Estimation of the number of aphids carrying Citrus tristeza virus that visit adult citrus trees

Aphid Species in Citrus Orchards in Crete: Key Vectors of Citrus Tristeza Virus and Automated Monitoring Innovations for Alate Aphids

Citrus tristeza virus (CTV) is a vector-borne virus that poses a significant threat to citrus production worldwide, inducing a variety of symptoms. Therefore, a detailed knowledge of local aphids, identification of viruliferous species, and the development of new monitoring tools are necessary to improve CTV control strategies. Herein, a 2-year survey was conducted to assess the frequency of aphid species infesting several citrus pilot orchards. Plot findings based on morphological and molecular identification revealed Aphis spiraecola (ranged from 44-100%) as the most abundant aphid species, followed by A. gossypii (<50%). Toxoptera aurantii, Myzus persicae, and A. craccivora were present in low numbers, and A. citricidus was not detected. Due to the absence of CTV detection in aphids and citrus trees from the pilot orchards, a complementary survey was conducted in CTV-infected fields. Three aphid species were identified as CTV-positive by RT-PCR, suggesting that they may be viruliferous, with A. spiraecola as predominant, followed by A. gossypii and T. aurantii. Additionally, we developed a non-invasive procedure for identifying aphid species using wingbeat analysis. This method provides a faster alternative to traditional identification techniques by taxonomic keys based on morphological features or PCR, although its accuracy is lower (approximately 95% for the two species tested). Overall, this work provides a detailed study of aphid species composition in citrus orchards, identifies the predominant local putative CTV vector, and introduces a novel sensor for aphid monitoring, contributing to improved epidemic forecasting and sustainable disease management strategies.

A Molecular Diagnostic Assay for the Discrimination of Aphid Species (Hemiptera: Aphididae) Infesting Citrus

Aphid species (Hemiptera: Aphididae) are among the most serious pests for citrus cultivation throughout the world causing substantial crop damages. Accurate identification of aphids to the species level can be difficult, though being crucial for their effective management. In this study, a molecular diagnostic assay for distinguishing eleven aphid species was developed. A fragment of the mitochondrial Cytochrome Oxidase I (mtCOI) gene was used and a Polymerase Chain Reaction-Restriction Fragment Length Polymorphism (PCR-FLP) analysis with five restriction enzymes, based on DNA sequence polymorphisms, was applied to differentiate the eleven aphid species. This molecular technique allows aphid species at any life stage to be discriminated accurately and simply and can be a useful tool for monitoring the populations of economically important aphid species.

Rootstocks with Different Tolerance Grade to Citrus Tristeza Virus Induce Dissimilar Volatile Profile in Citrus sinensis and Avoidance Response in the Vector Aphis gossypii Glover

The citrus tristeza virus (CTV) is an agent of devastating epidemics of the citrus plant grafted on Citrus aurantium, one of the main rootstocks still used in the Mediterranean area. Consequently, CTV-tolerant alternative citrus rootstocks are considered necessary to manage this disease and/or its vector; that in Mediterranean countries is the aphid Aphis gossypii. In this study, we analyzed the VOCs emitted from Citrus sinensis plants grafted on the CTV-susceptible C. aurantium and on the CTV-tolerant Volkamer lemon, Forner-Alcaide no. 5, and Carrizo citrange. Furthermore, the aphid preference/avoidance response toward these combinations was evaluated in a semi-field experiment. The VOC profiles recorded on the leaves of C. sinensis grafted on the four rootstocks listed above showed significant differences in the abundances and ratios of the compounds emitted. The behavioral experiments indicated that A. gossypii prefers to orient and establish on the C. sinensis plants grafted on C. aurantium rather than on that grafted on the three CTV-tolerant varieties. The possibility that this avoidance mechanism is triggered by the different profile of the VOC emitted by the different combinations and the consequent susceptibility/tolerance shown toward CTV is discussed.

Survey and detection for citrus tristeza virus in Florida groves with an unconventional tool: The Asian citrus psyllid

The citrus industry of Florida faces insurmountable challenges against the destructive diseases citrus tristeza and Huanglongbing (HLB, or citrus greening). Though the tristeza causal agent, citrus tristeza virus (CTV), has been in Florida decades longer than HLB, growers have concentrated most of their efforts on combating the more detrimental HLB. The Asian citrus psyllid (Diaphorina citri; ACP) is the insect vector of the bacterial pathogen Candidatus Liberibacter asiaticus and transmits the incurable HLB to all commercial citrus. During our searches for biological and viral controls against the ACP, we consistently detected sequences of CTV in Florida field populations of ACP. This unexpected finding led us to investigate whether ACPs collected from young shoots could be used as a tool to survey CTV in Florida citrus groves. We first surveyed for the most common CTV strains in Florida (T30, T36, and VT/T68) in citrus trees on mostly sour orange (Citrus aurantium) rootstock, the rootstock susceptible to CTV decline. Out of 968 trees sampled across five years (2018-2022), approximately 8.2% were positive for CTV, with more than half of the CTV-positive trees infected with strain T30. Simultaneously, we looked at CTV strains in ACPs during this time and found that approximately 88% of pooled adult and nymph ACPs also had CTV, with over half the positive samples having the T36 strain. As a result of the much higher CTV incidences in the ACPs, we conducted a second investigation into whether we could more easily detect the same CTV strains in ACP nymphs as in CTV-infected citrus tissue. After individually sampling 43 trees and pooling the nymphs from each tree, we detected CTV at about the same incidence in the citrus tissue and the nymphs, but with much less ACP tissue, time, and resources required for detection compared to citrus tissue. Results from this study illustrate the sustained threat of CTV to Florida citrus and demonstrate the ACP as a potential bioindicator for CTV.

A Comprehensive Analysis of Citrus Tristeza Variants of Bhutan and Across the World

Mandarin orange is economically one of the most important fruit crops in Bhutan. However, in recent years, orange productivity has dropped due to severe infection of citrus tristeza virus (CTV) associated with the gradual decline of citrus orchards. Although the disease incidence has been reported, very limited information is available on genetic variability among the Bhutanese CTV variants. This study used reverse transcription PCR (RT-PCR) to detect CTV in collected field samples and recorded disease incidence up to 71.11% in Bhutan’s prominent citrus-growing regions. To elucidate the extent of genetic variabilities among the Bhutanese CTV variants, we targeted four independent genomic regions (5′ORF1a, p25, p23, and p18) and analyzed a total of 64 collected isolates. These genomic regions were amplified and sequenced for further comparative bioinformatics analysis. Comprehensive phylogenetic reconstructions of the GenBank deposited sequences, including the corresponding genomic locations from 53 whole-genome sequences, revealed unexpected and rich diversity among Bhutanese CTV variants. A resistant-breaking (RB) variant was also identified for the first time from the Asian subcontinent. Our analyses unambiguously identified five (T36, T3, T68, VT, and HA16-5) major, well-recognized CTV strains. Bhutanese CTV variants form two additional newly identified distinct clades with higher confidence, B1 and B2, named after Bhutan. The origin of each of these nine clades can be traced back to their root in the north-eastern region of India and Bhutan. Together, our study established a definitive framework for categorizing global CTV variants into their distinctive clades and provided novel insights into multiple genomic region-based genetic diversity assessments, including their pathogenicity status.

Development of a real-time RT-PCR method for the detection of Citrus tristeza virus (CTV) and its implication in studying virus distribution in planta

Tristeza is an economically important disease of the citrus caused by Citrus tristeza virus (CTV) of genus Closterovirus and family Closteroviridae. The disease has caused tremendous losses to citrus industry worldwide by killing millions of trees, reducing the productivity and total production. Enormous efforts have been made in many countries to prevent the viral spread and the losses caused by the disease. To understand the reason behind this scenario, studies on virus distribution and tropism in the citrus plants are needed. Different diagnostic methods are available for early CTV detection but none of them is employed for in planta virus distribution study. In this study, a TaqMan RT-PCR-based method to detect and quantify CTV in different tissues of infected Mosambi plants (Citrus sinensis) has been standardized. The assay was very sensitive with the pathogen detection limit of > 0.0595 fg of in vitro-transcribed CTV-RNA. The assay was implemented for virus distribution study and absolute CTV titer quantification in samples taken from Tristeza-infected trees. The highest virus load was observed in the midribs of the symptomatic leaf (4.1 × 10⁷-1.4 × 10⁸/100 mg) and the lowest in partial dead twigs (1 × 10³-1.7 × 10⁴/100 mg), and shoot tip (2.3 × 10³-4.5 × 10³/100 mg). Interestingly, during the peak summer months, the highest CTV load was observed in the feeder roots (3 × 10⁷-1.1 × 10⁸/100 mg) than in the midribs of symptomatic leaf. The viral titer was highest in symptomatic leaf midrib followed by asymptomatic leaf midrib, feeder roots, twig bark, symptomatic leaf lamella, and asymptomatic leaf lamella. Overall, high CTV titer was primarily observed in the phloem containing tissues and low CTV titer in the other tissues. The information would help in selecting tissues with higher virus titer in disease surveillance that have implication in Tristeza management in citrus.

Detection and Molecular Characterization of 'Candidatus Liberibacter asiaticus' and Citrus Tristeza Virus Associated with Citrus Decline in Bhutan

Citrus, mainly mandarin (Citrus reticulata Blanco), is an economically important fruit crop in Bhutan. Despite having favorable agroclimatic conditions for citrus cultivation, the early decline of fruit-bearing orchards coupled with low crop productivity is a major concern among citrus growers. During a recent survey, an association of 'Candidatus Liberibacter asiaticus' (citrus greening) and citrus tristeza virus (CTV), either singly or as mixed infections in declined citrus trees, was recorded in all four major citrus-growing districts (Tsirang, Dagana, Zhemgang, and Sarpang). Using PCR-based diagnosis, a higher incidence of citrus greening (27.45%) and tristeza (70.58%) was observed in symptomatic field samples. Detection and characterization of 'Ca. L. asiaticus' was performed based on the 16S ribosomal DNA, prophage gene, 50S ribosomal rplA-rplJ gene, and tandem repeats of the CLIBASIA_01645 locus. Similarly, the coat protein, p23, and p18 genes were used as genetic markers for the detection and characterization of Bhutanese CTV. The 'Ca. L. asiaticus' isolates from Bhutan segregated into classes II and III based on the CLIBASIA_01645 locus, analogous to Indian isolates from the northeast region and Term-A based on the CLIBASIA_05610 locus. CTV isolates of Bhutan were observed as closely related to the VT strain, which is considered to be the most devastating. To the best of our knowledge, this is the first study on molecular characterization of 'Ca. L. asiaticus' and CTV isolates and their association with citrus decline in Bhutan.

Diversity of Citrus tristeza virus Strains in the Upper Gulf Coast Area of Texas

Citrus tristeza virus (CTV) in Texas was first reported in the 1950s and has since been sporadically reported in the residential areas in the Upper Gulf Coast region. Because the major rootstock for commercial citriculture in South Texas is sour orange, which is susceptible to CTV decline, the spread of CTV into South Texas can pose a great threat to Texas citrus industry. Thirty-six CTV-positive samples, collected during surveys conducted in the Upper Gulf Coast area of Texas from 2013 to 2018, were first analyzed by strain-specific real-time PCR (RT-PCR) targeting various regions of CTV Open reading frame (Orf) 1a and then by amplicon sequencing derived from p25 and p20 region of CTV genome. Among 36 samples, 33 were successfully genotyped by strain-specific RT-PCR and by amplicon sequencing followed by phylogenetic analysis. Variability in the detection of CTV strains was observed over a 6-year period. In 2013, T3 and T30 were the only strains detected in the Upper Gulf Coast of Texas, but in further surveys until 2018, additional strains were detected, including T36, VT, and RB. Mixed infections were also detected in 14 samples comprising about 42% of CTV samples examined in the study. Although genotyping mixed infection samples by targeting Orf 1a and full-length p25, residing in the 5' and 3' region of the CTV genome, respectively, confirmed the presence of multiple strains in these samples, incongruent genotyping data were observed. These findings suggested that the current status of CTV strain diversity in Texas Upper Gulf Coast region might have been established by multiple introductions of CTV-infected plant materials for propagation and with a potential recombination in planta.

Development of a reverse transcription recombinase polymerase based isothermal amplification coupled with lateral flow immunochromatographic assay (CTV-RT-RPA-LFICA) for rapid detection of Citrus tristeza virus

Tristeza is a highly destructive disease of citrus caused by the phloem-limited, flexuous filamentous Citrus tristeza virus (CTV) in the genus Closterovirus and the family Closteroviridae. It has been a major constraint for higher productivity and has destroyed millions of citrus trees globally. CTV is graft transmissible and spread through use of virus infected nursery plants. Therefore, virus detection by using specific and reliable diagnostic tools is very important to mitigate disease outbreaks. Currently, the standard molecular techniques for CTV detection include RT-PCR and RT-qPCR. These diagnostic methods are highly sensitive but time consuming, labor intensive and require sophisticated expensive instruments, thus not suitable for point-of-care use. In the present study, we report the development of a rapid, sensitive, robust, reliable, and highly specific reverse transcription-RPA technique coupled with a lateral flow immunochromatographic assay (CTV-RT-RPA-LFICA). RT-RPA technique was standardized to amplify the coat protein gene of CTV (CTV-p25) and detect double labeled amplicons on a sandwich immunoassay by designing specific labeled primer pair and probe combinations. The optimally performing primer set (CTRPA-F1/CTRPA-R9-Btn) and the corresponding TwistAmp nfo probe (CTRPA-Probe) was optimized for temperature and reaction time using purified cDNA and viral RNA as template. The sensitivity of the developed assay was compared with other detection techniques using in vitro-transcribed RNA. The efficacy and specificity of the assay was evaluated using CTV positive controls, healthy samples, field grown citrus plants of unknown status, and other virus and bacterial pathogens that infect citrus plants. The RT-RPA-LFICA was able to detect ≤ 141 fg of RNA when cDNA used as a template. The assay detected ≤ 0.23 ng/µl of CTV RNA when directly used as template without cross-reactivity with other citrus pathogens. Best results were achieved at the isothermal temperature of 40 °C within 15-20 min. The study demonstrated that RT-RPA-LFICA has potential to become an improved detection technique for end users in bud-wood certification and quarantine programs and a promising platform for rapid point-of-care diagnostics for citrus farmers and small nurseries in low resource settings.

Aphids Are Unable to Ingest Phloem Sap from the Peduncles of Lime Fruits

Citrus exports to Europe are regulated enforcing that fruits shall be free from peduncles and leaves, as they represent an important pathway for the entrance of non-European (non-EU) Citrus tristeza virus (CTV) isolates into the European Community. Aphids, are the vectors of CTV and could potentially feed on peduncles of imported fruits and thus spread non-EU isolates of CTV across Europe. We studied the probing behaviour of the main vectors of CTV (Aphis (Toxoptera) citricidus and Aphis gossypii) on lime leaves and peduncles to assess whether they could potentially transmit the virus. Aphids placed on peduncles rejected probing and feeding, tried to escape and spent most of their time on non-probing activities. Our work demonstrated that both A. citricidus and A. gossypii could not ingest sap from the phloem of lime peduncles, as phloem ingestion was never observed. This implies that aphids would not be able to acquire CTV from an infected fruit peduncle and transmit it to a susceptible plant. Our study supports that citrus exports with fruit peduncles to Europe may not be a real risk for the introduction of non-EU isolates of CTV to the European Community.

In-silico characterization and RNA-binding protein based polyclonal antibodies production for detection of citrus tristeza virus

Citrus tristeza virus (CTV) is the etiologic agent of the destructive Tristeza disease, a massive impediment for the healthy citrus industry worldwide. Routine indexing of CTV is an essential component for disease surveys and citrus budwood certification for production of disease-free planting material. Therefore, the present study was carried out to develop an efficient serological assay for CTV detection based on the RNA binding protein (CTV-p23), which is translated from a subgenomic RNA (sgRNA) that accumulates at higher levels in CTV-infected plants. CTV-p23 gene was amplified, cloned and polyclonal antibodies were raised against recombinant CTV-p23 protein. The efficacy of the produced polyclonal antibodies was tested by Western blots and ELISA to develop a quick, sensitive and economically affordable CTV detection tool and was used for indexing of large number of plant samples. The evaluation results indicated that the developed CTV-p23 antibodies had an excellent diagnostic agreement with RT-PCR and would be effective for the detection of CTV in field samples. Furthermore, CTV-p23 gene specific primers designed in the present study were found 1000 times more sensitive than the reported coat protein (CTV-p25) gene specific primers for routine CTV diagnosis. In silico characterizations of CTV-p23 protein revealed the presence of key conserved amino acid residues that involved in the regulation of protein stability, suppressor activity and protein expression levels. This would provide precious ground information towards understanding the viral pathogenecity and protein level accumulation for early diagnosis of virus.

Tissue-Print and Squash Capture Real-Time RT-PCR Method for Direct Detection of Citrus tristeza virus (CTV) in Plant or Vector Tissues

Direct systems to process samples allow high-throughput testing or identification of Citrus tristeza virus (CTV) by the sensitive real-time reverse transcription coupled to polymerase chain reaction (RT-PCR) neither with extract preparation nor nucleic acid purification. Immobilized CTV targets are amplified from fresh sections of plant tissues or squashes of fresh or already caught vectors onto paper, nitrocellulose, or positively charged nylon membranes. The printed or squashed support can be stored or mailed at room temperature. These validated user-friendly methods are recommended by IPPC-FAO standard for CTV diagnosis, detection, and identification. The methods are safe, not under current quarantine regulations because they do not involve any risk of introduction of exotic CTV isolates or vectors and are discrete and useful for epidemiological studies or screening for large-scale analyses. In this chapter, tissue-printing and squashing capture methods for direct sample preparation without extract preparation or nucleic acid extraction and purification were coupled with validated real-time RT-PCR detection protocols based on TaqMan chemistry for CTV detection.

Effects of Citrus Overwintering Predators, Host Plant Phenology and Environmental Variables on Aphid Infestation Dynamics in Clementine Citrus

The Spirea citrus aphid, Aphis spiraecola Patch, and the cotton aphid, Aphis gossypii Glover (Hemiptera: Aphididae), are key pests of clementine mandarines in the Mediterranean basin. Severity of aphid infestations is determined by environmental variables, host plant phenology patterns, and the biological control exerted by their associated natural enemies. However, there is no information about the role these limiting and regulating factors play. Aphid densities, citrus phenology, and associated predators that overwinter in the crop were monitored weekly throughout two flush growth periods (February to July) in four clementine mandarin groves; relationships between these parameters and environmental variables (temperature and precipitation) were studied. Our results show exponential increase in aphid infestation levels to coincide with citrus phenological stages B3 and B4; shoots offer more space and nutritional resources for colony growth at these stages. Duration of these phenological stages, which was mediated by mean temperature, seems to importantly determine the severity of aphid infestations in the groves. Among those studied, the micro-coccinellids, mostly Scymnus species, were the only group of predators with the ability to efficiently regulate aphid populations. These natural enemies had the highest temporal and spatial demographic stability. Aphid regulation success was only achieved through early presence of natural enemies in the grove, at the aphid colonization phase. Our results suggest that conservation strategies aimed at preserving and enhancing Scymnus sp. populations may make an important contribution to the future success of the biological control of these key citrus pests.

CTV Vectors and Interactions with the Virus and Host Plants

Citrus is a graft-propagated perennial crop, and Citrus tristeza virus (CTV) is readily graft-transmissible. CTV is comprised of a complex of strains and isolates and, in nature, is spread semi-persistently by aphid vectors. Therefore, citrus trees become infected with multiple CTV strains over time. An important step in characterizing a CTV field isolate is to use aphid vectors to "clean" up the CTV population of a source tree to separate strains and eliminate other graft-transmissible agents. Use of Toxoptera citricida or Aphis gossypii will expedite efficient CTV transmission. CTV vector studies require critical coordination of abundant robust and virus-free vector-competent aphid colonies and an insect-proof, climate-controlled greenhouse or growth chamber. CTV donor and healthy receptor plants with young flush growth must be available for virus acquisition and inoculation. Vector optimums for virus acquisition and inoculation are 24 h for each. CTV infection is readily determined by serology using a polyclonal antiserum or a monoclonal antiserum cocktail; whereas, molecular genotyping is conducted with reverse transcription polymerase chain (RT-PCR) or real time quantitavtive RT-PCR (RT-qPCR) with strain-specific primers and probes. However, the phenotype of the aphid-transmitted isolate still requires virus indexing by graft inoculation to a citrus host range and evaluating symptoms such as stem pitting, vein clearing, stunting, and chlorosis.

Application of Loop-Mediated Isothermal Amplification in an Early Warning System for Epidemics of an Externally Sourced Plant Virus

Restricting Turnip yellows virus (TuYV) spread in canola (Brassica napus) crops often relies upon the application of systemic insecticides to protect young vulnerable plants from wide-scale green-peach aphid (GPA; Myzus persicae) colonization and subsequent virus infection. For these to be applied at the optimal time to ensure they prevent epidemics, growers would need to be forewarned of incoming viruliferous aphid migration and colonization. This study was conducted to field validate a loop-mediated isothermal amplification (LAMP) protocol designed to detect TuYV in aphids caught on traps and develop an early warning system for TuYV epidemics. Double-sided yellow sticky traps were deployed at 30 sites sown with canola over a two-year period in the south-west Australian grainbelt. Using LAMP, the percentage (%) of trap sides with TuYV-carrying aphids was measured and related to TuYV infection incidence in the adjacent crop. When TuYV was detected in aphids on >30% trap sides in a six-week period from pre-emergence to GS15 (five-leaf stage), TuYV reached >60% crop incidence by GS30 (beginning of stem elongation). Whereas, TuYV detection in aphids on ≤15% trap sides during this period was associated with ≤6% TuYV incidence by GS30. Furthermore, when large numbers of aphids, including GPA, were caught during this period but no TuYV was detected in them, minimal TuYV spread (≤5%) occurred in the crop by GS30. Therefore, the LAMP TuYV protocol can be used in an early warning system for TuYV epidemics by providing detection of initial viruliferous aphid migration into a canola crop before they establish colonies throughout the crop and spread virus. This would enable proactive, non-prophylactic, and thereby more effective systemic insecticide applications to minimize seed yield and quality losses due to early season TuYV infection.

Titer Variation of Citrus Tristeza Virus in Aphids at Different Acquisition Access Periods and Its Association with Transmission Efficiency

Tristeza, caused by citrus tristeza virus (CTV; Closterovirus, Closteroviridae), is of significant economic importance. Tristeza epidemics have caused severe declines in productivity, and even death, of millions of citrus trees on sour orange rootstock in many regions all over the world. In the field, CTV is most efficiently vectored by the brown citrus aphid (Toxoptera citricida (Kirkaldy)) in a semipersistent manner. The transmission efficiency of the vector is influenced by its acquisition access period (AAP) for CTV. A real-time RT-PCR assay using SYBR Green fluorescent dye was used to estimate the CTV titers in groups of 15 aphids under AAPs after 0.5 to 48 h for three CTV isolates (CT11A, CT16-2, and CTLJ). Similar trends for CTV titer in viruliferous aphids were displayed for the three isolates. The maximum CTV titer was at AAP 6 h for isolates CT11A and CT16-2, and at 4 h for isolate CTLJ. During the AAPs from 0.5 to 6 h, the mean CTV titer of CT16-2 increased from 7.8 × 10⁴ to 1.71 × 10⁷ copies per 15 aphids, and was correlated with an increase in transmission rate from 20 to 90.9%. This suggests that the transmission efficiency is positively correlated with viral titer in the insect from 0.5 h until 6 h AAPs. While a downward trend in CTV titer was observed after a 6-h AAP, the transmission rate remained higher than 90% up to 48 h. These results indicate that factors other than the virus titer in the vector contribute to successful transmission under long acquisition conditions. This is the first detailed quantitative analysis of CTV in its main vector species following different AAPs and its association with transmission efficiency, and should enhance our understanding of T. citricida-CTV interactions.

Spatial and temporal spread of Citrus tristeza virus and its aphid vectors in the North western area of Morocco

First report of Citrus tristeza virus (CTV,Closterovirus) in Morocco datesback to 1961 in collections of citrus varieties. An exhaustive survey of citrus in the north of the country in 2009 revealed that CTV was spread all over the citrus production area. We attempted to evaluate the relative contribution of different aphid species in the spread of CTV disease in a Citrus reticulata orchard at the Loukkous region during 2 years (2012 and 2013). The overall CTV incidence estimated in the experimental site increased from 17.8% in 2012 to 31.15% in 2013. The most abundant aphid species colonising clementine trees was Aphis spiraecola and A. gossypii. Both aphid species reached their maximum peaks during the spring season. The rate of viruliferous aphids, estimated by real-time RT-PCR of single aphid, revealed that 35.4% of winged A. gossypii and 28.8% of winged A. spiraecola were viruliferous, confirming a high inoculum pressure in the area surrounding the experimental site. The aphid species Toxoptera citricida, which is able to transmit the aggressive isolates of CTV, was not found in the Loukkous region. The study of the spatial distribution of the CTV showed that in general, the disease was randomly distributed in the field. Overall, the results seem to indicate that A. spiraecola may be considered as the major aphid species contributing to CTV spread in our experimental conditions. The prevalence of mild strains in the region and the high level of aphid flight activity could explain the rapid evolution of CTV incidence in the experimental area.

Why the aphid Aphis spiraecola is more abundant on clementine tree than Aphis gossypii?

Aphis spiraecola and Aphis gossypii cause harmful damages on clementine tree orchards. Weekly surveys measured the abundance of aphids (larvae, winged and wingless adults) as well as of auxiliary insects and parameters of energy metabolism. Correlatively, soluble carbohydrates, total free amino acids, free proline and condensed tannins were quantified in control and infested leaves. Both aphid species showed parallel temporal variations, but A. spiraecola was consistently more abundant regardless of the stage. Amino acids had a positive effect on both aphid species abundance, but neither condensed tannins nor auxiliary insects seemed to modulate aphid populations. Interestingly, the leaf carbohydrate content was positively correlated with the abundance of A. spiraecola, but not with that of A. gossypii. Moreover, A. gossypii's abundance was significantly down-regulated by high proline concentrations. Thus, the higher abundance of A. spiraecola could be explained by a better tolerance to high proline contents and a better conversion of foliar energy metabolites.

Parasitoids and hyperparasitoids (Hymenoptera) on aphids (Hemiptera) infesting citrus in east Mediterranean region of Turkey

The aphids, aphid parasitoids, and hyperparasitoids found in citrus orchards, the parasitoids' and hyperparasitoids' seasonal abundance, and the plant-aphid-parasitoid relationships in Hatay, Osmaniye, Adana, and Mersin provinces of the east Mediterranean region of Turkey are presented in the present 2-yr study. Aphidius colemani Viereck, Binodoxys angelicae (Haliday), and Lysiphlebus confusus Tremblay and Eady (Hymenoptera: Braconidae: Aphidiinae) were encountered as the most common parasitoids among 10 identified aphidiine and aphelinid taxa on different citrus species. Hyperparasitoids belonging to the genera Alloxysta, Phaenoglyphis, Asaphes, Pachyneuron, Syrphophagus, and Dendrocerus are reported for the first time emerging from aphids feeding on citrus in Turkey. Among them, Asaphes spp., Pachyneuron spp., and Syrphophagus spp. were recorded as the most common ones. Citrus reticulata Blanco and Citrus limon (L.) Burm. fil. were recorded as main hosts for the aphid parasitoids and their hyperparasitoids.

A sensitive and reliable RT-nested PCR assay for detection of Citrus tristeza virus from naturally infected citrus plants

A specific and sensitive reverse transcriptase-nested polymerase chain reaction assay (RT-nPCR) was developed for the detection of Citrus tristeza virus (CTV) from naturally infected citrus samples. Two sets of primer pairs were designed by alignment of nucleotide sequences available in GenBank database for different genotypes of CTV. RT-nPCR reaction components and thermal cycling parameters were optimized and reaction conditions were standardized. Sequencing of the PCR products from direct and nested-PCR reactions confirmed the specificity of both primer pairs. Presence of CTV specific amplicons in asymptomatic samples which were collected from diseased orchards indicated the sensitivity of the test. As RT-nPCR technique, developed in the present study, is specific and efficient in detecting CTV, this could be envisioned for diagnostic applications and surveillance.

CPm gene diversity in field isolates of Citrus tristeza virus from Colombia

The nucleotide sequence diversity of the CPm gene from 28 field isolates of Citrus tristeza virus (CTV) was assessed by SSCP and sequence analyses. These isolates showed two major shared haplotypes, which differed in distribution: A1 was the major haplotype in 23 isolates from different geographic regions, whereas R1 was found in isolates from a discrete region. Phylogenetic reconstruction clustered A1 within an independent group, while R1 was grouped with mild isolates T30 from Florida and T385 from Spain. Some isolates contained several minor haplotypes, which were very similar to, and associated with, the major haplotype.

Quantitative estimation of plum pox virus targets acquired and transmitted by a single Myzus persicae

The viral charge acquired and inoculated by single aphids in a non-circulative transmission is estimated using plum pox virus (PPV). A combination of electrical penetration graph and TaqMan real-time RT-PCR techniques was used to establish the average number of PPV RNA targets inoculated by an aphid in a single probe (26,750), approximately half of the acquired ones. This number of PPV targets is responsible for a systemic infection of 20% on the inoculated receptor plants. No significant differences were found between the number of PPV RNA targets acquired after one and after five intracellular punctures (pd), but the frequency of infected receptor plants was higher after 5 pd. The percentage of PPV-positive leaf discs after just 1 pd of inoculation probe (28%/4,603 targets) was lower than after 5 pd (45.8%/135 x 10(6) targets). The methodology employed could be easily extended to other virus-vector-host combinations to improve the accuracy of models used in virus epidemiology.

Citrus tristeza virus: a pathogen that changed the course of the citrus industry

Citrus tristeza virus (CTV) (genus Closterovirus, family Closteroviridae) is the causal agent of devastating epidemics that changed the course of the citrus industry. Adapted to replicate in phloem cells of a few species within the family Rutaceae and to transmission by a few aphid species, CTV and citrus probably coevolved for centuries at the site of origin of citrus plants. CTV dispersal to other regions and its interaction with new scion varieties and rootstock combinations resulted in three distinct syndromes named tristeza, stem pitting and seedling yellows. The first, inciting decline of varieties propagated on sour orange, has forced the rebuilding of many citrus industries using tristeza-tolerant rootstocks. The second, inducing stunting, stem pitting and low bearing of some varieties, causes economic losses in an increasing number of countries. The third is usually observed by biological indexing, but rarely in the field. CTV polar virions are composed of two capsid proteins and a single-stranded, positive-sense genomic RNA (gRNA) of approximately 20 kb, containing 12 open reading frames (ORFs) and two untranslated regions (UTRs). ORFs 1a and 1b, encoding proteins of the replicase complex, are directly translated from the gRNA, and together with the 5' and 3'UTRs are the only regions required for RNA replication. The remaining ORFs, expressed via 3'-coterminal subgenomic RNAs, encode proteins required for virion assembly and movement (p6, p65, p61, p27 and p25), asymmetrical accumulation of positive and negative strands during RNA replication (p23), or suppression of post-transcriptional gene silencing (p25, p20 and p23), with the role of proteins p33, p18 and p13 as yet unknown. Analysis of genetic variation in CTV isolates revealed (1) conservation of genomes in distant geographical regions, with a limited repertoire of genotypes, (2) uneven distribution of variation along the gRNA, (3) frequent recombination events and (4) different selection pressures shaping CTV populations. Measures to control CTV damage include quarantine and budwood certification programmes, elimination of infected trees, use of tristeza-tolerant rootstocks, or cross protection with mild isolates, depending on CTV incidence and on the virus strains and host varieties predominant in each region. Incorporating resistance genes into commercial varieties by conventional breeding is presently unfeasible, whereas incorporation of pathogen-derived resistance by plant transformation has yielded variable results, indicating that the CTV-citrus interaction may be more specific and complex than initially thought. A deep understanding of the interactions between viral proteins and host and vector factors will be necessary to develop reliable and sound control measures.

Quantitative detection of Citrus tristeza virus in citrus and aphids by real-time reverse transcription-PCR (TaqMan®)

A quantitative and multiplex real-time RT-PCR assay was developed to detect Citrus tristeza virus (CTV) along with plant mRNA, which serves as an internal control to ascertain RNA extraction quality. The real-time technique was validated against 39 CTV strains from around the world as well as with the aphid vector, Aphis gossypii, given a 48 h acquisition access period on a CTV source plant. The assay was effective for quantitation of the viral template in infected plants and in single aphids. CTV detection was compared from different plant tissues and for different RNA isolation methods from aphids. Less than 1 fg was consistently detected when RNA transcripts were diluted in extracts from healthy plants while RNA copies carried by single aphids were estimated to be between 12,000 and 13,000,000. The assay was more sensitive and less time consuming than ELISA or traditional RT-PCR. The real-time RT-PCR assay developed is a valuable new tool for detection and titer quantitation of CTV.

Cytochrome c oxidase mRNA as an internal control for detection of Potato virus Y and Potato leafroll virus from single aphids by a co-amplification RT-PCR assay

Using cytochrome c oxidase subunit 1 (COX1) mRNA as the internal control, a triplex reverse transcription-polymerase chain reaction (RT-PCR) for detection of Potato virus Y (PVY) and Potato leafroll virus (PLRV) with co-amplification of COX1 from single specimens of various aphid species has been developed. Partial length cDNA of COX1 from green peach aphid, Myzus persicae (Sulzer), potato aphid, Macrosiphum euphorbiae (Thomas), buckthorn aphid, Aphis nasturtii (Kaltenbach), and pea aphid, Acyrthosiphom pisum (Harris), was cloned and sequenced. These sequences, together with existing COX1 sequences from other aphid species capable or suspected to be capable of transmitting PVY and/or PLRV, were analyzed. The sequence identity between any two aphid species ranged from 97 to 100% at the putative protein level, and 89 to 94% at the nucleic acid level. Two highly conserved COX1 nucleotide sequence stretches were selected to design universal primers Aph F and Aph R. This primer pair, together with two existing universal primer pairs (C1-J-2183 and C1-N-2329; Favret F and Favret R), were evaluated at the optimal annealing temperature using RNA from M. persicase, M. euphorbiae, and A. nasturtii. The Aph primer pair performed well in the monoplex RT-PCR but poorly in the triplex RT-PCR in the presence of the PVY- and PLRV-specific primers. On the other hand, the Favret and C1 primer pairs performed well in both monoplex and triplex RT-PCR formats using single aphids of M. persicase, M. euphorbiae and A. nasturtii, demonstrating their suitability to indicate the successfulness of RT-PCR assays for PVY and PLRV. Using the Favret, PVY and PLRV primer sets, single aphids of M. persicase, M. euphorbiae and A. nasturtii that had been exposed to PLRV-infected and/or PVY-infected potato plants were assessed for their acquisition of the viruses by the triplex RT-PCR assay. Although majority (175/180) of the aphid samples produced the COX1 fragment, five aphid samples failed to produce either the COX1- or the virus-specific band, indicating failed RT-PCR in these samples. This method offers a sensitive tool for detection of viruliferous aphids combined to an effective quality control measure.

Real-time assay for quantitative detection of non-persistently transmitted Plum pox virus RNA targets in single aphids

A TaqMan real-time RT-PCR was developed to detect and quantify RNA-targets from the non-circulative, non-persistently transmitted Plum pox virus (PPV) in individual fresh or aphids captured previously and squashed on paper. Reliable quantitation ranged from 40 up to 4 x 10(8) copies of control transcripts. This technique was applied successfully to plant material and to individual PPV vector (Myzus persicae) and non-vector of PPV (Aphis nerii) aphid species demonstrating acquisition of viral targets by both vector and non-vector aphids. The number of viruliferous aphids detected by real-time RT-PCR and nested RT-PCR in a single closed tube was similar in parallel assays, nevertheless the sensitivity provided by real-time RT-PCR was 100 times higher than nested RT-PCR and 1000 times higher than DASI-ELISA and conventional RT-PCR. The quantities of PPV-RNA targets detected in a single aphid ranged from 40 to more than 2 x 10(3) units. The combined system (immobilization of targets on paper by squash capture and real-time RT-PCR) allows, for the first time, reliable quantitation of PPV targets acquired by individual aphid species and constitute an excellent tool for understanding better PPV epidemiology.