and use in citrus canker management.

Seasonal and post-harvest population dynamics of the Asiatic citrus canker pathogen Xanthomonascitri subsp. citri on grapefruit in Florida

Asiatic citrus canker (ACC, caused by Xanthomonas citri subsp. citri, [Xcc]) is a serious disease of citrus, reducing yield and impacting marketability of fresh fruit. ACC has phytosanitary implications for fresh fruit trade. Lesions on fruit, leaves, and shoots in an orchard of ACC-susceptible grapefruit were monitored for production of Xcc from June to the following January in 2009 and 2010. Lesions on fruit post-harvest either packingline-treated or not treated were also monitored for production of Xcc on fruit harvested in November 2013 and January 2014. Shoot lesions generally produced fewer bacteria and had a smaller proportion of active lesions compared to leaf and fruit lesions. As the season progressed the proportion of active lesions on fruit and shoots gradually declined. Bacterial flux density (BFD) in surviving lesions was >103 on all organs at all sample times. A window-pane analysis revealed weather variables associated with bacterial survival within the lesion. The packingline treatment had no effect in reducing the proportion of active lesions, and had only a small, transient effect reducing BFD (~1.5 log units). Lesion size affected survival – small lesions (<10 mm2) declined in activity more rapidly compared to larger lesions, both in the field pre-harvest, and post-harvest in storage. Although lesions of ACC on fruit, leaves, and shoots are a constant source of inoculum, steps that can be taken to minimize early infection of fruit which often results in larger lesions will reduce the risk of inoculum dispersal. Current packingline treatments do not reduce activity of lesions of ACC. Based on these and other previously reported results, lesions on fruit post-harvest can be deemed a limited risk for spread of canker.

CRISPR elements provide a new framework for the genealogy of the citrus canker pathogen Xanthomonas citri pv. citri

BackgroundXanthomonads are an important clade of Gram-negative bacteria infecting a plethora of economically important host plants, including citrus. Knowledge about the pathogen’s diversity and population structure are prerequisite for epidemiological surveillance and efficient disease management. Rapidly evolving genetic loci, such as Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR), are of special interest to develop new molecular typing tools.ResultsWe analyzed CRISPR loci of 56 Xanthomonas citri pv. citri strains of world-wide origin, a regulated pathogen causing Asiatic citrus canker in several regions of the world. With one exception, 23 unique sequences built up the repertoire of spacers, suggesting that this set of strains originated from a common ancestor that already harbored these 23 spacers. One isolate originating from Pakistan contained a string of 14 additional, probably more recently acquired spacers indicating that this genetic lineage has or had until recently the capacity to acquire new spacers. Comparison of CRISPR arrays with previously obtained molecular typing data, such as amplified fragment length polymorphisms (AFLP), variable-number of tandem-repeats (VNTR) and genome-wide single-nucleotide polymorphisms (SNP), demonstrated that these methods reveal similar evolutionary trajectories. Notably, genome analyses allowed to generate a model for CRISPR array evolution in X. citri pv. citri, which provides a new framework for the genealogy of the citrus canker pathogen.ConclusionsCRISPR-based typing will further improve the accuracy of the genetic identification of X. citri pv. citri outbreak strains in molecular epidemiology analyses, especially when used concomitantly with another genotyping method.

The Xanthomonas citri pv. citri Type VI Secretion System is Induced During Epiphytic Colonization of Citrus.

Xanthomonas citri pv. citri (X. citri pv. citri) is the causal agent of Asiatic citrus canker and infects economically important citrus crops. X. citri pv. citri contains one type VI secretion system (T6SS) required for resistance to predation by the soil amoeba Dictyostelium discoideum and induced by the ECF sigma factor EcfK in the presence of amoeba. In this work, we describe the analysis of T6SS gene expression during interaction with host plants. We show that T6SS genes and the cognate positive regulator ecfK are upregulated during growth in the plant surface (epiphytic) and maintain low expression levels during growth inside plant mesophyll. In addition, expression of the virulence-associated T3SS is also induced during epiphytic growth and shows a temporal induction pattern during growth inside plant leaves. The T6SS is not required for adhesion to leaf surface and biofilm formation during the first stages of plant colonization nor for killing of yeasts cells. Since the phyllosphere is colonized by eukaryotic predators of bacteria, induction of the X. citri pv. citri anti-amoeba T6SS during epiphytic growth suggests the presence of an environmental signal that triggers the resistance phenotype.

Deciphering how plant pathogenic bacteria disperse and meet: Molecular epidemiology of Xanthomonas citri pv.citri at microgeographic scales in a tropical area of Asiatic citrus canker endemicity.

Although some plant pathogenic bacteria represent a significant threat to agriculture, the determinants of their ecological success and evolutionary potential are still poorly understood. Refining our understanding of bacterial strain circulation at small spatial scales and the biological significance and evolutionary consequences of co‐infections are key questions. The study of bacterial population biology can be challenging, because it requires high‐resolution markers that can be genotyped with a high throughput. Here, we overcame this difficulty for Xanthomonas citri pv. citri, a genetically monomorphic bacterium causing Asiatic citrus canker (ACC). Using a genotyping method that did not require cultivating the bacterium or purifying DNA, we deciphered the pathogen's spatial genetic structure at several microgeographic scales, down to single lesion, in a situation of ACC endemicity. In a grove where copper was recurrently applied for ACC management, copper‐susceptible and copper‐resistant X. citri pv. citri coexisted and the bacterial population structured as three genetic clusters, suggesting a polyclonal contamination. The range of spatial dependency, estimated for the two largest clusters, was four times greater for the cluster predominantly composed of copper‐resistant bacteria. Consistently, the evenness value calculated for this cluster was indicative of increased transmission. Linkage disequilibrium was high even at a tree scale, probably due to a combination of clonality and admixture. Approximately 1% of samples exhibited within‐lesion multilocus polymorphism, explained at least in part by polyclonal infections. Canker lesions, which are of major biological significance as an inoculum source, may also represent a preferred niche for horizontal gene transfer. This study points out the potential of genotyping data for estimating the range of spatial dependency of plant bacterial pathogens, an important parameter for guiding disease management strategies.

Antibacterial activity of monoacetylated alkyl gallates against Xanthomonas citri subsp. citri.

Asiatic citrus canker (ACC) is an incurable disease of citrus plants caused by the Gram-negative bacterium Xanthomonas citri subsp. citri (X. citri). It affects all the commercially important citrus varieties in the major orange producing areas around the world. Control of the pathogen requires recurrent sprays of copper formulations that accumulate in soil and water reservoirs. Here, we describe the improvement of the alkyl gallates, which are potent anti-X. citri compounds, intended to be used as alternatives to copper in the control of ACC. Acetylation of alkyl gallates increased their lipophilicity, which resulted in potentiation of the antibacterial activity. X. citri exposed to the acetylated compounds exhibited increased cell length that is consistent with the disruption of the cell division apparatus. Finally, we show that inhibition of cell division is an indirect effect that seemed to be caused by membrane permeabilization, which is apparently the primary target of the acetylated alkyl gallates.

Evidence-based controls for epidemics using spatio-temporal stochastic models in a Bayesian framework.

The control of highly infectious diseases of agricultural and plantation crops and livestock represents a key challenge in epidemiological and ecological modelling, with implemented control strategies often being controversial. Mathematical models, including the spatio-temporal stochastic models considered here, are playing an increasing role in the design of control as agencies seek to strengthen the evidence on which selected strategies are based. Here, we investigate a general approach to informing the choice of control strategies using spatio-temporal models within the Bayesian framework. We illustrate the approach for the case of strategies based on pre-emptive removal of individual hosts. For an exemplar model, using simulated data and historic data on an epidemic of Asiatic citrus canker in Florida, we assess a range of measures for prioritizing individuals for removal that take account of observations of an emerging epidemic. These measures are based on the potential infection hazard a host poses to susceptible individuals (hazard), the likelihood of infection of a host (risk) and a measure that combines both the hazard and risk (threat). We find that the threat measure typically leads to the most effective control strategies particularly for clustered epidemics when resources are scarce. The extension of the methods to a range of other settings is discussed. A key feature of the approach is the use of functional-model representations of the epidemic model to couple epidemic trajectories under different control strategies. This induces strong positive correlations between the epidemic outcomes under the respective controls, serving to reduce both the variance of the difference in outcomes and, consequently, the need for extensive simulation.