Distribution and Rate of Movement of the Curtovirus Beet mild curly top virus (Family Geminiviridae) in the Beet Leafhopper

Beet curly top virus affects vector biology: the first transcriptome analysis of the beet leafhopper

Curly top disease, caused by beet curly top virus (BCTV), is among the most serious viral diseases affecting sugar beets in western USA. The virus is exclusively transmitted by the beet leafhopper (BLH, Circulifer tenellus) in a circulative and non-propagative manner. Despite the growing knowledge on virus-vector interactions, our understanding of the molecular interactions between BCTV and BLH is hampered by limited information regarding the virus impact on the vector and the lack of genomic and transcriptomic resources for BLH. This study unveils the significant impact of BCTV on both the performance and transcriptome response of BLHs. Viruliferous BLHs had higher fecundity than non-viruliferous counterparts, which was evident by upregulation of differentially expressed transcripts (DETs) associated with development, viability and fertility of germline and embryos in viruliferous insects. Conversely, most DETs associated with muscle movement and locomotor activities were downregulated in viruliferous insects, implying potential behavioural modifications by BCTV. Additionally, a great proportion of DETs related to innate immunity and detoxification were upregulated in viruliferous insects. Viral infection also induced notable alterations in primary metabolisms, including energy metabolism, namely glucosidases, lipid digestion and transport, and protein degradation, along with other cellular functions, particularly in chromatin remodelling and DNA repair. This study represents the first comprehensive transcriptome analysis for BLH. The presented findings provide new insights into the multifaceted effects of viral infection on various biological processes in BLH, offering a foundation for future investigations into the complex virus-vector relationship and potential management strategies for curly top disease.

A Tomato-Free Period Delays and Reduces Begomovirus Disease in Processing Tomato Fields in a Complex Agroecosystem in Central Brazil

A mandatory tomato-free period (TFP) was implemented in the state of Goiás, Brazil, in 2007 to help manage diseases caused by whitefly-transmitted begomoviruses. The impact of the TFP was examined in five locations across three states in Central Brazil from 2013 to 2016. Surveys revealed significant differences in begomovirus disease incidence among locations, i.e., low in Guaíra-TFP and Patos de Minas-TFP; moderate-high in Itaberaí-TFP and Morrinhos-TFP; and high in the non-TFP (NTFP) control, Cristalina-NTFP. PCR tests and DNA sequencing were used to validate the symptoms and showed that all collected symptomatic plant samples were infected with tomato severe rugose virus (ToSRV), a common indigenous bipartite begomovirus. Early season surveys (20 to 40 days after transplants [DAT]) in Itaberaí-TFP and Morrinhos-TFP revealed significantly less begomovirus disease in fields established sooner after the TFP (0 to 2 months) compared with incidences in (i) equivalent early planted fields in the Cristalina-NTFP control and (ii) fields established longer after the end of the TFP (>2 to 5 months). Whitefly infestation of crops was detected year-round in all locations and years, and all tested adults were classified in the Bemisia tabaci MEAM1 cryptic species. Infestation levels were significantly higher during the summer but did not vary significantly among locations. Results of monthly monitoring of adult whiteflies for general begomovirus and ToSRV were positively correlated and were indicators of disease incidence in the field. Notably, ToSRV was not detected in whiteflies collected from nontomato plants during the TFP, and there was a longer lag period before detection in whiteflies collected from processing tomatoes for Itaberaí-TFP and Morrinhos-TFP compared with Cristalina-NTFP. Taken together with the low levels of ToSRV infection detected in potential nontomato reservoir hosts at all locations, our results revealed low levels of primary inoculum during the TFP. Thus, even in a complex agroecosystem with year-round whitefly infestation of crops, the TFP was beneficial due to delayed and reduced begomovirus disease pressure during a critical stage of plant development (first month) and for favoring low levels of primary inoculum. Thus, we concluded that the TFP should be part of a regional integrated pest management (IPM) program targeting ToSRV in Brazil.

Unusual outbreaks of curly top disease in processing tomato fields in northern California in 2021 and 2022 were caused by a rare strain of beet curly top virus and facilitated by extreme weather events

In the western United States, curly top disease (CTD) is caused by beet curly top virus (BCTV). In California, CTD causes economic loss to processing tomato production in central and southern areas but, historically, not in the north. Here, we document unusual CTD outbreaks in processing tomato fields in the northern production area in 2021 and 2022, and show that these were caused by the rare spinach curly top strain (BCTV-SpCT). These outbreaks were associated with proximity of fields to foothills and unusually hot, dry, and windy spring weather conditions, possibly by altering migrations of the beet leafhopper (BLH) vector from locations with BCTV-SpCT reservoirs. Support for this hypothesis came from the failure to observe CTD outbreaks and BLH migrations in 2023, when spring weather conditions were cool and wet. Our results show the climate-induced emergence of a rare plant virus strain to cause an economically important disease in a new crop and location.

Interactions of beet leafhopper (Hemiptera: Cicadellidae), vector of beet curly top virus, and hemp in New Mexico

The beet leafhopper, Circulifer tenellus (Baker 1896), is the sole vector of beet curly top virus (BCTV). Both the virus and the vector have very wide host ranges, including many crops and weeds. Industrial hemp (Cannabis sativa L.) has been reported as a host for both the virus and leafhopper in the past few years with the legal cultivation of the crop in the United States. This research assessed the interactions of the beet leafhopper and hemp in New Mexico by determining the natural infection of hemp with BCTV in 3 field plots in 2021 and 2022 and monitoring the numbers of leafhoppers using yellow sticky traps. The relative preference of beet leafhopper for hemp types and varieties of hemp was assessed using cafeteria-style choice tests. Higher numbers of beet leafhoppers were trapped in and around hemp fields in 2022 than in 2021 in all 3 locations. BCTV was found to infect all 3 types of hemp (cannabidiol or CBD, fiber, and grain) in 2022 in 1 location and only a single CBD variety of hemp in the other 2 locations. Two BCTV strains were identified in CBD hemp, while an additional BCTV strain was found infecting chile pepper grown at the same location.

New Assays for Rapid Detection of Beet Leafhopper-Associated Plant Pathogens, 'Candidatus Phytoplasma trifolii', Beet Curly Top Virus, and Spiroplasma citri

The beet leafhopper Circulifer tenellus is an important pest of agricultural crops in the United States, where it transmits beet curly top virus, beet leafhopper-transmitted virescence agent phytoplasma, and Spiroplasma citri to numerous crops, affecting yield and quality. Each of these pathogens have been linked to serious disease outbreaks within Washington State in the past century. To mitigate the risk of disease, growers target the beet leafhopper in their insect pest management programs. Knowledge of pathogen prevalence in beet leafhopper populations could help growers make better management decisions, but timely diagnostics is required. Four new assays were developed for the rapid detection of the beet leafhopper-associated pathogens. These include two assays that detect Beet leafhopper transmitted virescence agent (a PCR and a real-time PCR SYBR green assay), a duplex PCR assay that simultaneously detects beet curly top virus and Spiroplasma citri, and a multiplex real-time PCR assay for the simultaneous detection of all three pathogens. The screening of dilution series generated from plant total nucleic acid extracts with these new assays typically led to detection at levels 10- to 100-fold more sensitive than the conventional PCR assays currently used. These new tools will allow the rapid detection of beet leafhopper-associated pathogens in both plant and insect specimens and will have the potential to be used in diagnostic laboratories seeking to disseminate fast and accurate results to growers for implementation in their insect pest monitoring programs.

Curly Top Disease of Hemp (Cannabis sativa) in California Is Caused by Mild-Type Strains of Beet curly top virus Often in Mixed Infection

Interest in industrial hemp (Cannabis sativa) as a potential crop led to the establishment of commercial fields in a number of counties in California in 2019 and 2020. Plants in these fields developed different types of virus-like symptoms. The most prevalent type was stunted and bushy plants with distorted, upcurled, and yellowed leaves, which were similar to those associated with curly top disease (CTD) caused by the beet curly top virus (BCTV). This beet leafhopper-vectored virus is endemic in California and can cause economic losses to processing tomato production. Using a multiplex PCR test, BCTV infection was detected in 89% of hemp samples with CTD-like symptoms from Fresno, San Bernardino, and Ventura counties. Other symptom types had low incidence of BCTV infection and were associated with other factors. Hemp plants in California were infected only with the mild-type strains, BCTV-CO and BCTV-Wor, and often in mixed infection (43% of samples). Finally, using an infectious clone of a BCTV-CO isolate from hemp, we demonstrated that agroinoculated hemp plants developed these CTD-like symptoms, thereby fulfilling Koch's postulates for the disease.

Insect vector manipulation by a plant virus and simulation modeling of its potential impact on crop infection

There is widespread evidence of plant viruses manipulating behavior of their insect vectors as a strategy to maximize infection of plants. Often, plant viruses and their insect vectors have multiple potential host plant species, and these may not overlap entirely. Moreover, insect vectors may not prefer plant species to which plant viruses are well-adapted. In such cases, can plant viruses manipulate their insect vectors to preferentially feed and oviposit on plant species, which are suitable for viral propagation but less suitable for themselves? To address this question, we conducted dual- and no-choice feeding studies (number and duration of probing events) and oviposition studies with non-viruliferous and viruliferous [carrying beet curly top virus (BCTV)] beet leafhoppers [Circulifer tenellus (Baker)] on three plant species: barley (Hordeum vulgare L.), ribwort plantain (Plantago lanceolata L.), and tomato (Solanum lycopersicum L.). Barley is not a host of BCTV, whereas ribwort plantain and tomato are susceptible to BCTV infection and develop a symptomless infection and severe curly top symptoms, respectively. Ribwort plantain plants can be used to maintain beet leafhopper colonies for multiple generations (suitable), whereas tomato plants cannot be used to maintain beet leafhopper colonies (unsuitable). Based on dual- and no-choice experiments, we demonstrated that BCTV appears to manipulate probing preference and behavior by beet leafhoppers, whereas there was no significant difference in oviposition preference. Simulation modeling predicted that BCTV infection rates would to be higher in tomato fields with barley compared with ribwort plantain as a trap crop. Simulation model results supported the hypothesis that manipulation of probing preference and behavior may increase BCTV infection in tomato fields. Results presented were based on the BCTV-beet leafhopper pathosystem, but the approach taken (combination of experimental studies with complementary simulation modeling) is widely applicable and relevant to other insect-vectored plant pathogen systems involving multiple plant species.

Grapevine Red Blotch Virus Is Transmitted by the Three-Cornered Alfalfa Hopper in a Circulative, Nonpropagative Mode with Unique Attributes

The transmission mode of grapevine red blotch virus (GRBV, genus Grablovirus, family Geminiviridae) by Spissistilus festinus, the three-cornered alfalfa hopper, is unknown. By analogy with other members in the family Geminiviridae, we hypothesized circulative, nonpropagative transmission. Time-course experiments revealed GRBV in dissected guts, hemolymph, and heads with salivary glands after a 5-, 8-, and 10-day exposure to infected grapevines, respectively. After a 15-day acquisition on infected grapevines and subsequent transfer on alfalfa, a nonhost of GRBV, the virus titer decreased over time in adult insects, as shown by quantitative PCR. Snap bean proved to be a feeding host of S. festinus and a pseudosystemic host of GRBV after Agrobacterium tumefaciens-mediated delivery of an infectious clone. The virus was efficiently transmitted by S. festinus from infected snap bean plants to excised snap bean trifoliates (90%) or grapevine leaves (100%) but less efficiently from infected grapevine plants to excised grapevine leaves (10%) or snap bean trifoliates (67%). Transmission of GRBV also occurred trans-stadially but not via seeds. The virus titer was significantly higher in (i) guts and hemolymph relative to heads with salivary glands, and (ii) adults emanating from third compared with first instars that emerged on infected grapevine plants and developed on snap bean trifoliates. This study demonstrated circulative, nonpropagative transmission of GRBV by S. festinus with an extended acquisition access period compared with other viruses in the family Geminiviridae and marked differences in transmission efficiency between grapevine, the natural host, and snap bean, an alternative herbaceous host.

Development of an IPM Strategy for Thrips and Tomato spotted wilt virus in Processing Tomatoes in the Central Valley of California

Tomato spotted wilt virus (TSWV; species Tomato spotted wilt orthotospovirus; genus Orthotospovirus; family Tospoviridae) is a thrips-transmitted virus that can cause substantial economic losses to many crops, including tomato (Solanum lycopersicum). Since 2005, TSWV emerged as an economically important virus of processing tomatoes in the Central Valley of California, in part due to increased populations of the primary thrips vector, western flower thrips (WFT; Frankliniella occidentalis). To develop an understanding of the epidemiology of TSWV in this region, population densities of WFT and incidence of TSWV were monitored in California's processing tomato transplant-producing greenhouses and associated open fields from 2007 to 2013. Thrips were monitored with yellow sticky cards and in tomato flowers, whereas TSWV incidence was assessed with indicator plants and field surveys for virus symptoms. All thrips identified from processing tomato fields were WFT, and females were three-fold more abundant on sticky cards than males. Symptoms of TSWV infection were observed in all monitored processing tomato fields. Incidences of TSWV ranged from 1 to 20%, with highest incidence found in late-planted fields. There was no single primary inoculum source, and inoculum sources for thrips/TSWV varied depending on the production region. These results allowed us to develop a model for TSWV infection of processing tomatoes in the Central Valley of California. The model predicts that low levels of primary TSWV inoculum are amplified in early-planted tomatoes and other susceptible crops leading to highest levels of infection in later-planted fields, especially those with high thrips populations. Based upon these findings, an integrated pest management (IPM) strategy for TSWV in processing tomatoes in California was devised. This IPM strategy focuses on strategic field placement (identification of high-risk situations), planting TSWV- and thrips-free transplants, planting resistant varieties, monitoring for TSWV symptoms and thrips, roguing infected plants, thrips management targeting early generations, extensive sanitation after harvest, and strategic cropping to avoid overlap with winter bridge crops.

Virus-mediated export of chromosomal DNA in plants

The propensity of viruses to acquire genetic material from relatives and possibly from infected hosts makes them excellent candidates as vectors for horizontal gene transfer. However, virus-mediated acquisition of host genetic material, as deduced from historical events, appears to be rare. Here, we report spontaneous and surprisingly efficient generation of hybrid virus/host DNA molecules in the form of minicircles during infection of Beta vulgaris by Beet curly top Iran virus (BCTIV), a single-stranded DNA virus. The hybrid minicircles replicate, become encapsidated into viral particles, and spread systemically throughout infected plants in parallel with the viral infection. Importantly, when co-infected with BCTIV, B. vulgaris DNA captured in minicircles replicates and is transcribed in other plant species that are sensitive to BCTIV infection. Thus, we have likely documented in real time the initial steps of a possible path of virus-mediated horizontal transfer of chromosomal DNA between plant species.

Pest categorisation of Beet curly top virus (non-EU isolates)

The EFSA Panel on Plant Health performed a pest categorisation of non-EU isolates of Beet curly top virus (BCTV) for the European Union territory. The virus causes severe diseases in beet, tomatoes and pepper crops, occurs predominantly in warm and dry zones and is reported from many countries outside the EU in particular from western USA and Mexico. New data from complete virus genomes make BCTV a well characterised virus species of which currently 11 strains are known and for which diagnostic methods are available. BCTV has a very broad host range of more than 300 species some of which may remain symptomless. Aside from vegetative propagation of infected plants, the only mode of BCTV transmission and spread is by the leafhopper Circulifer tenellus which efficiently transmits the virus in a persistent mode and which is present in several southern EU Member States. No current reports of BCTV presence in the EU exist and because of doubts about the accuracy of older reports, BCTV likely is absent from the EU territory. BCTV can enter into the EU with viruliferous insects and with imports of plants not subject to specific EU regulation. Because both the virus and its vector have a wide host range, BCTV is expected to establish and spread in the Member States where its vector is present and to cause severe diseases in sugar beet and tomato as well as in other crops. Overall, BCTV non-EU isolates meet all the criteria evaluated by EFSA to qualify as a Union quarantine pest and do not meet the criterion of presence in the EU to qualify as a Union regulated non-quarantine pest (RNQP). The main uncertainties concern (1) the presence of BCTV in the EU, (2) the distribution of C. tenellus and (3) the main commodities for virus entry.

Beet curly top virus Strains Associated with Sugar Beet in Idaho, Oregon, and a Western U.S. Collection

Curly top of sugar beet is a serious, yield-limiting disease in semiarid production areas caused by Beet curly top virus (BCTV) and transmitted by the beet leafhopper. One of the primary means of control for BCTV in sugar beet is host resistance but effectiveness of resistance can vary among BCTV strains. Strain prevalence among BCTV populations was last investigated in Idaho and Oregon during a 2006-to-2007 collection but changes in disease severity suggested a need for reevaluation. Therefore, 406 leaf samples symptomatic for curly top were collected from sugar beet plants in commercial sugar beet fields in Idaho and Oregon from 2012 to 2015. DNA was isolated and BCTV strain composition was investigated based on polymerase chain reaction assays with strain-specific primers for the Severe (Svr) and California/Logan (CA/Logan) strains and primers that amplified a group of Worland (Wor)-like strains. The BCTV strain distribution averaged 2% Svr, 30% CA/Logan, and 87% Wor-like (16% had mixed infections), which differed from the previously published 2006-to-2007 collection (87% Svr, 7% CA/Logan, and 60% Wor-like; 59% mixed infections) based on a contingency test (P < 0.0001). Whole-genome sequencing (GenBank accessions KT276895 to KT276920 and KX867015 to KX867057) with overlapping primers found that the Wor-like strains included Wor, Colorado and a previously undescribed strain designated Kimberly1. Results confirm a shift from Svr being one of the dominant BCTV strains in commercial sugar beet fields in 2006 to 2007 to becoming undetectable at times during recent years.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant-virus-vector interactions has flourished in recent years, plant-bacteria-vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant-bacteria-vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and 'Candidatus Phytoplasma spp.'. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector-plant-bacteria interactions.

Localization and distribution of wheat dwarf virus in its vector leafhopper, Psammotettix alienus

Numerous virus pathogens are transmitted by specific arthropod vectors. Understanding the mechanism of transmission is a critical step in the epidemiology of plant viruses and is crucial for the development of effective disease control strategies. In this study, we describe the localization and distribution of Wheat dwarf virus (WDV), an economically important and widespread single-stranded DNA virus, in its leafhopper vector, Psammotettix alienus. The results suggest that WDV not only can move to the salivary glands from the anterior and middle midgut via the hemocoel but also can pass directly through the sheath of the filter chamber and be readily transmitted to healthy wheat plants within 5 min of an acquisition access period on infected plants. When a bacterial-expressed recombinant capsid protein (CP) was incubated with the internal organs of leafhoppers, CP-immunoreactive antigens were found at the anterior and middle midgut. Furthermore, when leafhoppers were fed with an antiserum raised against the CP, the accumulation of WDV in the gut cells, hemocoel, and salivary glands was significantly reduced. These data provide evidence that transmission of WDV is determined by a CP-mediated virion-vector retention mechanism.

Diversity of beet curly top Iran virus isolated from different hosts in Iran

Beet curly top Iran virus (BCTIV) is a major pathogen of sugar beet in Iran. In order to study diversity of BCTIV, we sampled 68 plants in Iran during the summer of 2010 with curly top disease symptoms on beans (Phaseolus vulgaris), cowpeas (Vigna unguiculata), tomatoes (Solanum lycopersicum L.), sea beets (Beta vulgaris subsp. maritima), and sugar beets (Beta vulgaris). Plant samples showing leaf curling, yellowing, and/or swelling of veins on the lower leaf surfaces were collected from various fields in Khorasan Razavi, Northern Khorasan (north-eastern Iran), East Azarbayejan, West Azarbayejan (north-western Iran), and Fars (southern Iran) provinces. Using rolling circle amplification coupled with restriction digests, cloning, and Sanger sequencing, we determined the genomes of nine new BCTIV isolates from bean, cowpea, tomato, sea beet, and sugar beet in Iran. Our analysis reveals ~11 % diversity amongst BCTIV isolates and we detect evidence of recombination within these genomes.

Genetic diversity and host range studies of turnip curly top virus

Turnip curly top virus (TCTV) is a unique geminivirus that has recently been characterised as infecting turnips in Iran. The genome of TCTV shares <68 % pairwise identity with other geminiviruses and has a genome organisation similar to that of curtoviruses and topocuvirus. The replication-associated protein (Rep) bears the highest similarity to curtovirus Reps (48.5-69.0 %); however, in the case of the capsid protein (CP), the extent of similarity is only 39.5-44.5 %. We constructed an agroinfectious clone of TCTV and undertook host range studies on ten plant species; in three species (turnip, sugar beet and cowpea), we detected infection which presents curly top symptoms in turnip and sugar beet. The efficiency of TCTV infection in agroinoculated turnip plants was 71.7 %, and the infection was successfully transmitted to 80 % of the healthy turnip plants used in the insect transmission studies by Circulifer haematoceps under greenhouse conditions. We also determined the genome sequence of 14 new TCTV isolates from southern Iran isolated from turnips. We observed ~13 % diversity amongst all the TCTV isolates and found evidence of recombination in the CP- and Rep-coding regions of the genomes.

Genetic diversity in curtoviruses: a highly divergent strain of Beet mild curly top virus associated with an outbreak of curly top disease in pepper in Mexico

A full-length curtovirus genome was PCR-amplified and cloned from peppers in Mexico with symptoms of curly top disease. The cloned DNA of this isolate, MX-P24, replicated in Nicotiana tabacum protoplasts and was infectious in N. benthamiana plants. Sequence analysis revealed that the MX-P24 isolate had a typical curtovirus genome organization and was most similar to beet mild curly top virus (BMCTV). However, sequence identities were at the threshold value for establishment of a new curtovirus species. To further investigate the biological properties of MX-P24, an agroinoculation system was generated. Agroinoculated shepherd's purse plants developed typical curly top symptoms, and virus from these plants was transmissible by the beet leafhopper (Circulifer tenellus). The host range of MX-P24 was similar to that of BMCTV, with curly top symptoms induced in common bean, pepper, pumpkin, shepherd's purse and tomato plants and mild or no symptoms induced in sugar beet plants. Together, these results indicate that MX-P24 is a highly divergent strain of BMCTV associated with an outbreak of curly top disease in peppers in Mexico.

Immunodetection of Two Curtoviruses Infecting Sugar Beet

Beet leafhopper-transmitted curly top virus is a serious problem in many different crops in the semiarid western United States, including sugar beet, tomatoes, and beans. Curly top is caused by a genetically diverse complex of phloem-limited curtoviruses. Due to the phloem restriction of curtoviruses and the lack of a convenient laboratory host-vector system for curly top virus propagation and purification, no commercial immunodetection tests are available for curtoviruses. Routine diagnostics for curly top rely either on visual symptoms or on polymerase chain reaction (PCR) tests. Lack of an enzyme-linked immunosorbent assay (ELISA) system is one of the factors hampering development and screening of the curly top resistant germplasm in, for instance, sugar beet and bean breeding programs. To fill in this gap, we developed an ELISA-based detection system for curtoviruses which utilizes virus-specific antibodies generated against bacterially expressed capsid protein (CP) of Beet mild curly top virus. Bacterially expressed CP was affinity purified and used as an antigen for antibody production in two animal species. Specificity of the resulting antisera was tested in Western blots and various triple-antibody sandwich (TAS)-ELISA formats with sugar beet, bean, and Nicotiana benthamiana leaf tissue. We demonstrate reliable detection of two curtoviruses in different crops in TAS-ELISA format, suitable for large-scale screening of germplasm in breeding programs.

Turnip curly top virus, a highly divergent geminivirus infecting turnip in Iran

From 2006 onwards turnip crops in Fars province, Iran, have been noted with unusual leaf curling and vein swelling symptoms which are characteristic of the leafhopper-transmitted viruses of the genus Curtovirus (family Geminiviridae). Rolling circle amplification was used to clone viruses from five turnip isolates exhibiting leaf curl symptoms. Analysis of the sequences showed them to have >93% sequence identity and to be distinct from all other geminiviruses previously characterised. Analysis of the sequence of this virus, for which we propose the name Turnip curly top virus (TCTV), showed it to have a genome arrangement in the complementary-sense similar to that of curtoviruses (consisting of four overlapping genes) but only two open reading frames in the virion-sense (the curtoviruses encode three). The complementary-sense genes are homologous to those of curtoviruses but show little sequence identity to their curtovirus homologs, with the exception of the product of the C4 open reading frame (ORF) which shows approximately 70.6% amino acid sequence identity to the C4 of the North American curtoviruses, Pepper curly top virus and Beet mild curly top virus. For curtoviruses the C4 protein is a symptom determinant, which likely explains the similarity of TCTV symptoms to those of curtoviruses. In the virion-sense the predicted product of the V2 ORF of TCTV shows no significant similarity with any proteins in the databases whereas the product of the V1 ORF (encoding the coat protein [CP] of geminiviruses) shows low levels of sequence identity to the CPs of curtoviruses. These findings show TCTV to be a highly divergent geminivirus with similarities to viruses of the genus curtovirus. The significance of these findings, particularly the taxonomic implications are discussed.

Characterization of Curtoviruses Associated with Curly Top Disease of Tomato in California and Monitoring for These Viruses in Beet Leafhoppers

Curly top disease is caused by a complex of curtoviruses (family Geminiviridae), and it continues to plague tomato production in California. To better understand the etiology of curly top of tomatoes in California, polymerase chain reaction (PCR)-based methods were developed and used to characterize the curtoviruses involved, and to monitor for these viruses in the beet leafhopper vector, Circulifer tenellus. From 2002 to 2008, 86 processing and fresh market tomato fields in the Central Valley of California were surveyed for the incidence of curly top symptoms. Representative samples with curly top symptoms were collected from the surveyed fields, as well as from another 24 fields. The incidence of curly top symptoms in most fields ranged from trace (<1%) to low (1 to 5%); however, in 2002, 2003, and 2008, some fields had medium (5 to 20%) or high (>20%) incidences. PCR with general and species-specific primers was used to establish that the predominant species associated with tomato curly top disease were Beet mild curly top virus (BMCTV) and, to a lesser extent, Beet severe curly top virus (BSCTV). The incidence, relative amount, and species of curly top virus in leafhoppers, collected at monthly intervals by the California Department of Food and Agriculture (CDFA) personnel during 2003 to 2008, was also determined. The predominant species detected were BMCTV and BSCTV. The highest incidences of curly top in tomato fields were associated with high populations of leafhoppers (e.g., in 2003 when populations were two times greater than average) having high incidences and levels of curly top virus early in the growing seasons (e.g., March to May 2003 and 2008). Detection of curly top virus in leafhoppers early in the growing season was consistent with acquisition of virus from reservoir hosts in the foothills. However, continued detection of curly top virus in leafhoppers throughout the growing season and development of curly top in late-planted fresh market tomatoes were consistent with the presence of inoculum sources (e.g., weeds or crop plants) in the agricultural areas of the Central Valley. Geographical locations or "hotspots" having higher proportions of curly top virus-positive leafhoppers were identified, which may reveal areas having high concentrations of curly top virus reservoir hosts. The application of these molecular tools has provided new insight into curly top of tomato in California, and may lead to improved curly top management.

Curtovirus-cucurbit interaction: acquisition host plays a role in leafhopper transmission in a host-dependent manner

Curly top disease (CTD) of vegetable crops is caused by viruses in the genus Curtovirus (family Geminiviridae). Cucurbits are reported to be susceptible to CTD; however, the disease is rare in California despite annual outbreaks in other hosts (e.g., common bean, pepper, sugar beet, and tomato). Consistent with these observations, no obvious curly top symptoms were observed in melon fields surveyed for CTD in Central California in 2004 and 2005, whereas the disease was readily observed in tomato plants in nearby fields. However, samples of cucurbits from Idaho with curly top-like symptoms, collected in 2005 and 2007, were confirmed to have the disease. The susceptibility of cucurbits (cantaloupe, honeydew melon, pumpkin, and watermelon) to the three curtoviruses known to occur in California (Beet curly top virus, BCTV; Beet severe curly top virus, BSCTV; and Beet mild curly top virus, BMCTV) was evaluated by agroinoculation or leafhopper transmission. Irrespective of the curtovirus species and inoculation method, low rates of infection and mild or symptomless disease phenotypes were observed in cucurbits. In contrast, all inoculated tomato, Nicotiana benthamiana, or shepherd's purse plants were infected and developed severe symptoms. In leafhopper transmission experiments, BMCTV infected cucurbits when leafhoppers acquired the virus from a symptomatic host with a high viral titer (shepherd's purse), whereas no infection occurred when the acquisition host had mild symptoms and a low viral titer (sugar beet); in contrast, the acquisition host did not influence transmission of BMCTV to tomato or shepherd's purse (all plants were infected). This revealed an influence of the acquisition host on leafhopper transmission in a host-specific manner. Our results also indicate that, although cucurbits can develop CTD, they are relatively poor hosts for these curtoviruses.

Influence of Host Resistance and Insecticide Seed Treatments on Curly Top in Sugar Beets

Curly top on sugar beets (Beta vulgaris) caused by Beet severe curly top virus or closely related species is a considerable problem in arid growing regions of the western United States. Two insecticide seed treatments, Poncho Beta (60 g a.i. clothianidin + 8 g a.i. beta-cyfluthrin/100,000 seed) and Gaucho (45 g a.i. imidacloprid/100,000 seed), and four sugar beet hybrids varying in curly top resistance were evaluated for their influence on the control of curly top in comparison with untreated checks. Plots were established at two locations in southern Idaho in 2005 and evaluated for curly top. Moderate to severe curly top due to natural inoculum and leafhopper infestations occurred at both locations. Untreated, the four hybrids performed as expected with the fewest curly top symptoms on PM21 and the most on Monohikari. Both insecticide treatments lowered curly top ratings compared with the untreated check, but Poncho Beta reduced symptoms more than Gaucho as the season progressed. Poncho Beta led to increased yield and estimated recoverable sugar across all hybrids at harvest, particularly on the more susceptible hybrids. When considering the yield parameters for only the most resistant hybrids individually, Poncho Beta did not always outperform Gaucho. Poncho Beta provided a level of control that would justify its application as a supplement to host resistance under Idaho conditions.

Identification of regions of the Beet mild curly top virus (family Geminiviridae) capsid protein involved in systemic infection, virion formation and leafhopper transmission

Plant viruses in the genus Curtovirus (family Geminiviridae) are vectored by the beet leafhopper (Circulifer tenellus) and cause curly top disease in a wide range of dicotyledonous plants. An infectious clone of an isolate of Beet mild curly top virus (BMCTV-[W4]), associated with an outbreak of curly top in pepper and tomato crops, was characterized and used to investigate the role of the capsid protein (CP) in viral biology and pathogenesis. Frameshift mutations were introduced into the overlapping CP and V2 genes, and a series of CP alanine scanning mutations were generated. All mutants replicated in tobacco protoplasts or systemically infected plants, consistent with these gene products not being required for viral DNA replication. The CP frameshift mutant and most C-terminal alanine scanning mutants did not systemically infect Nicotiana benthamiana plants or form detectable virions, and were not leafhopper-transmitted. In contrast, most N-terminal alanine scanning mutants systemically infected N. benthamiana and induced disease symptoms, formed virions and were leafhopper-transmissible; thus, these substitution mutations did not significantly alter the functional properties of this region. One N-terminal mutant (CP49-51) systemically infected N. benthamiana, but did not form detectable virions; whereas another (CP25-28) systemically infected N. benthamiana and formed virions, but was not insect-transmissible. These mutants may reveal regions involved in virus movement through the plant and/or leafhopper vector. Together, these results indicate an important role for virions in systemic infection (long-distance movement) and insect transmission, and strongly suggest that virions are the form in which BMCTV moves, long distance, in the phloem.

Exploiting chinks in the plant's armor: evolution and emergence of geminiviruses

The majority of plant-infecting viruses utilize an RNA genome, suggesting that plants have imposed strict constraints on the evolution of DNA viruses. The geminiviruses represent a family of DNA viruses that has circumvented these impediments to emerge as one of the most successful viral pathogens, causing severe economic losses to agricultural production worldwide. The genetic diversity reflected in present-day geminiviruses provides important insights into the evolution and biology of these pathogens. To maximize replication of their DNA genome, these viruses acquired and evolved mechanisms to manipulate the plant cell cycle machinery for DNA replication, and to optimize the number of cells available for infection. In addition, several strategies for cell-to-cell and long-distance movement of the infectious viral DNA were evolved and refined to be compatible with the constraints imposed by the host endogenous macromolecular trafficking machinery. Mechanisms also evolved to circumvent the host antiviral defense systems. Effectively combatting diseases caused by geminiviruses represents a major challenge and opportunity for biotechnology.

Spinach curly top virus: A Newly Described Curtovirus Species from Southwest Texas with Incongruent Gene Phylogenies

ABSTRACT A curtovirus associated with a disease of spinach was isolated in southwest Texas during 1996. Disease symptoms included severe stunting and chlorosis, with younger leaves curled, distorted, and dwarfed. Viral DNA was purified and an infectious clone obtained. Agroinoculation using a construct bearing full-length tandem repeats of the cloned viral genome resulted in systemic infection of species in six of seven plant families tested, indicating that the virus has a wide host range. Symptoms produced in spinach agroinoculated with cloned viral DNA were similar to those observed in the field. Viral single-stranded and double-stranded DNA forms typical of curtovirus infection were detected in host plants by Southern blot hybridization. The complete sequence of the infectious clone comprised 2,925 nucleotides, with seven open reading frames encoding proteins homologous to those of other curtoviruses. Complete genome comparisons revealed that the spinach curtovirus shared 64.2 to 83.9% nucleotide sequence identity relative to four previously characterized curtovirus species: Beet curly top virus, Beet severe curly top virus, Beet mild curly top virus, and Horseradish curly top virus. Phylogenetic analysis of individual open reading frames indicated that the evolutionary history of the three virion-sense genes was different from that of the four complementary-sense genes, suggesting that recombination among curtoviruses may have occurred. Collectively, these results indicate that the spinach curtovirus characterized here represents a newly described species of the genus Curtovirus, for which we propose the name Spinach curly top virus.