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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. [ABSTRACT FROM AUTHOR]

Applications of formulated bacteriophages with skim milk and sucrose or nonformulated bacteriophages combined with acibenzolar-S-metiyl (ASM) were compared with copper bactericides applications for suppressing Asiatic citrus canker (ACC) caused by Xanthomonas citri subsp. citri (Xcc) on leaves under greenhouse and field conditions in Saudi Arabia. Bacteriophages were applied one day prior to inoculation of Mexican l:me (Citrus aurantifolia) plants with Xcc, then twice a week until the end of the trials. Copper hydroxide was applied once prior to inoculation and then every seven days afterward, whereas ASM was applied one week prior tc inoculation and then every 21 days afterward. Under greenhouse conditions, the incidence of ACC on leaves was reduced significantly from 75.2 to 12.8% or 18.3% for plants treated with copper hydroxide or bacteriophages in combination with ASM, respectively. Applications of formulated phages in combination with ASM as soil drench under field conditions significantly decreased disease incidence by 14.8% (Trial 1) and 16.8% (Trial 2) compared with untreated control plants. Overall, the Xcc-inoculated plants treated with bacteriophages + ASM combination showed significant ACC reduction under greenhouse and field conditions. The bacteriophages + ASM combination tested in these trials can be an effective tool in the integrated management programs of Asiatic citrus canker disease. [ABSTRACT FROM AUTHOR]

Xanthomonas citri subsp. citri, which causes Asiatic citrus canker (ACC), is an important pathogen of citrus ill Brazil and elsewhere. The genetic diversity of X. citri subsp. citri pathotype 'A has not been studied in Brazil at a local scale (up to 300 km). Forty isolates were sampled from lesions of ACC on citrus in three orchards in Paraná state, Brazil. Twelve minisatellite markers were used to characterize the genetic diversity of the isolates. An Unweighted Paired Group Method of Arithmetic Averages tree was used for identifying unique multilocus haplotypes but there was no association between haplotypes and source locations. An analysis of molecular variance among populations showed that 98% of the variance was accounted for within the populations, and only 2% was accounted for among populations. Differences among populations was not significant (0=0.018, P=0.2). The relatively high, yet uniform, genetic diversity among isolates and low degree of spatial differentiation between populations of X. citri subsp. citri suggests that the populations in Paraná state have a common origin and strong historical epidemiological links. [ABSTRACT FROM AUTHOR]

Asiatic citrus canker (ACC, caused by the bacterium Xanthomonas citri subsp. citri) is a destructive disease of citrus in Brazil and in several other citrus-producing countries. ACC management is problematic, and bactericides such as copper can be reasonably efficacious but do not completely control the disease. Furthermore, injury by citrus leaf miner (CLM) can exacerbate severity of ACC. Host resistance is the most desirable solution for management of ACC; however, evaluations of germplasm indicate that resistance is limited in many popular species and cultivars that are grown commercially. Limited evaluations have been made of sweet orange (Citrus sinensis) selections. We evaluated resistance of 25 Pera sweet orange selections to X. citri subsp. citri by wound inoculation and measuring lesion diameter under greenhouse conditions (wound inoculation indicates mesophyll resistance which will be valuable in areas where CLM exists). ACC severity was assessed on the same 25 selections at three locations in the field in Brazil, relying on natural inoculum and conditions to cause disease. In the greenhouse experiments, the selections EEL. Bianchi/CC. Ipigua, Olimpia, IAC 2000/1, and Ovale Siracusa consistently had the smallest diameter lesions, indicating greatest resistance, although differences in lesion diameter were small. Results from the field experiments were less conclusive, although EEL and Ovale Siracusa were consistently numerically least affected by ACC. These results indicate selections of sweet orange that might be preferable to consider in canker-prone areas in Brazil and elsewhere. [ABSTRACT FROM AUTHOR]

Asiatic citrus canker ( ACC), caused by Xanthomonas citri subsp. citri, has been controlled by exclusion and eradication measures in São Paulo state ( SP), Brazil, since it was first detected in 1957. Several protocols of eradication have been used since then, but the protocol applied from 1999 to 2009 was the most successful and well documented of these. During that time, if the incidence of trees with symptoms in a citrus block was ≤0.5%, the infected trees and the symptomless trees within a 30 m radius were removed. However, if the incidence was >0.5%, the entire block was eliminated. Subsequently, inspections were carried out to monitor the affected blocks during quarantine. Although this protocol did not eliminate the disease from SP, it suppressed the incidence of affected blocks to very low annual levels (<0.20%) for a decade. Of over 5000 foci detected, 65.1% were eliminated by removing the block, either when disease was first detected (47.8%) or at a subsequent detection (17.3%), and 28.4% were eradicated by removing trees within a 30 m radius. The latter method was not an effective stand-alone measure for ACC eradication and the 0.5% threshold was the key factor in suppression of disease statewide. Currently, a re-emergence of ACC is occurring in SP. The information presented here will aid the establishment of protocols for the eradication of ACC in other citrus-growing areas with low incidence of the disease or at risk of pathogen introduction. [ABSTRACT FROM AUTHOR]

Citrus canker, caused by Xanthomonas citri, affects most commercial citrus varieties. All X. citri strains possess at least one transcription activator-like effector of the PthA family that activates host disease susceptibility ( S) genes. The X. citri strain 306 encodes four PthA effectors; nevertheless, only PthA4 is known to elicit cankers on citrus. As none of the PthAs act as avirulence factors on citrus, we hypothesized that PthAs 1-3 might also contribute to pathogenicity on certain hosts. Here, we show that, although PthA4 is indispensable for canker formation in six Brazilian citrus varieties, PthAs 1 and 3 contribute to canker development in 'Pera' sweet orange, but not in 'Tahiti' lemon. Deletions in two or more pthA genes reduce bacterial growth in planta more pronouncedly than single deletions, suggesting an additive role of PthAs in pathogenicity and bacterial fitness. The contribution of PthAs 1 and 3 in canker formation in 'Pera' plants does not correlate with the activation of the canker S gene, LOB1 ( LATERAL ORGAN BOUNDARIES 1), but with the induction of other PthA targets, including LOB2 and citrus dioxygenase ( DIOX). LOB1, LOB2 and DIOX show differential PthA-dependent expression between 'Pera' and 'Tahiti' plants that appears to be associated with nucleotide polymorphisms found at or near PthA-binding sites. We also present evidence that LOB1 activation alone is not sufficient to elicit cankers on citrus, and that DIOX acts as a canker S gene in 'Pera', but not 'Tahiti', plants. Our results suggest that the activation of multiple S genes, such as LOB1 and DIOX, is necessary for full canker development. [ABSTRACT FROM AUTHOR]

This study was conducted at National Agricultural Research Center, Islamabad. The experiment was performed on randomized complete block design (RCBD) consisting of eleven treatments (cultivars) replicated three times during 2015. The results showed that Tarocco attained maximum height (156.13 cm), followed by Mexican Lemon (146.80 cm). Similarly, maximum plant spread and leaf area were recorded in Mexican Lemon 124.07 cm and 11.35 cm², respectively. However, Hamlin showed the highest stem girth (9.90 cm) and number of branches (66.53). The internode length was the greatest in Kinnow cultivar (2.66 cm). In case of disease incidence maximum lesion on leaves were found in Kinnow (14.2) while in Cara Cara, Salustiana and Arnold Blood, lesser lesions were found. On the basis of results and overall performance Tarocco and Hamlin performed best in all aspects while Arnold Blood and Cara Cara revealed poor growth. [ABSTRACT FROM AUTHOR]

Xanthomonas citri subsp . citri (Xcc) is the causal agent of citrus canker, a disease that affects almost all types of citrus crops. Production of particular Xcc pathogenicity factors is controlled by a gene cluster rpf, which encodes elements of a cell-cell communication system called quorum sensing ( QS), mediated by molecules of the diffusible signal factor ( DSF) family. Interference with cell-cell signalling, also termed quorum quenching, either by signal degradation or over-production, has been suggested as a strategy to control bacterial disease. In this study, three bacterial strains were isolated from citrus leaves that displayed the ability to disrupt QS signalling in Xcc. Pathogenicity assays in sweet orange ( Citrus sinensis) showed that bacteria of the genera Pseudomonas and Bacillus also have a strong ability to reduce the severity of citrus canker disease. These effects were associated with alteration in bacterial attachment and biofilm formation, factors that are known to contribute to Xcc virulence. These quorum-quenching bacteria may represent a highly valuable tool in the process of biological control and offer an alternative to the traditional copper treatment currently used to treat citrus canker disease. [ABSTRACT FROM AUTHOR]

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is an important pest of all citrus varieties. Larvae damage young leaves, thereby reducing photosynthesis and tree vigor, and may impact yield. Wounds opened by P. citrella larvae may also increase susceptibility to citrus canker disease caused by the bacterium Xanthomonas axonopodis pv. citri (Xanthomonadales: Xanthomonadaceae). Sex pheromones coupled with appropriate traps are used as monitoring tools for this and other lepidopteran pests. Information compiled from trap captures is indicative of seasonal population fluctuations and may be used to guide management practices. Trap density and pheromone dispenser longevity are factors affecting the accuracy of trapping data. Our objectives were to evaluate capture of P. citrella in relation to trap density and duration under field conditions. Almost 2 yr of citrus leafminer monitoring demonstrated that a density of 1 trap per approximately 2 ha yielded similar results to the higher recommended density of 1 trap per 0.4 to 1.6 ha. Trap catch with the 2 pheromone brands tested declined by 25% after 3 to 6 wk and 50% after 6 to 10 wk during the spring through fall growing season in Florida. Therefore, correction factors are required if traps are replaced at 8 to 13 wk intervals. Results of the present study will help optimize monitoring programs that can serve as early warning of potential damaging populations of P. citrella. [ABSTRACT FROM AUTHOR]

Citrus leprosis is one of the most destructive diseases of Citrus spp. and is associated with two unrelated virus groups that produce particles primarily in either the cytoplasm or nucleus of infected plant cells. Symptoms of leprosis, including chlorotic spots surrounded by yellow haloes on leaves and necrotic spots on twigs and fruit, were observed on leprosis-affected mandarin and navel sweet orange trees in the state of Queretaro, Mexico. Serological and molecular assays showed that the cytoplasmic types of Citrus leprosis virus (CiLV-C) often associated with leprosis symptomatic tissues were absent. However, using transmission electron microscopy, bullet-shaped rhabdovirus-like virions were observed in the nuclei and cytoplasm of the citrus leprosis-infected leaf tissues. An analysis of small RNA populations from symptomatic tissue was carried out to determine the genome sequence of the rhabdovirus-like particles observed in the citrus leprosis samples. The complete genome sequence showed that the nuclear type of CiLV (CiLV-N) present in the samples consisted of two negative-sense RNAs: 6,268-nucleotide (nt)- long RNA1 and 5,847-nt-long RNA2, excluding the poly(A) tails. CiLV-N had a genome organization identical to that of Orchid fleck virus (OFV), with the exception of shorter 5' untranslated regions in RNA1 (53 versus 205 nt) and RNA2 (34 versus 182 nt). Phylogenetic trees constructed with the amino acid sequences of the nucleocapsid (N) and glycoproteins (G) and the RNA polymerase (L protein) showed that CiLV-N clusters with OFV. Furthermore, phylogenetic analyses of N protein established CiLV-N as a member of the proposed genus Dichorhavirus. Reversetranscription polymerase chain reaction primers for the detection of CiLV-N were designed based on the sequence of the N gene and the assay was optimized and tested to detect the presence of CiLV-N in both diseased and symptom-free plants. [ABSTRACT FROM AUTHOR]

Asiatic citrus canker (ACC) is caused by Xanthomonas citri subsp. citri. The disease results in yield loss and renders fruit unfit for the fresh market. A 6-year study in Paraná State, Brazil, was conducted to compare the susceptibility of 186 genotypes of citrus representing sweet orange (Citrus sinensis), mandarin (C. reticulata), Mediterranean mandarin (C. deliciosa), Clementine mandarin (C. clementina), Satsuma mandarin (C. unshiu), sour orange (C. aurantium), lemon (C. limon), sweet lime (C. aurantifolia), grapefruit (C. paradisi), and four hybrids (C. reticulata × Citrus sp., C. reticulata × C. paradisi, C. reticulata × C. sinensis, and C. unshiu × C. sinensis). Sweet orange (C. sinensis) was represented by the most genotypes (n = 141). The number of lesions per leaf was assessed 18 times from 2005 to 2010 (up to 4 times per year). The data were analyzed using mixed-model analysis of fixed and random effects, which showed a total of six resistance-susceptibility groupings of species and hybrids. Based on species, the most resistant genotypes, on average, included Satsuma and lemon (mean lesions per leaf = 4.32 and 4.26, respectively), and the most susceptible genotypes were grapefruit and sweet lime, with 14.84 and 10.96 lesions per leaf, respectively. Genotypes of mandarin, sour orange, Mediterranean mandarin, and sweet orange had intermediate severity (5.48 to 9.56 lesions per leaf). The hybrids also showed a range of ACC severity but all were in the more resistant groupings (5.26 to 7.35 lesions per leaf). No genotype was immune to ACC. The most resistant genotype was 'Muscia' (C. reticulata) and the most susceptible was 'Valencia Frost' (C. sinensis) (1.86 and 14.78 lesions per leaf, respectively). Approximately one-sixth of the genotypes showed a negative relationship of mean lesions per leaf with time, suggesting increasing resistance as they aged, due to a reduction in either new flush or plant size and structure. These results of the relative susceptibility of different citrus genotypes can be used in future research and to assist in varietal selection or for breeding purposes both within Brazil and other regions where ACC is an issue. [ABSTRACT FROM AUTHOR]

Field evaluations have shown that Satsuma mandarin (Citrus unshiu) 'Okitsu' is one of the mandarin cultivars that shows substantial resistance to Xanthomonas citri subsp. citri (X citri), the causal agent of citrus bacterial canker disease. However, the mechanisms underlying this resistance are not well understood. In this study, we have shown that 'Okitsu' leaves are nevertheless susceptible to X. citri infection during a period of their development; however, this period is shorter than that seen in the susceptible mandarin 'Clemenules' (C. Clementina). Under controlled growth conditions, the resistance of 'Okitsu' to X. citri was associated with the age of the leaf and was evident in spray-inoculated plants but not in those inoculated by infiltration. Furthermore, X. citri showed reduced attachment and biofilm formation in 'Okitsu' leaves compared with 'Clemenules'. Taken together, our data suggest that structural features of the 'Okitsu' leaf surface, such as the physical properties of the cuticle, are involved in the resistance to X. citri. [ABSTRACT FROM AUTHOR]

Xanthomonas citri ssp. citri (Xcc) is the causal agent of Asiatic citrus canker (ACC), a commercially important disease in Florida citrus as well as in many other regions. In this study we evaluated occurrence of foliar lesions from ACC on progenies of 94 seed-source genotypes (hereafter called "parent genotypes") of Citrus and Citrus relatives in the field in east-central Florida to identify the relative susceptibility to ACC. Eight seedlings per parent genotype were planted in a randomized complete block planting, but the number of plants assessed in some genotype groups was reduced by mortality at some or all sampling dates. Plants experienced ambient exposure to high Xcc inoculum pressure and plants were assessed in Sept. 2010, July 2011, Oct. 2011, May 2012, and Sept. 2012. The incidence and severity of ACC lesions were assessed and evaluated using non-parametric analyses to compare progeny from the 94 parent genotypes. Progeny of 14 parent genotypes did not exhibit ACC symptoms at any date. All were in genera other than Citrus with only Microcitrus and Eremocitrus being crosscompatible with Citrus. The kumquat hybrid C. halimii, two accessions of C. reticulata, C. nobilis, and C. sunki were the only Citrus species in the group that had a low severity (percent total leaf area showing symptoms) on each date of assessment. The aforementioned accessions had an average incidence and severity of ACC lesions of less than 4% in 2011 and 2012, but 26% to 38% in 2010 when no chemical control for ACC was applied in the adjoining citrus groves at our field site. Fourteen of the 16 progeny of C. reticulata and related parent genotypes were in the group with the lowest incidence and severity of ACC on two or more assessment dates. However, for analysis of only the most symptomatic leaves on each plant, 10 C. reticulata parent genotype progenies were in the most resistant category on the Sept. 2012 assessment despite having a low incidence and severity of ACC symptoms overall. Progeny of Poncirus and its hybrids as well as those of C. maxima, C. limon, and related species were the most severely diseased at all assessment dates. There were few instances in which progeny of different accessions of the same species had markedly different responses to ACC: progeny of C. reticulata 'Fremont' displayed more severe ACC compared with several other C. reticulata groups and C. aurantium 'Zhuluan' displayed much lower incidence and severity of ACC compared with several other accessions designated C. aurantium. Information on ACC susceptibility in diverse Citrus and Citrus relatives may prove useful for breeding programs aimed at reducing ACC susceptibility and will be of value to researchers interested in mechanisms of ACC resistance and susceptibility. [ABSTRACT FROM AUTHOR]

Citrus is an economically important fruit crop that is severely afflicted by Asiatic citrus bacterial canker (CBC), a disease caused by the phytopathogen Xanthomonas citri subsp. citri (X. citri). To gain insight into the molecular epidemiology of CBC, 42 Xanthomonas isolates were collected from a range of Citrus spp. across 17 different orchards in Tucumán, Argentina and subjected to molecular, biochemical, and pathogenicity tests. Analysis of genome-specific X. citri markers and DNA polymorphisms based on repetitive elements-based polymerase chain reaction showed that all 42 isolates belonged to X. citri. Interestingly, pathogenicity tests showed that one isolate, which shares >90% genetic similarity to the reference strain X. citri T, has host range specificity. This new variant of X. citri subsp. citri, named X. citri AT, which is deficient in xanthan production, induces an atypical, noncankerous chlorotic phenotype in Citrus limon and C. paradisi and weak cankerous lesions in C. aurantifolia and C. clementina leaves. In C. limon, suppression of canker development is concomitant with an oxidative burst; xanthan is not implicated in the phenotype induced by this interaction, suggesting that other bacterial factors would be involved in triggering the defense response. [ABSTRACT FROM AUTHOR]

Abstract: Citrus canker is one of the most important agricultural citrus diseases worldwide. It is caused by Xanthomonas citri subsp. citri (Xcc) bacterium that infects leaves and the fruits produce a cysteine peptidase (CPXaC), which makes it a potential target for the development of effective and rapid detection methods for citrus canker. We report here the studies on the development of piezoelectric immunoassay for CPXaC using a polyclonal antibody against CPXaC (anti-CPXaC). Three different strategies for covalent immobilization of anti-CPXaC on gold surfaces were evaluated by monitoring the frequency (Δf) and energy dissipation (ΔD) variation in real time when 64.5×10−8 molL−1 CPXaC was added. Anti-CPXaC immobilized with 11-mercaptoundecanoic acid (MUA) showed the best relation between the frequency and dissipation factor variation, and strong values for the kinetic and equilibrium binding constant were obtained. The immunosensor showed a detection limit of 13.0nmolL−1 with excellent specificity, showing no response for different proteins that include another cysteine peptidase that is used as a target to detect Xylella fastidiosa bacterium, responsible for another important citrus disease. These results provide good perspectives for the use of CPXaC as a new biomarker for citrus canker. [Copyright &y& Elsevier]

The plant-pathogenic bacterium Xanthomonas citri subsp. citri is the causal agent of Asiatic citrus canker, a serious disease that affects all the cultivars of citrus in subtropical citrus-producing areas worldwide. There is no curative treatment for citrus canker; thus, the eradication of infected plants constitutes the only effective control of the spread ofX. citri subsp. citri. Since the eradication program in the state of Sâo Paulo, Brazil, is under threat, there is a clear risk of X citri subsp. citri becoming endemic in the main orange-producing area in the world. Here we evaluated the potential use of alkyl gallates to prevent X. citri subsp. citri growth. These esters displayed a potent anti-X. citri subsp. citri activity similar to that of kanamycin (positive control), as evaluated by the resazurin microtiter assay (REMA). The treatment of X citri subsp. citri cells with these compounds induced altered cell morphology, and investigations of the possible intracellular targets using X. citri subsp. citri strains labeled for the septum and centromere pointed to a common target involved in chromosome segregation and cell division. Finally, the artificial inoculation of citrus with X. citri subsp. citri cells pretreated with alkyl gallates showed that the bacterium loses the ability to colonize its host, which indicates the potential of these esters to protect citrus plants against X. citri subsp. citri infection. [ABSTRACT FROM AUTHOR]

Xanthomonas axonopodis pv. citri ( Xac), the bacterium that causes citrus canker, contains a gene in the hrp [for hypersensitive response (HR) and pathogenicity] cluster that encodes a harpin protein called Hpa1. Hpa1 produced HR in the nonhost plants tobacco, pepper and Arabidopsis, whereas, in the host plant citrus, it elicited a weak defence response with no visible phenotype. Co-infiltrations of Xac with or without the recombinant Hpa1 protein in citrus leaves produced a larger number of cankers in the presence of the protein. To characterize the effect of Hpa1 during the disease, an XacΔhpa1 mutant was constructed, and infiltration of this mutant caused a smaller number of cankers. In addition, the lack of Hpa1 hindered bacterial aggregation both in solution and in planta. Analysis of citrus leaves infiltrated with Hpa1 revealed alterations in mesophyll morphology caused by the presence of cavitations and crystal idioblasts, suggesting the binding of the harpin to plant membranes and the elicitation of signalling cascades. Overall, these results suggest that, even though Hpa1 elicits the defence response in nonhost plants and, to a lesser extent, in host plants, its main roles in citrus canker are to alter leaf mesophyll structure and to aggregate bacterial cells, and thus increase virulence and pathogen fitness. We expressed the N-terminal and C-terminal regions and found that, although both regions elicited HR in nonhost plants, only the N-terminal region showed increased virulence and bacterial aggregation, supporting the role of this region of the protein as the main active domain. [ABSTRACT FROM AUTHOR]

Abstract: Histopathological assays gain physiological insights into the mechanisms by which kumquat and calamondin are resistant to citrus canker caused by Xanthomonas citri subsp. citri (Xcc). Unlike Mexican lime, which is highly susceptible to Xcc, calamondin and kumquat leaves challenged with Xcc developed rapid necrosis, resembling a hypersensitive reaction. Fluorescent microscopy examination identified fluorescent materials around the necrotic lesions in kumquat and calamondin leaves challenged with Xcc, but not in Mexican lime. The resistant cultivars inoculated with Xcc exhibited lower antioxidant enzymatic activities and accumulated less H2O2. These modifications serve as physical barriers to ward off pathogen, thereby suppressing lesion development. [Copyright &y& Elsevier]

Xanthomonas axonopodis pv. citri (Xac) causes citrus canker disease, a major threat to citrus production worldwide. Accumulating evidence suggests that the formation of biofilms on citrus leaves plays an important role in the epiphytic survival of this pathogen prior to the development of canker disease. However, the process of Xac biofilm formation is poorly understood. Here, we report a genome-scale study of Xac biofilm formation in which we identified 92 genes, including 33 novel genes involved in biofilm formation and 7 previously characterized genes, colR, fhaB, fliC, galU, gumD, wxacO, and rbfC, known to be important for Xac biofilm formation. In addition, 52 other genes with defined or putative functions in biofilm formation were identified, even though they had not previously reported been to be associated with biofilm formation. The 92 genes were isolated from 292 biofilm-defective mutants following a screen of a transposon insertion library containing 22,000 Xac strain 306 mutants. Further analyses indicated that 16 of the novel genes are involved in the production of extracellular polysaccharide (EPS) and/or lipopolysaccharide (LPS), 7 genes are involved in signaling and regulatory pathways, and 5 genes have unknown roles in biofilm formation. Furthermore, two novel genes, XAC0482, encoding a haloacid dehalogenase-like phosphatase, and XAC0494 (designated as rbfS), encoding a twocomponent sensor protein, were confirmed to be biofilm-related genes through complementation assays. Our data demonstrate that the formation of mature biofilm requires EPS, LPS, both flagellum-dependent and flagellum-independent cell motility, secreted proteins and extracellular DNA. Additionally, multiple signaling pathways are involved in Xac biofilm formation. This work is the first report on a genome-wide scale of the genetic processes of biofilm formation in plant pathogenic bacteria. The report provides significant new information about the genetic determinants and regulatory mechanism of biofilm formation. [ABSTRACT FROM AUTHOR]

The diseases huanglongbing [HLB, associated with Candidatus Liberibacter asiaticus (CLas)] and Asian citrus canker [ACC, caused by Xanthomonas cirri (Xcc)] are widespread in Florida and many other citrus-growing areas, presenting unprecedented challenges for citrus breeding. Because HLB and ACC weaken trees and compromise cropping, breeding is much less efficient using seed parents that have been exposed to these diseases. Therefore, it would be highly desirable to use unique disease-exposed selections only as pollen parents with pollen applied to disease-free trees. However, there may be a risk of introducing these diseases using such pollen sources. To assess this potential, abundance of the pathogens associated with these diseases was assessed in anthers and flowers using quantitative polymerase chain reaction. Because CLas is systemic, levels on mature leaves from the flower source trees were assessed to see if the presence of CLas in flowers was associated with leaf levels. Disease-exposed trees were tested in 10 genotypes from each of three broad genotypic categories, which reflect different levels of susceptibility to the diseases associated with the pathogens studied: Poncirus trifoliata hybrids (most resistant to HLB), Citrus maxima and hybrids (susceptible to both diseases), and C. reticulata and hybrids (considerable resistance to ACC). Of the 30 samples of each tissue type analyzed for CLas, 88% of mature leaves, 69% of flowers, and 88% of anthers had one or more CLas bacterium per sample. The trifoliate genotypic group had significantly lower levels of CLas than the pummelo and mandarin groups in mature leaf samples, but CLas levels were more similar between groups in anther and flower samples, and the pathogen was present in most of the trifoliate hybrids tested. Mean numbers of CLas detected per nanogram nucleic acid were 100 to 800 times higher in mature leaf samples, most characteristic of HLB symptoms, compared with anther samples. Xcc DNA was detected in 30% of flower samples and 23% of anther samples. No significant differences in Xcc levels were found between tissue type or genotypic group. However, regressions between Xcc levels in flowers and percent of plant pedigree derived from mandarin had a negative correlation and an r² of 0.159 (P= 0.029). The biology of CLas is consistent with the pathogen being present in anthers from unopened flowers, whereas the ACC pathogen detected inside flowers was likely the result of contamination despite great care in sample collection and handling. Where exceptional diligence to exclude HLB and ACC is appropriate, results suggest that there may be a risk of spreading these pathogens through use of pollen from trees on infected farms. [ABSTRACT FROM AUTHOR]

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is a worldwide pest of citrus crops and is responsible for proliferation of citrus bacterial canker, Xanthomonas axonopodis (Hasse) pv. citri (Gamma Proteobacteria: Xanthomonadaceae). We developed and evaluated an attracticide formulation, termed MalEx, for control of P. citrella. MalEx is a viscous paste with UV-protective properties that is dispensed as 50-μl droplets using custom-made calibrated pumps. A formulation containing 0.016% P. citrella pheromone [3:1 blend of ( Z, Z, E)-7,11,13-hexadecatrienal and ( Z, Z)-7,11-hexadecadienal] and 6% permethrin was found to suppress male response to pheromone in the field better than formulations containing 10× less pheromone. Although formulations without permethrin showed some suppression of male activity because of mating disruption, addition of 6% permethrin to the formulation was required for optimal efficacy. When MalEx, containing 0.016% pheromone and 6% permethrin, was applied at 3 000 point sources ha−1, the application height did not influence efficacy of male P. citrella suppression within mature 4-m tall citrus trees. Decreasing the rate of MalEx from 3 000 to 1 500 droplets ha−1 reduced efficacy as measured by both male P. citrella activity and larval infestation. Although 4 500 droplets ha−1 did not result in statistically better efficacy than 3 000 droplets ha−1, there was a noticeable trend for higher efficacy as droplet density increased. Continuous treatment of 0.5-ha blocks of citrus with MalEx over the course of 112 days reduced larval infestation of new flush, as compared with those in untreated control plots, by 3.6–7.2× depending on droplet application density. In laboratory behavioral bioassays, the attractiveness of MalEx droplets to male P. citrella was drastically reduced after 21 days of field aging. However, our laboratory investigation confirmed that 100% of males contacting MalEx droplets, aged up to 35 days in the field, were killed within 24 h. Direct observation of male P. citrella behavior in the field confirmed that attracted males made contact with droplets. Control of P. citrella with MalEx should reduce the number of required broad spectrum sprays for leafminer management in both field and citrus nursery settings. [ABSTRACT FROM AUTHOR]

In nursery trial conducted at Citrus Research Institute, Sargodha, Pakistan during 2005-2006, various chemicals i.e. streptomycin sulphate, Bac-Off (copper based fungicide), copper oxychloride, and Score (dipheno conazole) were sprayed against Xanthomonas compestris pv. citri on sweet orange (cv. Blood Red). Although all chemicals exhibited non-significant differences, yet these reduced the disease considerably. Score remained at top reducing 67.29 percent disease incidence followed by streptomycin sulphate (62%) after 45 days with three sprays. Bac-Off offered minimum disease control (55.56%). Regarding reduction of lesions per leaf, copper oxychloride gave good results after 45 days with three sprays by exhibiting 70 percent control over disease lesions on leaves. It was followed by Score (66.70%), streptomycin sulphate (59.80%). Bac-Off, gave poor results after each spray. All other chemicals behaved differently by in each spray. [ABSTRACT FROM AUTHOR]

Abstract: Citrus canker is one of the most devastating diseases that threaten marketability of citrus crops. This research was aimed to develop a hyperspectral imaging approach for detecting canker lesions on citrus fruit. A hyperspectral imaging system was developed for acquiring reflectance images from citrus samples in the spectral region from 450 to 930nm. Ruby Red grapefruits with cankerous, normal and other common peel diseases including greasy spot, insect damage, melanose, scab, and wind scar were tested. Spectral information divergence (SID) classification method, which was based on quantifying the spectral similarities by using a predetermined canker reference spectrum, was performed on the hyperspectral images of the grapefruits for differentiating canker from normal fruit peels and other citrus surface conditions. The overall classification accuracy was 96.2% using an optimized SID threshold value of 0.008, which was determined under the condition that the errors of false negative and false positive were weighted equally. Considering the high economic impact of missing a cankerous fruit, zero false negative error was achieved by using a threshold value of 0.009, under which the classification accuracy was 95.2%. This research demonstrated that hyperspectral imaging technique coupled with the SID based image classification method could be used for discriminating citrus canker from other confounding diseases. [Copyright &y& Elsevier]

Citrus canker (Xanthomonas citri subsp. citri) is destructive in many citrus production regions in tropical and subtropical parts of the world. Assessment of canker symptoms is required for diverse reasons, including monitoring epidemics, evaluating the efficacy of control strategies, and disease response in breeding material. The objectives were to compare the ability of experienced and inexperienced raters at assessing citrus canker, to identify factors that affect the quality of the assessment, to determine common sources of error, and to discern how error is related to actual disease magnitude. Two-hundred digital leaf images (0 to 37% area infected) were assessed once by 28 raters, five of whom were experienced plant pathologists (PPs), and 23 who had no experience in disease severity assessment (NPPs). True disease (lesion number [LN], % necrotic area [%N], and % chlorotic+necrotic area [%CN]) was measured using image analysis on a leaf-by-leaf basis, and each parameter was estimated by the 28 raters. LN was neither severely over- nor underestimated, while %N was greatly overestimated, with a lesser tendency to overestimate %CN over the true severity range of these two symptom types. A linear relationship existed between estimate of the disease and true disease for all measures of severity. Data were heteroscedastic and error was not constant with increasing true disease. Agreement between rater estimates and true disease was measured with Lin's concordance correlation coefficient (pc). LN showed greatest agreement (pc = 0.88 to 0.99), followed by %CN (pc = 0.80 to 0.95) and %N (pc = 0.19 to 0.84). Greater lesion number resulted in overestimation of area infected for both %N and %CN. Overestimation was particularly noticeable at low disease seventies. There was a linear relationship between log variance and log true disease for LN (r² = 0.71), %N (r² = 0.85), and %CN (r² = 0.88), and raters tended to estimate disease above 10% to the nearest 5 or 10%. GLM analysis showed differences between PP and NPP groups in assessing disease. For LN, precision of assessment for both groups was similar (r² > 0.92 and 0.94, respectively), but for estimates of %N and %CN, the PPs were more precise (%N and %CN, r² = 0.61 and 0.73, respectively) compared to NPPs (%N and %CN, r² = 0.45 and 0.58, respectively). Absolute error for mean LN was low. The absolute error of %N and %CN showed overestimation to approximately 8% area infected. Above 8%, absolute error increased, but comprised both over- and underestimation. For %N and %CN, relative error was almost exclusively positive and dramatic at severity <8% (up to approximately 600%), but at severity >10% it was relatively small. Error in rater estimates of canker severity is ubiquitous. Understanding these sources of error will aid in the development of both appropriate training and relevant rating aids. [ABSTRACT FROM AUTHOR]

Asiatic citrus canker is a major disease worldwide, and its causal agent, Xanthomonas citri pv. citri, is listed as a quarantine organism in many countries. Analysis of the molecular epidemiology of this bacterium is hindered by a lack of molecular typing techniques suitable for surveillance and outbreak investigation. We report a comparative evaluation of three typing techniques, amplified fragment length polymorphism (AFLP) analysis, insertion sequence ligation-mediated PCR (IS-LM-PCR) typing, and multilocus variable-number tandem-repeat analysis (MLVA), with 234 strains originating from Asia, the likely center of origin of the pathogen, and reference strains of pathotypes A, A*, and Aw, which differ in host range. The typing techniques were congruent in describing the diversity of this strain collection, suggesting that the evolution pattern of the bacterium may be clonal. Based on a hierarchical analysis of molecular variance, the AFLP method best described the genetic variation found among pathotypes whereas MLVA best described the variation found among individual strains from the same countries or groups of neighboring countries. IS-LM-PCR data suggested that the transposition of insertion sequences in the genome of X. citri pv. citri occurs rarely enough not to disturb the phylogenetic signal. This technique may be useful for the global surveillance of non-epidemiologically related strains. Although pathological characteristics of strains could be most often predicted from genotyping data, we report the occurrence in the Indian peninsula of strains genetically related to pathotype A* strains but with a host range similar to that of pathotype A, which makes the classification of this bacterium even more complicated. [ABSTRACT FROM AUTHOR]

Bacteriophages, alone or in combination with copper bactericides, were evaluated for managing Asiatic citrus canker and citrus bacterial spot incited by Xanthomonas axonopodis pathovars citri and citrumelo, respectively. In a set of five greenhouse experiments, phage treatment provided consistent control of citrus canker, causing an average of 59% reduction in disease severity. However, treatment with phage was ineffective if applied with skim milk, a protective formulation, which increases phage residual activity. In nursery settings, phage treatment also reduced disease but was less effective than copper-mancozeb, a chemical bactericide. The integration of phage and copper-mancozeb resulted in equal or less control than copper-mancozeb application alone. Phage treatments were evaluated in a commercial citrus nursery for reducing citrus bacterial spot caused by natural inoculum. Phage treatment provided significant disease reduction on moderately sensitive Valencia oranges in two trials (48 and 35%); however, on the highly susceptible grapefruit host it was ineffective. In an experimental citrus nursery, phage treatment provided significant control of citrus bacterial spot caused by a phage-sensitive strain, but was equally or less effective than copper-mancozeb. The combination of phage and copper-mancozeb did not increase control compared with copper-mancozeb alone. [ABSTRACT FROM AUTHOR]

Citrus canker is a disease of citrus and is caused by the bacterial pathogen Xanthomonas citri subsp. citri. Ways of managing the disease are being sought, and accurate, precise, reproducible disease assessment is needed for monitoring epidemics. The objective of this study was to investigate the characteristics of visual assessment of citrus canker symptoms compared with actual disease measured using image analysis (IA). Images of 210 citrus leaves with a range of incidence and severity of citrus canker were assessed by three plant pathologists (VR 1-3) and by IA. The number of lesions (L), % area necrotic (%AN), and % area necrotic+chlorotic (%ANC) were assessed. The best relationships were found between %AN and %ANC (r² = 0.41 to 0.87), and the worst between L and %AN (r² = 0.27 to 0.66). Bland-Altman plots showed various sources of rater error in assessments, including under- and over-estimation, proportional error, and heterogeneity of variation dependent on actual disease magnitude. There was a tendency to overestimate area diseased, but not lesion counts, and this tendency was pronounced at lower disease severity, with a leaf having more lesions tending to be assessed as having greater area infected compared with a leaf with fewer lesions but equal actual area infected. The rater estimations of disease were less accurate or precise with increasing actual disease severity as indicated by the fit of a normal probability density function—the incidence of extreme values increases with increasing actual disease. For example, for %ANC the kurtosis of the distribution ranged from 17.92 to 1.18, 0.51, and 0.22 in actual disease category ranges of 0 to 10, 11 to 20, 21 to 30, and 31 to 40% area infected, respectively. The log variance of the estimates plotted against log actual disease for all three raters over two assessment occasions gave a linear relationship for L, %AN, and %ANC (r² = 0.74, 0.65, and 0.74, respectively). Training should improve the accuracy, precision, and reproducibility of raters, and knowledge of the characteristics of disease assessment should help develop and target the training more appropriately and address specific causes and sources of error. [ABSTRACT FROM AUTHOR]

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is an important world-wide pest of citrus. Larval mining within leaf flush impacts yield and predisposes trees to infection by citrus canker, Xanthomonas axonopodis pv. citri. The present series of studies sought to identify factors affecting male P. citrella catch in pheromone-baited traps with the intent of developing effective monitoring. A commercially available pheromone lure (Citralure, ISCA Technologies, Riverside, CA, USA) was highly effective in attracting male P. citrella to traps. Pherocon VI Delta (Trécé Inc., Adair, OK, USA) traps baited with a Citralure captured more male P. citrella than identically baited Pherocon IC Wing traps (Trécé Inc.). The superiority of the Delta-style trap was found to be due to a 3 cm long closing latch that likely prevents males from flying directly through the trap without capture. Within canopies of mature citrus trees (approximately 3.5 m high), traps at mid-canopy height (2.0 m) captured more males than traps placed higher (3.5 m) or lower (0.6 m). On the canopy perimeter and in between canopies, traps near ground level (0.6 m height) captured more males than traps at 2.0 and 3.5 m heights. Male catch was greater within the tree canopy or on the canopy perimeter than 2.0 away from the canopy. Traps deployed in trees on the edge of groves captured more males than traps placed 120 and 240 m away from the grove edge and within the grove interior. In non-pheromone-treated grove plots, the optimal dosage for catching males was between 0.1 and 1.0 mg of the 3 : 1 blend of ( Z, Z, E)-7,11,13-hexadecatrienal and ( Z, Z)-7,11-hexadecadienal; however, in pheromone-treated plots a higher 10.0 mg dosage lure was most effective. Male catch in pheromone-baited traps exhibited a diel rhythm with most males captured during scotophase (22:00–23:00 h) and no males captured during photophase. [ABSTRACT FROM AUTHOR]

Considerable losses of citrus trees have been observed in the major citrus-growing areas of Spain. Samples were collected from 132 orchards, and isolations and pathogencity tests were conducted to determine the aetiology of a serious canker disease. Affected trees showed cankers on the scion that frequently began on the branches. Three Phytophthora species were identified based on their morphological, cultural, physiological and molecular profiles. Phytophthora citrophthora was the main species associated with this new syndrome in 114 orchards. Phytophthora nicotianae (syn. P. parasitica) was isolated from nine orchards as the sole Phytophthora species and in coinfection with P. citrophthora from another nine orchards. Phytophthora citricola was isolated only from one orchard. In stem-inoculation studies conducted under greenhouse conditions, clementine mandarin cv. Hernandina and sweet orange cv. Navel Late were more susceptible to P. citrophthora than sour orange and Carrizo citrange rootstocks. Clementine cv. Hernandina was also highly susceptible in field inoculation experiments. In agreement with field surveys, clementine mandarin cultivars were the most affected, their rootstocks remaining healthy. Phytophthora citrophthora was found to be the predominant species in orchard soils; however, P. nicotianae was also isolated. This information changes the scenario of diseases caused by Phytophthora spp. in Spain and consequently, the present knowledge of epidemiology and the effectiveness of the current control measures should be reassessed. [ABSTRACT FROM AUTHOR]

Citrus bacterial canker (CBC) caused by Xanthomonas axonopodis pv. citri (Xac) was first documented in India and Java in the mid 19th century. Since that time, the known distribution of the disease has steadily increased. Concurrent with the dispersion of the pathogen, the diversity of described strains continues to increase, with novel strains appearing in Saudi Arabia, Iran, and Florida in the last decade. Herbarium specimens of infected plants provide an historical record documenting both the geographic distribution and genetic diversity of the pathogen in the past. However, no method was available to assess the genetic diversity within these herbarium samples. We have developed a method, insertion event scanning (IES), and applied the method to characterize the diversity present within CBC populations documented as herbarium specimens over the past century. IES is based on the specific amplification of junction fragments that define insertion events. The potential for IES in current forensic applications is demonstrated by finding an exact match of pathogen genotypes preserved in herbarium specimens from Japan and Florida, demonstrating the source of the original outbreak of citrus canker in Florida in 1911. IES is a very sensitive technique for differentiating bacterial strains and can be applied to any of the several hundred bacteria for which full genomic sequence data are available. [ABSTRACT FROM AUTHOR]

Citrus canker disease caused by Xanthomonas campestris pv. citri is reported on lime in the present study for the first time in United Arab Emirates. The disease was found only on lime in 32 orchards out of 4456 citrus orchards inspected during 1984-85. The citrus canker organism in U.A.E. has a host range similar to "A" strain. Copper hydroxide was significantly superior to all other treatments in decreasing the incidence of citrus canker disease that developed on inoculated lime seedlings. Streptocycline, Kasumin, copper oxychloride and Bordeaux mixture were not effective in reducing the incidence of the disease. [ABSTRACT FROM AUTHOR]

Population density of citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), was monitored in a Florida citrus grove for 5 years by scouting weekly for larval-induced mines (leafminer-created tunnels in the leaves) in a replicated citrus plot treated with seven insect control regimes: Admire (imidacloprid) applied at 12, 6, 3, or 2-month intervals; Temik (aldicarb) applied annually; Metasystox-R (oxydemeton-methyl) applied annually; or no insect control. Leafminer populations were highest during the warmer months (April to September) and lowest during the cooler months (November to March). Populations peaked during June in all 5 years monitored. Trees treated with Temik or Metasystox-R had the same number of mines as the untreated controls. A biannual treatment with Admire reduced leafminer damage (number of mines) all 5 years compared with the controls. Additional Admire applications further reduced damage during some, but not all, years. A single application of Admire significantly reduced mines in 3 of the 5 years. [ABSTRACT FROM AUTHOR]

Telomerase (TERT), a specialized reverse transcriptase, mediates maintenance of telomere length and is closely associated with cellular proliferation capacity. Because disordered cell division and cell enlargement are crucial events for symptom development with citrus canker, the involvement of telomerase in the disease was tested. Southern analysis using probes of AtTERT confirmed that citrus has its homolog as a single copy. The telomeric repeat amplification protocol assay revealed that the telomerase activity was induced specifically in citrus leaves infected with Xanthomonas axonopodis pv. citri but not in mock-inoculated leaves, indicating a possible role for telomerase in citrus canker development. [ABSTRACT FROM AUTHOR]

Citrus leafminer (CLM), Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae), is one of the most important insect pests of Pakistan's citrus nursery stock and caused extensive damage to young flushes. The organic compost is a widespread technique used to manage insect pests and plant diseases. Different composts (biofert, tara root and vermicompost) at 0.5 and 0.25 kg/plant concentration in comparison to NPK fertilizer at 0.4 and 0.2 g/plant were evaluated for CLM infestation and the associated citrus canker disease in nursery plantations of Citrus reticulata Blanco. Application of biofert at 0.5 kg/plant reduced the CLM infestation up to 54.5% during Fall-2016 and 39.1% during Summer-2017 in comparison to control treatment. The CLM larval density was also found lower by the application of biofert followed by vermicompost during both seasons. Both concentrations of biofert followed by vermicompost at 0.5 kg/plant resulted in remarkable protection against citrus canker disease in both flushes. The incidence of canker associated with CLM infested leaves was also studied and found lower by the application of biofert and vermicompost compared with control treatment. Conclusively, the soil amendment using biofert and vermicompost affects the CLM population and canker infection in nursery plantations. These organic fertilizers can be used in future citrus IPM programs as a tool to suppress the CLM population and citrus canker disease. [ABSTRACT FROM AUTHOR]

The article focuses on the plant disease Asiatic Citrus Canker caused by Xanthomonas citri pv. citri-A in and has been reported in Mayotte and mentions that the disease is of major economic importance as it can destroy wide range of citrus species leading into premature fruit and leaf drop.

Examines the early-warning system developed by Agricultural Research Service researchers at the United States Horticultural Research Laboratory to monitor Asiatic citrus cancer disease (ACC). Market value of the citrus industry in Florida; Advantages of the tree sentinel grid system in detecting ACC in citrus production areas; How the system works.