Temporal Progress of Huanglongbing Epidemics and the Effect of Noncommercial Inoculum Sources on Citrus Orchards in São Paulo State, Brazil

Huanglongbing (HLB) incidence is increasing and threatening citrus production in São Paulo State, Brazil, despite multiple efforts to control the disease and its vector, the Asian citrus psyllid (ACP) (Diaphorina citri). The objective of this research was to study the temporal dynamics of HLB epidemics, under intensive disease management, in 177 individual commercial citrus blocks on a single property in São Paulo State. The effect of internal and external sources of HLB-associated bacteria and its vector were explored based on the disease epidemics and vector dynamics in the studied area. To manage HLB, the property owner used healthy nursery plants, eradicated symptomatic trees, and used insecticides to control ACPs. Logistic and Gompertz models were fitted to the data to describe dynamics of HLB incidence for all blocks. The average number of ACPs per yellow sticky trap was determined for the same blocks for a period of four consecutive years. Both logistic and Gompertz models described the HLB epidemics well, although the Gompertz model provided a slightly better fit. Disease progress rates, HLB incidences, and average ACP count per trap in the 177 blocks were low compared with reports in the literature. HLB incidence and number of ACPs per trap were higher (P ≤ 0.05) in some citrus blocks located on the periphery of the property. A large number of noncommercial trees were found near the property and were a potential primary inoculum source of HLB-associated bacteria, accounting for the higher incidence of HLB and ACPs per trap in blocks located on the periphery of the property. These results support the recommended preventive measures to HLB management and the necessity of external actions, to include trees in commercial orchards, and noncommercial trees located near commercial citrus properties, in an attempt to maximize the effectiveness of these preventive measures.

Relative Contribution of Windbreak, Copper Sprays, and Leafminer Control for Citrus Canker Management and Prevention of Crop Loss in Sweet Orange Trees

The management of citrus canker, caused by Xanthomonas citri subsp. citri, has been widely studied in endemic areas because of the importance of the disease in several citrus-producing countries. A set of control measures is well established, but no study has investigated the efficiency of each measure individually and their combination for disease suppression. This study comprised a 3-year field study to assess the relative contribution of three measures for the control of citrus canker and reduction of crop losses. Windbreak (Wb), copper sprays (Cu), and leafminer control (Lc) were assessed in eight different combinations in a split-split plot design. The orchard was composed of 'Valencia' sweet orange trees grafted onto 'Rangpur' lime. Casuarina cunninghamiana trees were used as Wb. Cu and Lc sprays were performed every 21 days throughout the year. Individually, Cu showed the highest contribution for canker control, followed by Wb. Lc had no effect on reducing citrus canker. Wb+Cu showed the highest efficiency for control of the disease. This combination reduced the incidence of diseased trees by approximately 60%, and the incidence of diseased leaves and fruit by ≥90% and increased the yield in 2.0- to 2.6-fold in comparison with the unmanaged plots. Cu sprays were important for reducing disease incidence and crop losses, whereas Wb had an additional contribution in minimizing the incidence of cankered, non-marketable fruit. The results indicated that the adoption of these measures of control may depend on the characteristics of the orchard and destination of the production.

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.

CitrusKB: a comprehensive knowledge base for transcriptome and interactome of Citrus spp. infected by Xanthomonas citri subsp. citri at different infection stages

Citrus canker type A is a serious disease caused by Xanthomonas citri subsp. citri (X. citri), which is responsible for severe losses to growers and to the citrus industry worldwide. To date, no canker-resistant citrus genotypes are available, and there is limited information regarding the molecular and genetic mechanisms involved in the early stages of the citrus canker development. Here, we present the CitrusKB knowledge base. This is the first in vivo interactome database for different citrus cultivars, and it was produced to provide a valuable resource of information on citrus and their interaction with the citrus canker bacterium X. citri. CitrusKB provides tools for a user-friendly web interface to let users search and analyse a large amount of information regarding eight citrus cultivars with distinct levels of susceptibility to the disease, with controls and infected plants at different stages of infection by the citrus canker bacterium X. citri. Currently, CitrusKB comprises a reference citrus genome and its transcriptome, expressed transcripts, pseudogenes and predicted genomic variations (SNPs and SSRs). The updating process will continue over time by the incorporation of novel annotations and analysis tools. We expect that CitrusKB may substantially contribute to the field of citrus genomics. CitrusKB is accessible at http://bioinfo.deinfo.uepg.br/citrus. Users can download all the generated raw sequences and generated datasets by this study from the CitrusKB website.

Analyses of Seven New Genomes of Xanthomonas citri pv. aurantifolii Strains, Causative Agents of Citrus Canker B and C, Show a Reduced Repertoire of Pathogenicity-Related Genes

Xanthomonas citri pv. aurantifolii pathotype B (XauB) and pathotype C (XauC) are the causative agents respectively of citrus canker B and C, diseases of citrus plants related to the better-known citrus canker A, caused by Xanthomonas citri pv. citri. The study of the genomes of strains of these related bacterial species has the potential to bring new understanding to the molecular basis of citrus canker as well as their evolutionary history. Up to now only one genome sequence of XauB and only one genome sequence of XauC have been available, both in draft status. Here we present two new genome sequences of XauB (both complete) and five new genome sequences of XauC (two complete). A phylogenomic analysis of these seven genome sequences along with 24 other related Xanthomonas genomes showed that there are two distinct and well-supported major clades, the XauB and XauC clade and the Xanthomonas citri pv. citri clade. An analysis of 62 Type III Secretion System effector genes showed that there are 42 effectors with variable presence/absence or pseudogene status among the 31 genomes analyzed. A comparative analysis of secretion-system and surface-structure genes showed that the XauB and XauC genomes lack several key genes in pathogenicity-related subsystems. These subsystems, the Types I and IV Secretion Systems, and the Type IV pilus, therefore emerge as important ones in helping explain the aggressiveness of the A type of citrus canker and the apparent dominance in the field of the corresponding strain over the B and C strains.

Detection and identification of Xanthomonas pathotypes associated with citrus diseases using comparative genomics and multiplex PCR

Background

In Citrus cultures, three species of Xanthomonas are known to cause distinct diseases. X. citri subsp. citri patothype A, X. fuscans subsp. aurantifolii pathotypes B and C, and X. alfalfae subsp. citrumelonis, are the causative agents of cancrosis A, B, C, and citrus bacterial spots, respectively. Although these species exhibit different levels of virulence and aggressiveness, only limited alternatives are currently available for proper and early detection of these diseases in the fields. The present study aimed to develop a new molecular diagnostic method based on genomic sequences derived from the four species of Xanthomonas.

Results

Using comparative genomics approaches, primers were synthesized for the identification of the four causative agents of citrus diseases. These primers were validated for their specificity to their target DNA by both conventional and multiplex PCR. Upon evaluation, their sensitivity was found to be 0.02 ng/µl in vitro and 1.5 × 10⁴ CFU ml⁻¹ in infected leaves. Additionally, none of the primers were able to generate amplicons in 19 other genomes of Xanthomonas not associated with Citrus and one species of Xylella, the causal agent of citrus variegated chlorosis (CVC). This denotes strong specificity of the primers for the different species of Xanthomonas investigated in this study.

Conclusions

We demonstrated that these markers can be used as potential candidates for performing in vivo molecular diagnosis exclusively for citrus-associated Xanthomonas. The bioinformatics pipeline developed in this study to design specific genomic regions is capable of generating specific primers. It is freely available and can be utilized for any other model organism.

Origin and diversification of Xanthomonas citri subsp. citri pathotypes revealed by inclusive phylogenomic, dating, and biogeographic analyses

BACKGROUND

Xanthomonas citri subsp. citri pathotypes cause bacterial citrus canker, being responsible for severe agricultural losses worldwide. The A pathotype has a broad host spectrum, while A* and Aw are more restricted both in hosts and in geography. Two previous phylogenomic studies led to contrasting well-supported clades for sequenced genomes of these pathotypes. No extensive biogeographical or divergence dating analytic approaches have been so far applied to available genomes.

RESULTS

Based on a larger sampling of genomes than in previous studies (including six new genomes sequenced by our group, adding to a total of 95 genomes), phylogenomic analyses resulted in different resolutions, though overall indicating that A + AW is the most likely true clade. Our results suggest the high degree of recombination at some branches and the fast diversification of lineages are probable causes for this phylogenetic blurring effect. One of the genomes analyzed, X. campestris pv. durantae, was shown to be an A* strain; this strain has been reported to infect a plant of the family Verbenaceae, though there are no reports of any X. citri subsp. citri pathotypes infecting any plant outside the Citrus genus. Host reconstruction indicated the pathotype ancestor likely had plant hosts in the family Fabaceae, implying an ancient jump to the current Rutaceae hosts. Extensive dating analyses indicated that the origin of X. citri subsp. citri occurred more recently than the main phylogenetic splits of Citrus plants, suggesting dispersion rather than host-directed vicariance as the main driver of geographic expansion. An analysis of 120 pathogenic-related genes revealed pathotype-associated patterns of presence/absence.

CONCLUSIONS

Our results provide novel insights into the evolutionary history of X. citri subsp. citri as well as a sound phylogenetic foundation for future evolutionary and genomic studies of its pathotypes.

Detection of citrus canker and Huanglongbing using fluorescence imaging spectroscopy and support vector machine technique

Citrus canker and Huanglongbing (HLB) are citrus diseases that represent a serious threat to the citrus production worldwide and may cause large economic losses. In this work, we combined fluorescence imaging spectroscopy (FIS) and a machine learning technique to discriminate between these diseases and other ordinary citrus conditions that may be present at citrus orchards, such as citrus scab and zinc deficiency. Our classification results are highly accurate when discriminating citrus canker from citrus scab (97.8%), and HLB from zinc deficiency (95%). These results show that it is possible to accurately identify citrus diseases that present similar symptoms.

Temperature stress promotes cell division arrest in Xanthomonas citri subsp. citri

Citrus canker is an economically important disease that affects orange production in some of the most important producing areas around the world. It represents a great threat to the Brazilian and North American citriculture, particularly to the states of São Paulo and Florida, which together correspond to the biggest orange juice producers in the world. The etiological agent of this disease is the Gram‐negative bacterium Xanthomonas citri subsp. citri (Xcc), which grows optimally in laboratory cultures at ~30°C. To investigate how temperatures differing from 30°C influence the development of Xcc, we subjected the bacterium to thermal stresses, and afterward scored its recovery capability. In addition, we analyzed cell morphology and some markers of essential cellular processes that could indicate the extent of the heat‐induced damage. We found that the exposure of Xcc to 37°C for a period of 6 h led to a cell cycle arrest at the division stage. Thermal stress might have also interfered with the DNA replication and/or the chromosome segregation apparatuses, since cells displayed an increased number of sister origins side‐by‐side within rods. Additionally, Xcc treated at 37°C was still able to induce citrus canker symptoms, showing that thermal stress did not affect the ability of Xcc to colonize the host citrus. At 40–42°C, Xcc lost viability and became unable to induce disease symptoms in citrus. Our results provide evidence about essential cellular mechanisms perturbed by temperature, and can be potentially explored as a new method for Xanthomonas citri synchronization in cell cycle studies, as well as for the sanitation of plant material.

Comparison of Resistance to Asiatic Citrus Canker Among Different Genotypes of Citrus in a Long-Term Canker-Resistance Field Screening Experiment in Brazil

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.

Asymmetric chromosome segregation in Xanthomonas citri ssp. citri

This study was intended to characterize the chromosome segregation process of Xanthomonas citri ssp. citri (Xac) by investigating the functionality of the ParB factor encoded on its chromosome, and its requirement for cell viability and virulence. Using TAP tagging we show that ParB is expressed in Xac. Disruption of parB increased the cell doubling time and precluded the ability of Xac to colonize the host citrus. Moreover, Xac mutant cells expressing only truncated forms of ParB exhibited the classical phenotype of aberrant chromosome organization, and seemed affected in cell division judged by their reduced growth rate and the propensity to form filaments. The ParB-GFP localization pattern in Xac was suggestive of an asymmetric mode of replicon partitioning, which together with the filamentation phenotype support the idea that Xac may control septum placement using mechanisms probably analogous to Caulobacter crescentus, and perhaps Vibrio cholerae, and Corynebacterium glutamicum. Xac exhibits asymmetric chromosome segregation, and the perturbation of this process leads to an inability to colonize the host plant.

Antibacterial activity of alkyl gallates against Xanthomonas citri subsp. citri

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 of X. citri subsp. citri. Since the eradication program in the state of São 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.

Recombinant expression and characterization of a cysteine peptidase from Xanthomonas citri subsp citri

Xanthomonas citri subsp citri (Xac) is the bacterium responsible for citrus canker disease in citrus plants. The aim of this study was to describe the recombinant expression, purification, and characterization of a cysteine peptidase from Xac strain 306, which is a candidate for involvement in the pathogenicity of this bacterium. The gene was cloned and expressed in Pichia pastoris, and the cysteine peptidase was successfully expressed, secreted, and purified using affinity chromatography with a yield of approximately 10 mg/L. A polyclonal antibody produced against cysteine peptidase from X. citri subsp citri fused with HIS tag ((HIS)CPXAC) recognized the purified recombinant cysteine peptidase (HIS)CPXAC, confirming the correct production of this protein in P. pastoris. The same antibody detected the protein in the culture supernatant of Xac grown in pathogenicity-inducing medium. Kinetic analysis revealed that (HIS)CPXAC hydrolyzed the carbobenzoxy-Leu-Arg-7-amido-4-methylcoumarin substrate with a catalytic efficiency (k(cat)/K(m)) of 47 μM(-1)∙s(-1). The purified ((HIS))CPXAC displayed maximal catalytic activity at pH 5.5 and 30°C. The recombinant enzyme was inhibited by the specific cysteine peptidase inhibitor E-64, as well as by the recombinant cysteine peptidase inhibitors CaneCPI-1, CaneCPI-2, CaneCPI-3, and CaneCPI-4, with K(i) values of 1.214, 84.64, 0.09, 0.09, and 0.012 nM, respectively. Finally, the N-terminal sequencing of the purified protein enabled the identification of the first 5 amino acid residues (AVHGM) immediately after the putative signal peptide, thereby enabling the identification of the cleavage point and corroborating previous studies that have identified this sequence in a secreted protein from Xanthomonas spp.

Genetic Diversity of a Brazilian Strain Collection of Xanthomonas citri subsp. citri Based on the Type III Effector Protein Genes

Exclusion and eradication or management based on an integrated approach with less susceptible varieties, copper-based bactericides, and windbreaks are the two main strategies used to prevent or control citrus canker. Field tolerance or resistance to citrus canker is not found in the most important commercial sweet orange cultivars, and pathogen-derived resistance has been developed and applied in different crops to obtain resistant genotypes to plant pathogens. We describe the development of DNA primers and probes based on the type III effector genes avrXacE1, avrXacE2, avrXacE3, avrBs2, pthA4, hpaF, and XAC3090 (leucine rich protein), and their application in the evaluation of the genetic diversity of the pathogen. A total of 49 haplotypes were identified in 157 strains by Southern blot analysis. No genetic polymorphism was detected by BOX elements - and enterobacterial repetitive intergenic consensus-polymerase chain reaction (ERIC-PCR) analysis, nor with the genes avrBs2, XAC3090, and hpaF. Nei's genetic diversity indexes varied from 0.65 to 0.96 for subcollections of the pathogen. One or few haplotypes were most frequent in the strain collection, but several haplotypes were represented by solely one or few strains. The PthA4 probe resulted in the higher number of haplotypes identified in the Brazilian subcollections. Greater variation in the frequency of haplotypes occurred within subcollections (93.7%) than among subcollections. Only some haplotypes were genetically distant from all others, especially those originated from Rio Grande do Sul and Santa Catarina states. These bacterial effectors are widely spread in the collections and are useful for a better understanding of the host-pathogen interaction and the search for resistance genes in host and nonhost plants.

Optical fiber laser induced fluorescence spectroscopy as a citrus canker diagnostic

Citrus canker is a serious disease caused by Xanthomonas citri subsp. citri bacteria, which infects citrus plants (Citrus spp.) leading to large economic losses in citrus production worldwide. In this work, laser induced fluorescence spectroscopy (LIF) was investigated as a diagnostic technique for citrus canker disease in citrus trees at an orchard using a portable optical fiber based spectrometer. For comparison we have applied LIF to leaves contaminated with citrus canker, citrus scab, citrus variegates chlorosis, and Huanglongbing (HLB, Greening). In order to reduce the noise in the data, we collected spectra from ten leaves with visual symptoms of diseases and from five healthy leaves per plant. This procedure is carried out in order to minimize the environmental effect on the spectrum (water and nutrient supply) of each plant. Our results show that this method presents a high sensitivity (approximately 90%), however it does present a low specificity (approximately 70%) for citrus canker diagnostic. We believe that such poor performance is due to the fact that the optical fiber collects light from only a small part of the leaf. Such results may be improved using the fluorescence imaging technique on the whole leaf.

Detection of a New Bacterium Related to Xanthomonas fuscans subsp. aurantifolii Infecting Swingle Citrumelo in Brazil

In March 2009, in a sweet orange orchard (Citrus sinensis) cv. Valencia grafted on Swingle citrumelo (C. paradisi Macf. × Poncirus trifoliata L. Raf.) rootstock in Severínia County, São Paulo State, Brazil, approximately 40 trees were detected with small, necrotic, dark brown leaf spots. These lesions occurred whether or not citrus leafminer (Phyllocnistis citrella) was present and they were only found on leaves from branches arising from the rootstock. Sweet orange foliage was not affected even when in contact with infected rootstock branches. Symptoms were unusual and distinct from typical citrus canker lesions because the lesions were smaller and did not have erumpent margins. Typical yellow Xanthomonas colonies were isolated from the lesions on nutrient agar. The isolates were aerobic, gram negative, rod shaped, and they produced a dark pigment, which is characteristic of some Xanthomonas fuscans subsp. aurantifolii strains. Two reference strains were tested for pathogenicity on not fully expanded leaves of sweet orange, Swingle citrumelo, and key/Mexican lime (C. aurantifolia) plants by wound inoculation with a sterile needle previously dipped in a bacterial suspension (approximately 10⁶ ml^-1). Two plants of each species were used for inoculations in greenhouse conditions and six leaves were inoculated per plant. Each inoculated leaf received six point inoculations. These tests confirmed that the host range of this pathogen was restricted to Swingle citrumelo. Symptoms similar to those in the orchard were observed 3 weeks after inoculation and Koch's postulates were completed by reisolation of the bacterium and comparing it with the original isolates. Molecular fingerprinting with PCR-restriction fragment length polymorphism (RFLP) of the 16S-23S spacer region polymorphism (1) and ERIC- and BOX-PCR (2) was used to compare the new strain with 26 reference strains of Xanthomonas citri subsp. citri types A, A* and A^w, X. fuscans subsp. aurantifolii types B and C, and X. alfalfae subsp. citrumelonis. PCR-RFLP and ERIC-PCR showed that this new pathogen had the same profile as X. fuscans subsp. aurantifolii (B and C types). In BOX-PCR, this new strain had a unique profile, but it was still most similar to X. fuscans subsp. aurantifolii and very distinct from X. citri subsp. citri (A, A*, and A^w) and X. alfalfae subsp. citrumelonis strains. During the rainy season in Brazil, this new Xanthomonas strain is less aggressive than X. citri subsp. citri on Swingle citrumelo, inducing fewer lesions without erumpent margins even in young leaves severely infested by the citrus leafminer. The disease only occurred on trees that were separated from each other by 3 to 20 m, suggesting that the bacterium is spread by windblown rain and/or cultural practices. Xanthomonads pathogenic to citrus are of great importance for regulatory purposes worldwide. X. fuscans subsp. aurantifolii is only known to be pathogenic on lemons and limes in the field, and until now, has only been reported to infect lemons and limes in Argentina and key/Mexican lime in São Paulo (Brazil) (3). To our knowledge, this is the first report of a strain of this subspecies that infects Swingle citrumelo but not key/Mexican lime. References: (1) S. A. L. Destéfano and J. Rodrigues Neto. Summa Phytopathol. 28:167, 2002. (2) F. J. Louws et al. Appl. Environ. Microbiol. 60:2286, 1994. (3) N. W. Schaad et al. Syst. Appl. Microbiol. 28:494, 2005.

Detection of mechanical and disease stresses in citrus plants by fluorescence spectroscopy

We have investigated the detection of mechanical and disease stresses in citrus plants (Citrus limonia [L.] Osbeck) using laser-induced fluorescence spectroscopy. Due to its economic importance we have chosen to investigate the citrus canker disease, which is caused by the Xanthomonas axonopodis pv. citri bacteria. Mechanical stress was also studied because it plays an important role in the plant's infection by such bacteria. A laser-induced fluorescence spectroscopy system, composed of a spectrometer and a 532 nm 10 mW excitation laser was used to perform fluorescence spectroscopy. The ratio of two chlorophyll fluorescence bands allows us to detect and discriminate between mechanical and disease stresses. This ability to discriminate may have an important application in the field to detect citrus canker infected trees.

Adult Citrus Leafminers (Phyllocnistis citrella) Are Not Efficient Vectors for Xanthomonas axonopodis pv. citri

Interest in the interaction between the citrus leafminer (Phyllocnistis citrella) and citrus bacterial canker, caused by Xanthomonas axonopodis pv. citri, has increased as a greater incidence and severity of canker-diseased plants was observed in groves infested with the citrus leafminer. To determine whether adults of the citrus leafminer could act as vectors of citrus canker, we investigated two potential mechanisms for direct spread by leafminer adults using experimental microcosms. First, adult leafminers were raised on canker-infected foliage and were allowed to mate and lay eggs on healthy plants. These plants then were observed for development of citrus canker symptoms. In a second set of experiments, adults raised on healthy plants were given free access to canker-diseased plants during the period in which they mated and laid eggs on healthy plants. In all, 3,119 mines were produced by developing larvae on a total of 2,384 leaves examined for citrus canker symptoms. No symptoms of citrus bacterial canker disease were observed on any of the healthy test plants in 37 independent experimental trials conducted to test these two potential mechanisms of spread of citrus canker, and the pathogen was not recovered from insects exposed to symptomatic Rangpur lime plants. The upper limit on the rate of transmission was estimated to be less than 0.2% per oviposition event based on the binomial probability distribution. However, when adult P. citrella insects were artificially contaminated with high levels of X. axonopodis pv. citri, transmission to Rangpur lime plants with the induction of citrus canker was observed. This suggests that the ability of P. citrella to transmit X. axonopodis pv. citri is limited by the rate at which it can acquire inoculum from infected plants. The results support the conclusion that adult citrus leafminers are not efficient vectors for citrus canker bacteria, and the disease is unlikely to be spread this way.