Severe Yellowing Outbreaks in Tomato in Spain Associated with Infections of Tomato chlorosis virus

Genetic diversity, distribution, and structure of Bemisia tabaci whitefly species in potential invasion and hybridization regions of East Africa

Outbreaks of whitefly, Bemisia tabaci species in East and Central Africa, have become increasingly prevalent during the previous 25 years and are responsible for driving the spread of plant-virus diseases, such as cassava mosaic disease and cassava brown steak disease. Epidemics of these diseases have expanded their ranges over the same period, spreading from Uganda into other sub-Saharan African countries. It was hypothesised that a highly abundant 'invader' population of B. tabaci was responsible for spreading these diseases from Uganda to neighbouring countries and potentially hybridising with the resident cassava B. tabaci populations. Here, we test this hypothesis by investigating the molecular identities of the highly abundant cassava B. tabaci populations from their supposed origin in Uganda, to the northern, central, eastern and coastal regions of Tanzania. Partial mitochondrial cytochrome oxidase I (mtCOI) barcoding sequences and nuclear microsatellite markers were used to analyse the population genetic diversity and structure of 2734 B. tabaci collected from both countries and in different agroecological zones. The results revealed that: (i) the putative SSA1 species is structured according to countries, so differ between them. (ii) Restricted gene flow occurred between SSA1-SG3 and both other SSA1 subgroups (SG1 and SG2), even in sympatry, demonstrating strong barriers to hybridization between those genotypes. (iii) Not only B. tabaci SSA1-(SG1 and SG2) was found in highly abundant (outbreak) numbers, but B. tabaci SSA1-SG3 and the Indian Ocean (IO) species were also recorded in high numbers in several sites in Tanzania. (iv) The SSA1-(SG1 and SG2) species was distributed in both countries, but in Tanzania, the B. tabaci IO and SSA1-SG3 species predominated. These data confirm that multiple, local Tanzanian B. tabaci species produce highly abundant populations, independent of the spread of the putative invasive B. tabaci SSA1-(SG1 and SG2) populations.

Antagonism of tomato spotted wilt virus against tomato yellow leaf curl virus in Nicotiana benthamiana detected by transcriptome analysis

BACKGROUND

Tomato spot wilt virus (TSWV) and tomato yellow leaf curl virus (TYLCV) are highly harmful viruses in agricultural production, which can cause serious economic losses to crops and even devastating consequences for vegetable yield in some countries and regions. Although the two viruses belong to different families and have different transmission vectors, they share most hosts.

OBJECTIVE

This study aimed to examine the transcriptomic expression of single and mixed inoculations of TSWV and TYLCV, leading to antagonism using high-throughput RNA sequencing.

METHODS

We confirmed the single and mixed infections of these viruses in Nicotiana benthamiana (N. benthamiana) by artificial inoculation. And the expression changes of related genes and their biological functions and pathways during the mixed infection of TSWV and TYLCV were analyzed by comparative transcriptome.

RESULTS

Basically, similar symptoms were observed in the plants singly infected with TSWV and co-infected with TYLCV; the symptoms of TYLCV in the co-infected plants were not obvious compared with single TYLCV infections. When inoculated with TYLCV, the accumulation of the virus significantly reduced in single and mixed infections with TSWV; the TSWV accumulated slightly less in co-infection with TYLCV, whereas this reduction was much smaller than that of TYLCV. The results suggested that TSWV had an antagonistic effect on the accumulation of TYLCV in N. benthamiana. It mainly focused on the changes in unique differentially expressed genes (DEGs) caused by the co-infection of TSWV and TYLCV. The eight pathways enriched by upregulated DEGs mainly included amino acid biosynthesis, citrate cycle (or tricarboxylic acid cycle, TCA cycle), and so on. However, only pentose phosphate pathway (PPP) and peptidoglycan biosynthesis could be downregulated in the Kyoto Encyclopedia of Genes and Genomes pathway in which peptidoglycan biosynthesis was involved in upregulated and downregulated pathways.

CONCLUSIONS

The antagonistic effect of TSWV on TYLCV in N.benthamiana and the change trends and specific pathways of DEGs in this process were found. Our study provided new insights into the host regulation and competition between viruses in response to TSWV and TYLCV mixed infection.

Suppression of Bta11975, an α-glucosidase, by RNA interference reduces transmission of tomato chlorosis virus by Bemisia tabaci

BACKGROUND

Tomato chlorosis virus (ToCV) is mainly vectored by Bemisia tabaci in China, which has a worldwide distribution, and greatly reduces the yields of tomato and other vegetables. At present, control of ToCV has been focused mainly by the use of insecticides to control whitefly populations. Transcriptome sequencing showed high expression of the B. tabaci Bta11975 gene, an α-glucosidase (AGLU) during ToCV acquisition by whitefly Mediterranean (MED) species. To investigate the role of Bta11975 gene in ToCV acquisition and transmission by B. tabaci MED, we used RNA interference (RNAi) to reduce the expression of the Bta11975 gene.

RESULTS

The relative expression of the Bta11975 gene was correlated with the ToCV content in B. tabaci. The AGLU is highly expressed in primary salivary gland and gut. After the Bta11975 gene was silenced, the gene expression of B. tabaci was reduced and B. tabaci mortality was increased. Besides, ToCV acquisition by B. tabaci at 48 and 72 h AAP was reduced, and ToCV transmission was significantly reduced by 25 or 50 of B. tabaci.

CONCLUSIONS

These results indicate that suppression of expression of the Bta11975 gene in B. tabaci MED by RNAi can reduce acquisition and transmission of ToCV by B. tabaci MED.

A simplified RT-PCR assay for the simultaneous detection of tomato chlorosis virus and tomato yellow leaf curl virus in tomato

Tomato chlorosis virus (ToCV), a species of single-stranded RNA virus belonging to the Crinivirus genus, and Tomato yellow leaf curl virus (TYLCV), a species of single-stranded circular DNA virus belonging to the Begomovirus genus, are two major emerging viruses transmitted by whiteflies and are causing huge losses to tomato production worldwide. To facilitate the simultaneous detection of both viruses in co-infected plants for disease control, a duplex reverse-transcription PCR assay was developed. The assay used three primers, a degenerate reverse primer targeting a conserved region of TYLCV and the RNA2 of ToCV, and two virus-specific forward primers targeting the minor coat protein gene of ToCV and the C3 gene of TYLCV, respectively, to amplify a 762-bp and a 338-bp fragment from ToCV and TYLCV, respectively, in a single reaction. The concentration of the primers, annealing temperature and amplification cycles used in the assay were optimized, and the sensitivity of the assay was assessed. Using this assay, 150 tomato leaf samples collected from the field during 2018 were tested. The results showed that both viruses could be detected simultaneously in co-infected field samples. The assay should benefit the rapid detection of these two viruses in tomato crops and would facilitate early warning of infections for the control of the two virus diseases.

Infectious Clones of Tomato Chlorosis Virus: Toward Increasing Efficiency by Introducing the Hepatitis Delta Virus Ribozyme

Tomato chlorosis virus (ToCV) is an emergent plant pathogen that causes a yellow leaf disorder in tomato and other solanaceous crops. ToCV is a positive-sense, single stranded (ss)RNA bipartite virus with long and flexuous virions belonging to the genus Crininivirus (family Closteroviridae). ToCV is phloem-limited, transmissible by whiteflies, and causes symptoms of interveinal chlorosis, bronzing, and necrosis in the lower leaves of tomato accompanied by a decline in vigor and reduction in fruit yield. The availability of infectious virus clones is a valuable tool for reverse genetic studies that has been long been hampered in the case of closterovirids due to their genome size and complexity. Here, attempts were made to improve the infectivity of the available agroinfectious cDNA ToCV clones (isolate AT80/99-IC from Spain) by adding the hepatitis delta virus (HDV) ribozyme fused to the 3′ end of both genome components, RNA1 and RNA2. The inclusion of the ribozyme generated a viral progeny with RNA1 3′ ends more similar to that present in the clone used for agroinoculation. Nevertheless, the obtained clones were not able to infect tomato plants by direct agroinoculation, like the original clones. However, the infectivity of the clones carrying the HDV ribozyme in Nicotiana benthamiana plants increased, on average, by two-fold compared with the previously available clones.

Natural Infection Rate of Known Tomato chlorosis virus-Susceptible Hosts and the Influence of the Host Plant on the Virus Relationship With Bemisia tabaci MEAM1

Tomato chlorosis virus (ToCV; genus Crinivirus, family Closteroviridae) was identified in tomato crops in São Paulo State, Brazil, in 2006. Management strategies to control external sources of inoculum are necessary, because chemical control of the whitefly vector Bemisia tabaci Middle East-Asia Minor 1 (MEAM1) has not efficiently prevented virus infections and no commercial tomato varieties or hybrids are resistant to this crinivirus. We first evaluated the natural infection rate of some known wild and cultivated ToCV-susceptible hosts and their attractiveness for B. tabaci MEAM1 oviposition. Physalis angulata was the most susceptible to natural infection in all six exposures in 2018 and 2019. No plants of Capsicum annuum 'Dahra' or Chenopodium album became infected. Solanum melongena 'Napoli' had only two infected plants of 60 exposed. Capsicum annuum and Chenopodium album were the least preferred, and Nicotiana tabacum and S. melongena were the most preferred for whitefly oviposition. In addition, from 2016 to 2019, we surveyed different tomato crops and the surrounding vegetation to identify ToCV in weeds and cultivated plants in the region of Sumaré, São Paulo State. Only S. americanum, vila vila (S. sisymbriifolium), and Chenopodium album were found naturally infected, with incidences of 18, 20, and 1.4%, respectively. Finally, we estimated the ToCV titer (U.S. and Brazilian isolates ToCV-FL and ToCV-SP, respectively) by quantitative reverse transcription PCR in different ToCV-susceptible host plants and evaluated the relationship between virus acquisition and transmission by B. tabaci MEAM1. The results clearly showed significant differences in ToCV concentrations in the tissues of ToCV-susceptible host plants, which appeared to be influenced by the virus isolate. The concentration of the virus in plant tissues, in turn, directly influenced the ToCV-B. tabaci MEAM1 relationship and subsequent transmission to tomato plants. To minimize or prevent damage from tomato yellowing disease through management of external sources of ToCV, it is necessary to correctly identify potentially important ToCV-susceptible hosts in the vicinity of new plantings.

Transcriptome analysis and comparison reveal divergence between the Mediterranean and the greenhouse whiteflies

Both the Mediterranean (MED) species of the Bemisia tabaci whitefly complex and the greenhouse whitefly (Trialeurodes vaporariorum, TV) are important agricultural pests. The two species of whiteflies differ in many aspects such as morphology, geographical distribution, host plant range, plant virus transmission, and resistance to insecticides. However, the molecular basis underlying their differences remains largely unknown. In this study, we analyzed the genetic divergences between the transcriptomes of MED and TV. In total, 2,944 pairs of orthologous genes were identified. The average identity of amino acid sequences between the two species is 93.6%. The average nonsynonymous (Ka) and synonymous (Ks) substitution rates and the ratio of Ka/Ks of the orthologous genes are 0.0389, 2.23 and 0.0204, respectively. The low average Ka/Ks ratio indicates that orthologous genes tend to be under strong purified selection. The most divergent gene classes are related to the metabolisms of xenobiotics, cofactors, vitamins and amino acids, and this divergence may underlie the different biological characteristics between the two species of whiteflies. Genes of differential expression between the two species are enriched in carbohydrate metabolism and regulation of autophagy. These findings provide molecular clues to uncover the biological and molecular differences between the two species of whiteflies.

Foliar Spraying of Tomato Plants with Systemic Insecticides: Effects on Feeding Behavior, Mortality and Oviposition of Bemisia tabaci (Hemiptera: Aleyrodidae) and Inoculation Efficiency of Tomato Chlorosis Virus

Simple Summary

The whitefly Bemisia tabaci (Hemiptera: Aleyrodidae) causes serious losses to vegetable, ornamental and fiber crops, including tomato plants, mainly as a vector of economically important viruses. Among the most important viruses affecting tomato is the tomato chlorosis virus (ToCV) (Closteroviridae: Crinivirus), which is semi-persistently transmitted by whiteflies. Effective management of this pest is crucial to reduce the spread of vector-borne diseases and to reduce crop damage and losses. In this study we evaluated the effect of systemic insecticides (cyantraniliprole, acetamiprid and flupyradifurone) on the feeding behavior, mortality and oviposition of B. tabaci MEAM1 and their ability to interfere with the inoculation of ToCV in tomato plants. Our findings indicate that systemic insecticides cause high mortality when compared to untreated plants. Also, we found that flupyradifurone affects stylet activities of B. tabaci and significantly reduce phloem ingestion, a behavior that is closely linked to the transmission of ToCV. Overall, our findings indicate that flupyradifurone may contribute to management of this pest and ToCV in tomato crops.

Abstract

Tomato chlorosis virus (ToCV) is a phloem-limited crinivirus transmitted by whiteflies and seriously affects tomato crops worldwide. As with most vector-borne viral diseases, no cure is available, and the virus is managed primarily by the control of the vector. This study determined the effects of the foliar spraying with the insecticides, acetamiprid, flupyradifurone and cyantraniliprole, on the feeding behavior, mortality, oviposition and transmission efficiency of ToCV by B. tabaci MEAM1 in tomato plants. To evaluate mortality, oviposition and ToCV transmission in greenhouse conditions, viruliferous whiteflies were released on insecticide-treated plants at different time points (3, 24 and 72 h; 7 and 14 days) after spraying. Insect mortality was higher on plants treated with insecticides; however, only cyantraniliprole and flupyradifurone differed from them in all time points. The electrical penetration graph (DC-EPG) technique was used to monitor stylet activities of viruliferous B. tabaci in tomato plants 72 h after insecticide application. Only flupyradifurone affected the stylet activities of B. tabaci, reducing the number and duration of intracellular punctures (pd) and ingestion of phloem sap (E2), a behavior that possibly resulted in the lower percentage of ToCV transmission in this treatment (0–60%) in relation to the control treatment (60–90%) over the periods evaluated. Our results indicate that flupyradifurone may contribute to management of this pest and ToCV in tomato crops.

Pest categorisation of non-EU viruses and viroids of potato

Following a request from the EU Commission, the Panel on Plant Health has addressed the pest categorisation of those viruses and viroids (hereafter referred to as viruses) of Solanum tuberosum and other tuber-forming Solanum spp. (hereafter referred to as potato) which are considered to be either non-EU or of undetermined standing based on a previous EFSA opinion. These viruses belong to different families and genera and either have an established identity or produce consistent symptoms. Plants for planting is the main pathway for entry for all categorised viruses as they can all be transmitted by vegetative propagation. Several categorised viruses have a relatively wide host range and/or are vector-transmitted, increasing the potential for entry. The information currently available on geographical distribution, biology, epidemiology, impact and potential entry pathways has been evaluated with regard to the criteria to qualify as potential Union quarantine pest or as Union regulated non-quarantine pest (RNQP). Since this opinion addresses specifically the non-EU potato viruses, in general these viruses do not meet the criteria assessed by EFSA to qualify as potential Union regulated non-quarantine pests. The following viruses meet the criteria to qualify as potential Union quarantine pest: APLV, APMMV, APMoV, ChiLCV, CYSDV, PAMV, PBRSV, PVH, PVP, PVT, PYDV, PYMV, PYV, PYVV, RCVMV, SALCV, SB26/29, ToCV, ToLCNDV, ToMHaV, ToMoTV, ToSRV and ToYVSV. With the exception of the criterion regarding the potential for consequences in the EU territory, for which the Panel is unable to conclude because of lack of information, AVB, CPSbV, PaLCrV, PapMV, PVB, PVU, SB41 and TVBMV meet all the other criteria to qualify as potential Union quarantine pest. PotLV and WPMV do not qualify as potential Union quarantine pest, since they are not reported to have any impact. For most of the categorised viruses, the conclusions of the Panel have inherent uncertainties, due to the lack of quantitative data on their impact and/or absence or limited availability of information on the biology, epidemiology and geographical distribution.

Design, Synthesis, and Anti-ToCV Activity of Novel 4(3H)-Quinazolinone Derivatives Bearing Dithioacetal Moiety

Tomato chlorosis virus (ToCV) has caused great harm to the production of tomato worldwide. To develop efficient anti-ToCV agents, some novel 4(3H)-quinazolinone derivatives containing dithioacetal were designed and synthesized, and their anti-ToCV activities were evaluated by microscale thermophoresis (MST) using ToCV coat protein (ToCV-CP) as a new target. The results showed that some compounds had a strong binding capacity to ToCV-CP. In particular, compounds C5 and C22 have an excellent binding capacity to ToCV-CP, with binding constant values of 0.24 and 0.25 μM, respectively. Additionally, reduced ToCV-CP gene expression levels of 81.05 and 87.59% could be achieved when tomato was treated with compounds C5 and C22, respectively, which were obviously higher than the levels after ningnanmycin (NNM) treatment (43.88%) and lead compound Xiangcaoliusuobingmi (XCLSBM) treatment (63.56%). Therefore, this work indicates that 4(3H)-quinazolinone derivatives containing dithioacetal moiety can be used as novel anti-ToCV agents.

Epidemiology and control of emerging criniviruses in bean

During the last two decades, new criniviruses emerged in green bean crops in the south-east of Spain. Bean yellow disorder virus (BnYDV) was first detected in 2003 and caused major economic damage in crops grown in greenhouses. It was characterized as the first crinivirus to infect a member species of the Leguminosae family. Symptoms induced during BnYDV infection include interveinal chlorosis and yellowing on leaves, and reduced fruit yield and quality. Similar symptoms, although more severe, were observed in bean crops in the same region during the fall of 2011. From that moment on, BnYDV was not detected anymore in diseased plants, but instead lettuce chlorosis virus (LCV) was associated with the diseased plants. Previously, LCV was detected only in California, USA, infecting lettuce and sugarbeets. The host range and partial genomic sequences lead to the description of the new strain, LCV-SP. The complete sequence of its genome revealed the virus as a recombinant of BnYDV and LCV, in which the latter had lost two ORFs in the RNA1 of the bipartite genome and had acquired two homologue ORFs from BnYDV. Both viruses are transmitted by the whitefly Bemisia tabaci. When compared with other crinivirus pathosystems, the transmission efficiency of BnYDV to its primary host bean, is among the highest, and its persistence in the vector among the longest, up to 9 days. The host range of BnYDV s restricted to several crop species of the Leguminosae: common bean (Phaseolus vulgaris), pea (Pisum sativum), tirabeque (P. sativum subsp. sativum var. macrocarpon), lentil (Lens culinaris) and faba bean (Vicia faba). LCV-SP is also able to infect green bean plants but not lettuce, its original host, probably following its recombinant nature. Symptoms and epidemiology of the bean criniviruses are compared with similar pathosystems that occur in the same region and that involve cucurbit yellow stunting disorder virus and tomato chlorosis virus, infecting cucurbitaceous and solanaceous crops, respectively. Control of the criniviruses in bean crops will depend on efficient control of the vector. Physical control with greenhouses that prevent viruliferous whiteflies from gaining access to crops reduces BnYDV infection in plants and loss of production. Integrated pest management in beans would be preferred and the use of natural enemies to reduce secondary spread within greenhouses must be investigated.

Long-Term Cocirculation of Two Strains of Pepino Mosaic Virus in Tomato Crops and Its Effect on Population Genetic Variability

Mixed viral infections are common in plants, and the evolutionary dynamics of viral populations may differ depending on whether the infection is caused by single or multiple viral strains. However, comparative studies of single and mixed infections using viral populations in comparable agricultural and geographical locations are lacking. Here, we monitored the occurrence of pepino mosaic virus (PepMV) in tomato crops in two major tomato-producing areas in Murcia (southeastern Spain), supporting evidence showing that PepMV disease-affected plants had single infections of the Chilean 2 (CH2) strain in one area and the other area exhibited long-term (13 years) coexistence of the CH2 and European (EU) strains. We hypothesized that circulating strains of PepMV might be modulating the differentiation between them and shaping the evolutionary dynamics of PepMV populations. Our phylogenetic analysis of 106 CH2 isolates randomly selected from both areas showed a remarkable divergence between the CH2 isolates, with increased nucleotide variability in the geographical area where both strains cocirculate. Furthermore, the potential virus-virus interaction was studied further by constructing six full-length infectious CH2 clones from both areas, and assessing their viral fitness in the presence and absence of an EU-type isolate. All CH2 clones showed decreased fitness in mixed infections and although complete genome sequencing indicated a nucleotide divergence of those CH2 clones by area, the magnitude of the fitness response was irrespective of the CH2 origin. Overall, these results suggest that although agroecological cropping practices may be particularly important for explaining the evolutionary dynamics of PepMV in tomato crops, the cocirculation of both strains may have implications on the genetic variability of PepMV populations.

Transmission of Begomoviruses and Other Whitefly-Borne Viruses: Dependence on the Vector Species

Most plant viruses require a biological vector to spread from plant to plant in nature. Among biological vectors for plant viruses, hemipteroid insects are the most common, including phloem-feeding aphids, whiteflies, mealybugs, planthoppers, and leafhoppers. A majority of the emerging diseases challenging agriculture worldwide are insect borne, with those transmitted by whiteflies (Hemiptera: Aleyrodidae) topping the list. Most damaging whitefly-transmitted viruses include begomoviruses (Geminiviridae), criniviruses (Closteroviridae), and torradoviruses (Secoviridae). Among the whitefly vectors, Bemisia tabaci, now recognized as a complex of cryptic species, is the most harmful in terms of virus transmission. Here, we review the available information on the differential transmission efficiency of begomoviruses and other whitefly-borne viruses by different species of whiteflies, including the cryptic species of the B. tabaci complex. In addition, we summarize the factors affecting transmission of viruses by whiteflies and point out some future research prospects.

Infection by the semi-persistently transmitted Tomato chlorosis virus alters the biology and behaviour of Bemisia tabaci on two potato clones

Insect-borne plant viruses usually alter the interactions between host plant and insect vector in ways conducive to their transmission ('host manipulation hypothesis'). Most studies have tested this hypothesis with persistently and non-persistently transmitted viruses, while few have examined semi-persistently transmitted viruses. The crinivirus Tomato chlorosis virus (ToCV) is semi-persistently transmitted virus by whiteflies, and has been recently reported infecting potato plants in Brazil, where Bemisia tabaci Middle East Asia Minor 1 (MEAM1) is a competent vector. We investigated how ToCV infection modifies the interaction between potato plants and B. tabaci in ways that increase the likelihood of ToCV transmission, in two clones, one susceptible ('Agata') and the other moderately resistant (Bach-4) to B. tabaci. Whiteflies alighted and laid more eggs on ToCV-infected plants than mock-inoculated plants of Bach-4. When non-viruliferous whiteflies were released on ToCV-infected plants near mock-inoculated plants, adults moved more intensely towards non-infected plants than in the reverse condition for both clones. Feeding on ToCV-infected plants reduced egg-incubation period in both clones, but the egg-adult cycle was similar for whiteflies fed on ToCV-infected and mock-inoculated plants. Our results demonstrated that ToCV infection in potato plants alters B. tabaci behaviour and development in distinct ways depending on the host clone, with potential implications for ToCV spread.

Tomato chlorosis virus, an emergent plant virus still expanding its geographical and host ranges

Tomato chlorosis virus (ToCV) causes an important disease that primarily affects tomato, although it has been found infecting other economically important vegetable crops and a wide range of wild plants. First described in Florida (USA) and associated with a 'yellow leaf disorder' in the mid-1990s, ToCV has been found in 35 countries and territories to date, constituting a paradigmatic example of an emergent plant pathogen. ToCV is transmitted semipersistently by whiteflies (Hemiptera: Aleyrodidae) belonging to the genera Bemisia and Trialeurodes. Whitefly transmission is highly efficient and cases of 100% infection are frequently observed in the field. To date, no resistant or tolerant tomato plants are commercially available and the control of the disease relies primarily on the control of the insect vector.

TAXONOMY

Tomato chlorosis virus is one of the 14 accepted species in the genus Crinivirus, one of the four genera in the family Closteroviridae of plant viruses.

VIRION AND GENOME PROPERTIES

The genome of ToCV is composed of two molecules of single-stranded positive-sense RNA, named RNA1 and RNA2, separately encapsidated in long, flexuous, rod-like virions. As has been shown for other closterovirids, ToCV virions are believed to have a bipolar structure. RNA1 contains four open reading frames (ORFs) encoding proteins associated with virus replication and suppression of gene silencing, whereas RNA2 contains nine ORFs encoding proteins putatively involved in encapsidation, cell-to-cell movement, gene silencing suppression and whitefly transmission.

HOST RANGE

In addition to tomato, ToCV has been found to infect 84 dicot plant species belonging to 25 botanical families, including economically important crops.

TRANSMISSION

Like all species within the genus Crinivirus, ToCV is semipersistently transmitted by whiteflies, being one of only two criniviruses transmitted by members of the genera Bemisia and Trialeurodes.

DISEASE SYMPTOMS

Tomato 'yellow leaf disorder' syndrome includes interveinal yellowing and thickening of leaves. Symptoms first develop on lower leaves and then advance towards the upper part of the plant. Bronzing and necrosis of the older leaves are accompanied by a decline in vigour and reduction in fruit yield. In other hosts the most common symptoms include interveinal chlorosis and mild yellowing on older leaves.

CONTROL

Control of the disease caused by ToCV is based on the use of healthy seedlings for transplanting, limiting accessibility of alternate host plants that can serve as virus reservoirs and the spraying of insecticides for vector control. Although several wild tomato species have been shown to contain genotypes resistant to ToCV, there are no commercially available resistant or tolerant tomato varieties to date.

Prevalence, epidemiology and molecular studies of Tomato chlorosis virus (ToCV) in South Africa

Criniviruses accumulate in the phloem tissue and damage crops by reducing chlorophyll which is essential for plant growth and development. Tomato chlorosis crinivirus (ToCV) is vectored by several whitefly species that damage tomato crops throughout the world. In South Africa, ToCV is a poorly studied pathogen of global economic importance. Therefore, a national survey was initiated to investigate the occurrence and distribution of criniviruses infecting tomato crops in South Africa. Whitefly infested tomato crops exhibiting interveinal leaf chlorosis and chlorotic flecking symptoms were assayed for crinivirus infections using a multiplex reverse transcription polymerase reaction (RT-PCR) approach to assess for the presence of crinivirus species that are known to infect solanaceous hosts. Next-generation sequencing (NGS) was used to generate the complete genome of ToCV from South Africa. Results from the survey indicated that ToCV is presently the only crinivirus species infecting tomatoes in South Africa. Blast analysis showed that the RNA-1 segment of ToCV from South Africa (ToCR1-186) matched 99% to Spanish isolates. On the other hand, the RNA-2 (ToCR2-186) segment matched 98% to a South Korean isolate and three Spanish isolates. Although recombination events were not detected, phylogenetic studies showed inconsistencies in the grouping of RNA-1 and RNA-2 segments for some of the ToCV isolates analyzed in this study. Therefore, we suggest the possibility of intraspecific reassortment. This is the first comprehensive study and full genome sequence of ToCV from South Africa. The information generated from this study is intended to raise awareness of ToCV infections on tomato crops in South Africa.

Expanding Knowledge of the Host Range of Tomato chlorosis virus and Host Plant Preference of Bemisia tabaci MEAM1

The crinivirus Tomato chlorosis virus (ToCV) is often found infecting tomato crops in Brazil, with variable incidence, but associated with prevalence of its primary vector, Bemisia tabaci MEAM1. ToCV control is difficult because there are no resistant commercial tomato varieties or hybrids available and chemical spray for control of the whitefly vector has not been effective. The present study evaluated the partial host range of a Brazilian isolate of ToCV and the preference of B. tabaci MEAM1 for oviposition on those species identified as susceptible to the virus. Subsequently, transmission tests were performed using plants of each ToCV host species as sources of inoculum to elucidate the epidemiological importance of nontomato sources of inoculum for infection of tomato. Among 80 species experimentally inoculated, 25 were susceptible, including 6 previously not known to be hosts (Jaltomata procumbens, Physalis pruinosa, Solanum aculeatissimum, S. viarum, Beta vulgaris var. cicla, and Chenopodium quinoa). Preference of whitefly for oviposition and infection by ToCV under free-choice transmission tests varied among the susceptible species. When ToCV-infected tomato, eggplant, and C. quinoa were used separately as sources of inoculum for virus transmission to tomato plants, mean percentages of infected plants were 76.6, 3, and 0%, respectively. Average oviposition of Bemisia tabaci on these three hosts were 2.7, 10.6, and 0.0 eggs/cm², respectively. Additional studies will be necessary to evaluate the importance of ToCV host plants under field conditions and their efficiency as sources of inoculum for virus acquisition and transmission to tomato crops.

Biochemical, Serological, Molecular and Natural Host Studies on Tomato Chlorosis Virus in Egypt

BACKGROUND AND OBJECTIVE

Tomato Chlorosis Virus (ToCV) is a white fly-transmitted and phloem-limited crinivirus reported in this study for the first time in Egypt. ToCV caused drastic reduction in tomato yield since 2013. The aim of this study is to characterize the virus incidence using biological, serological and molecular tools.

MATERIAL AND METHODS

The B. tabaci MEAM1 white fly was used for virus isolation and propagation. Identity of ToCV , its natural hosts were confirmed with RT-PCR using a specific primer pair for ToCV-heat shock protein 70 homologue (HSP70h) gene, sequencing and phylogenetic studies. ToCV was purified using the innovative electro-elution technique. The induced antiserum for the Egyptian isolate of the virus (ToCV-Giza) was used for DAS-ELISA and dot blotting immuno-assays to evaluate the virus presence in tomato and other natural hosts.

RESULTS

The ToCV-Giza isolate was donated an accession number "MH667315.1" from the GenBank. Blastx sequence analysis of the HSP70h gene indicated 97-99% of amino acid similarities with many tested ToCV isolates. Phylogenetic studies showed the clustering of all ToCV isolates including ToCV-Giza in a separate group from the other tested criniviruses. The virus had a UV spectrum of a nucleoprotein with Amax and Amin at 260 and 240 nm, respectively and A260/280 ratio of 1.33. Out of 52 different tested plant species within 22 families, 44 were positive hosts for ToCV. Thirty seven out of these 44 plant species were considered as new hosts for ToCV in the present study. These included Ammi majus and Coriandrum sativum (Apiaceae), cabbage (Brassicaceae), sweet potato (Convolvulaceae), melon, cucumber, luffa (Cucurbitaceae), soybean, cowpea, faba bean (Fabaceae), Egyptian and American Cotton (Malvaceae). Several ornamentals either herbal type or woody trees belonging to Acanthaceae, Amaranthaceae, Euophorbiaceae, Moraceae and Rubiaceae were also recognized for the first time as hosts for ToCV.

CONCLUSION

The obtained results confirmed the wide distribution of ToCV in its natural hosts in Egypt. Hygienic measures including control of the virus vector and removing of natural hosts should be strictly implicated.

Development of a graft inoculation method and a real-time RT-PCR assay for monitoring Tomato chlorosis virus infection in tomato

A graft inoculation method coupled with RT-qPCR was developed for monitoring ToCV infection in tomato plants. Ten seed-grown tomato seedlings were graft inoculated with phloem tissue-containing stem segments from a ToCV-infected tomato plants. Another group of tomato seedling were grafted with similar stem segments from a healthy tomato plant as mock inoculated control. The CP gene of ToCV was cloned under the control of T7 promoter and in vitro synthesized RNA was used as a standard for quantification. Total RNA was isolated from leaf samples of ToCV-inoculated and mock-inoculated control plants before the inoculation and 1-60 days post inoculation (dpi). The presence and the titer of ToCV were determined from all ToCV-inoculated or mock-inoculated control plants by RT-qPCR. After 15 dpi, ToCV was detected in 20-30% of graft-inoculated plants. The infection rate then increased progressively and reached to 70-80% by 60 dpi. Titer of ToCV was at the detectable level at 15 dpi and increased and reached to maximum level by 40 dpi and then started to decrease. The results showed that patch grafting is a simple and efficient method for experimental inoculation of ToCV and can be used as an alternative and/or complementary to vector transmission in the laboratories. The patch grafting could be combined with RT-qPCR and used for infecting and quantitative monitoring of ToCV or other phloem-limited viruses in tomato or in other plants.

Molecular Characterization and Detection of a Genetically Distinct Tomato Chlorosis Virus Strain in Taiwan

The whitefly-transmitted tomato chlorosis virus (ToCV) belonging to the genus Crinivirus (family Closteroviridae) affects tomato production worldwide. ToCV was first recorded in Taiwan in 1998 affecting tomato production. In this study, a local virus isolate XS was obtained, after serial whitefly transmissions from a diseased tomato plant displaying general chlorosis were collected in central Taiwan. The whole genome sequence of XS was determined from cDNA fragments amplified by reverse transcription (RT)-PCR, first using the degenerate primers for viruses of Closteroviridae and followed by degenerate and specific primers designed on available sequences of the ToCV isolates. The nucleotide (nt) sequences of RNA-1 and RNA-2 of the XS shared low identities of 77.8 to 78% and 78 to 78.1%, respectively, with genome segments of other ToCV isolates. Nevertheless, the viral RNA-dependent RNA polymerase (RdRp), heat shock protein 70 homolog (Hsp70h), and major capsid protein (CP) shared 88.3 to 96.2% amino acid (aa) identities with other ToCV isolates, indicating that XS is a new strain of this virus. Phylogenetic analyses of these three proteins indicated that all ToCV isolates from different counties outside Taiwan are closely related and clustered in the same clade, whereas the XS isolate is distinct and forms a unique branch. A one tube RT-PCR assay using primers designed from the genomic sequence of the XS was able to detect the ToCV-XS in infected tomato plants and in individual whiteflies. A field survey during 2013 to 2016 revealed a high ToCV-XS prevalence of 60.5% in 172 tested tomato samples, demonstrating that ToCV-XS is becoming an emerging threat for tomato production in Taiwan.

Transmission Efficiency, Preference and Behavior of Bemisia tabaci MEAM1 and MED under the Influence of Tomato Chlorosis Virus

Tomato chlorosis virus (ToCV, genus Crinivirus, family Closteroviridae) is an economically important virus in more than 20 countries. In China, ToCV was first detected in 2013 and has already spread throughout the country. ToCV is transmitted in a semi-persistent manner by the whitefly, Bemisia tabaci, but not seed. In the past two decades, the most invasive MEAM1 and MED have replaced the indigenous B. tabaci in China, and currently MED is the most dominant cryptic species. To better understand the prevalence of ToCV with their vectors, we tested the hypothesis that the rapid spread of ToCV in China is closely related to the dominance of MED. ToCV acquisition and accumulation rate following transmission was significantly higher by MED than MEAM1. In addition, ToCV persisted for more than 4 days in MED but only 2 days in MEAM1. Viruliferous MED preferred non-infected over virus-infected plants, although MED performed better on infected than on non-infected plants. Our combined results support the initial hypothesis that the rapid spread of ToCV is associated with the spread of B. tabaci MED in China.

Molecular dissection of distinct symptoms induced by tomato chlorosis virus and tomato yellow leaf curl virus based on comparative transcriptome analysis

The viral infection of plants may cause various physiological symptoms associated with the reprogramming of plant gene expression. However, the molecular mechanisms and associated genes underlying disease symptom development in plants infected with viruses are largely unknown. In this study, we employed RNA sequencing for in-depth molecular characterization of the transcriptional changes associated with the development of distinct symptoms induced by tomato chlorosis virus (ToCV) and tomato yellow leaf curl virus (TYLCV) in tomato. Comparative analysis of differentially expressed genes revealed that ToCV and TYLCV induced distinct transcriptional changes in tomato and resulted in the identification of important genes responsible for the development of symptoms of ToCV (i.e., chlorosis and anthocyanin accumulation) and TYLCV (i.e., yellowing, stunted growth, and leaf curl). Our comprehensive transcriptome analysis can provide molecular strategies to reduce the severity of disease symptoms as well as new insights for the development of virus-resistant crops.

Detection and epidemic dynamic of ToCV and CCYV with Bemisia tabaci and weed in Hainan of China

BACKGROUND

In recent years, two of the crinivirus, Tomato chlorosis virus (ToCV) and Cucurbit chlorotic yellows virus (CCYV) have gained increasing attention due to their rapid spread and devastating impacts on vegetable production worldwide. Both of these viruses are transmitted by the sweet potato whitefly, Bemisia tabaci (Gennadius), in a semi-persistent manner. Up to now, there is still lack of report in Hainan, the south of China.

METHODS

We used observational and experimental methods to explore the prevalence and incidence dynamic of CCYV and ToCV transmitted by whiteflies in Hainan of China.

RESULTS

In 2016, the chlorosis symptom was observed in the tomato and cucumber plants with a large number of B. tabaci on the infected leaves in Hainan, China, with the incidence rate of 69.8% and 62.6% on tomato and cucumber, respectively. Based on molecular identification, Q biotype was determined with a viruliferous rate of 65.0% and 55.0% on the tomato and cucumber plants, respectively. The weed, Alternanthera philoxeroides near the tomato and cucumber was co-infected by the two viruses. Furthermore, incidence dynamic of ToCV and CCYV showed a close relationship with the weed, Alternanthera philoxeroides, which is widely distributed in Hainan.

CONCLUSION

Our results firstly reveal that the weed, A. philoxeroides is infected by both ToCV and CCYV. Besides, whiteflies showed a high viruliferous rate of ToCV and CCYV. Hainan is an extremely important vegetable production and seed breeding center in China. If the whitefly can carry these two viruses concurrently, co-infection in their mutual host plants can lead to devastating losses in the near future.

Application of Genomics for Understanding Plant Virus-Insect Vector Interactions and Insect Vector Control

The relationships between plant viruses and their vectors have evolved over the millennia, and yet, studies on viruses began <150 years ago and investigations into the virus and vector interactions even more recently. The advent of next generation sequencing, including rapid genome and transcriptome analysis, methods for evaluation of small RNAs, and the related disciplines of proteomics and metabolomics offer a significant shift in the ability to elucidate molecular mechanisms involved in virus infection and transmission by insect vectors. Genomic technologies offer an unprecedented opportunity to examine the response of insect vectors to the presence of ingested viruses through gene expression changes and altered biochemical pathways. This review focuses on the interactions between viruses and their whitefly or thrips vectors and on potential applications of genomics-driven control of the insect vectors. Recent studies have evaluated gene expression in vectors during feeding on plants infected with begomoviruses, criniviruses, and tospoviruses, which exhibit very different types of virus-vector interactions. These studies demonstrate the advantages of genomics and the potential complementary studies that rapidly advance our understanding of the biology of virus transmission by insect vectors and offer additional opportunities to design novel genetic strategies to manage insect vectors and the viruses they transmit.

Transmission of Tomato chlorosis virus (ToCV) by Bemisia tabaci Biotype Q and Evaluation of Four Weed Species as Viral Sources

Tomato chlorosis virus (ToCV) is implicated in tomato yellows disease in many countries worldwide. It has a wide host range, including cultivated species as well as arable weeds, and it is transmitted in a semipersistent manner by at least five whitefly species or biotypes of the genera Trialeurodes and Bemisia. ToCV is not seed transmitted and more than 36 weed species have been recorded as natural reservoirs, acting as unique sources both for the virus and its vectors when susceptible crops are harvested. In this study, experiments were conducted to determine the transmission parameters of ToCV by biotype Q, the most abundant biotype of Bemisia tabaci in Greece. Results showed that biotype Q is an efficient vector of ToCV and it is able to retain the virus for at least 6 days. This vector was then used for the evaluation of four widespread weed species (Solanum nigrum, Sonchus oleraceus, Amaranthus retroflexus, and Chenopodium album) as ToCV sources through transmission experiments. Solanum nigrum was shown to be the most significant viral source among the tested weeds, followed by Sonchus oleraceus, A. retroflexus, and, lastly, C. album. Nevertheless, none of them was as efficient a ToCV source as tomato. This variation could be attributed to differences in virus concentration in each plant species or possible host preference by the whitefly vector.

A Primary Screening and Applying of Plant Volatiles as Repellents to Control Whitefly Bemisia tabaci (Gennadius) on Tomato

With the goal of finding a new way to reduce population densities of Bemisia tabaci biotype Q in greenhouses, seven repellent volatile chemicals and their combinations were screened. The mixture of DLCO (D-limonene, citral and olive oil (63:7:30)) had a better cost performance(SC50 = 22.59 mg/ml)to repel whiteflies from settling than the other mixtures or single chemicals. In the greenhouse, in both the choice test and the no-choice tests, the number of adult whiteflies that settled on 1% DLCO-treated tomato plants was significantly lower than those settling on the control plants for the different exposure periods (P < 0.01). In the choice test, the egg amount on the treated tomato plants was significantly lower (P < 0.01) than that on the control plants, but there was no significant difference (P > 0.05) between the number of eggs on treated and control plants in the no-choice test. Compared with the controls, 1% DLCO did not cause significantly statistic mortality rates (P > 0.05) out of different living stages of B. tabaci. The tests for evaluating the repellent efficacy, showed that a slow-releasing bottle containing the mixture had a period of efficacy of 29 days, and the application of this mixture plus a yellow board used as a push-pull strategy in the greenhouse was also effective.

Reverse transcription loop-mediated isothermal amplification assay for detecting tomato chlorosis virus

A betaine-free reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed and optimised for detecting tomato chlorosis virus (ToCV), one of the most important viruses that infect tomato crops worldwide. A set of four specific primers was designed against the RNA-dependent RNA polymerase (RdRp) gene. The betaine-free RT-LAMP procedure could be completed within 40 min under isothermal conditions at 60 °C without a thermal cycler, and no cross-reactivity was seen with other tomato viral pathogens. Sensitivity analysis showed that RT-LAMP could detect viral dilutions up to 2.0×10(-7)ng, which is 100-times more sensitive than reverse transcription-polymerase chain reaction (RT-PCR). In addition, naked-eye observation after staining in-tube RT-LAMP products with SYBR Green I facilitated detection of ToCV by avoiding the requirement for ethidium staining following gel electrophoresis. These results suggest that ToCV RT-LAMP is a rapid, sensitive, and affordable diagnostic tool that is more suitable than RT-PCR for the detection and surveillance of ToCV in field samples.

Transmission specificities of plant viruses with the newly identified species of the Bemisia tabaci species complex

Bemisia tabaci has had a colorful nomenclatural past and is now recognized as a species complex. This new species framework has added many new areas of research including adding new insight into the virus transmission specificity of the species in the B. tabaci species complex. There is a wide disparity in what is known about the transmission of plant viruses by different members of the B. tabaci species complex. In this paper, we have synthesized the transmission specificities of the plant viruses transmitted by species belonging to the complex. There are five genera of plant viruses with members that are transmitted by species of the B. tabaci species complex. The transmission of viruses belonging to two of these, Begomovirus and Crinivirus, are well studied and much is known in regards to the relationship between species and transmission and etiology. This is in contrast to viruses of the genera, Torradovirus and Carlavirus, for which very little is known inregards to their transmission. This is the first attempt to integrate viral data within the new B. tabaci species complex framework. It is clear that matching historical transmission data with the current species framework is difficult due to the lack of awareness of the underlying genetic diversity within B. tabaci. We encourage all researchers to determine which species of B. tabaci they are using to facilitate association of phenotypic traits with particular members of the complex.

Major tomato viruses in the Mediterranean basin

Tomato (Solanum lycopersicum L.) originated in South America and was brought to Europe by the Spaniards in the sixteenth century following their colonization of Mexico. From Europe, tomato was introduced to North America in the eighteenth century. Tomato plants show a wide climatic tolerance and are grown in both tropical and temperate regions around the world. The climatic conditions in the Mediterranean basin favor tomato cultivation, where it is traditionally produced as an open-field plant. However, viral diseases are responsible for heavy yield losses and are one of the reasons that tomato production has shifted to greenhouses. The major tomato viruses endemic to the Mediterranean basin are described in this chapter. These viruses include Tomato yellow leaf curl virus, Tomato torrado virus, Tomato spotted wilt virus, Tomato infectious chlorosis virus, Tomato chlorosis virus, Pepino mosaic virus, and a few minor viruses as well.

Emerging virus diseases transmitted by whiteflies

Virus diseases that have emerged in the past two decades limit the production of important vegetable crops in tropical, subtropical, and temperate regions worldwide, and many of the causal viruses are transmitted by whiteflies (order Hemiptera, family Aleyrodidae). Most of these whitefly-transmitted viruses are begomoviruses (family Geminiviridae), although whiteflies are also vectors of criniviruses, ipomoviruses, torradoviruses, and some carlaviruses. Factors driving the emergence and establishment of whitefly-transmitted diseases include genetic changes in the virus through mutation and recombination, changes in the vector populations coupled with polyphagy of the main vector, Bemisia tabaci, and long distance traffic of plant material or vector insects due to trade of vegetables and ornamental plants. The role of humans in increasing the emergence of virus diseases is obvious, and the effect that climate change may have in the future is unclear.

Methods for detection and differentiation of existing and new crinivirus species through multiplex and degenerate primer RT-PCR

A method was developed for rapid identification and differentiation of both known and novel crinivirus species involving both multiplex and degenerate reverse transcription-polymerase chain reaction (RT-PCR). The multiplex method can discriminate among known criniviruses infecting vegetable and small fruit crops, and rapidly identify viruses associated with disease symptoms, as well as identification of mixed crinivirus infections. Four host groups for multiplex detection of criniviruses were selected based on the types of crops where specific criniviruses would be expected to occur. Each detection group contained three to four crop-specific primers designed to the same region of the gene encoding the highly conserved RNA-dependent RNA polymerase gene (RdRp) of criniviruses for rapid, single-reaction determination of which crinivirus(es) may be infecting a plant. Degenerate reverse primers used for RT and in PCR were designed to amplify all members of each host group, and were coupled with species-specific forward primers resulting in four separate single-reaction cocktails for detection of most criniviruses sequenced to date, whether present in single or mixed virus infections. Additional viruses can be added to multiplex detection by adjustment of primer concentration for balanced detection of target viruses. In order to identify unknown putative criniviruses or those for which sequence information is not yet available, a genus-wide, universal degenerate primer set was developed. These primers also targeted the crinivirus RdRp gene, and amplify a wide range of crinivirus sequences. Both detection systems can be used with most RNA extraction methods, and with RT-PCR reagents common in most laboratories.

Resistance to Tomato chlorosis virus in wild tomato species that impair virus accumulation and disease symptom expression

Tomato chlorosis virus (ToCV) (genus Crinivirus, family Closteroviridae) is an emerging threat to tomato crops worldwide. Although symptoms on fruits are not obvious, yield losses occur through decreased fruit size and number. Control of ToCV epidemics is difficult because the virus is transmitted by several whitefly vector species and its relatively wide host range facilitates establishment in local wild reservoirs. Therefore, breeding for ToCV resistance offers the best control alternative. However, no sources for resistance are available thus far. Here, a screen of tomatoes and wild species relatives was performed in search of ToCV resistance. Two sources of resistance to ToCV were identified in this work, lines '802-11-1' and '821-13-1', each derived by two self-pollinations from ToCV asymptomatic plants of the population 'IAC CN RT' (derived from an interspecific hybrid Solanum lycopersicum x S. peruvianum accession LA0444) and accession LA1028 (S. chmielewskii), respectively. The resistance was expressed by impairing virus accumulation and disease symptom expression, both under natural infection and after challenging with ToCV in controlled inoculations. Genetic control of resistance to ToCV infection in '821-13-1' was conferred by a major locus with mainly additive effects but also partial dominance for higher susceptibility. Also, an additive x dominance epistatic interaction with at least one additional gene was evident.

Emerging viral diseases of tomato crops

Viral diseases are an important limiting factor in many crop production systems. Because antiviral products are not available, control strategies rely on genetic resistance or hygienic measures to prevent viral diseases, or on eradication of diseased crops to control such diseases. Increasing international travel and trade of plant materials enhances the risk of introducing new viruses and their vectors into production systems. In addition, changing climate conditions can contribute to a successful spread of newly introduced viruses or their vectors and establishment of these organisms in areas that were previously unfavorable. Tomato is economically the most important vegetable crop worldwide and many viruses infecting tomato have been described, while new viral diseases keep emerging. Pepino mosaic virus is a rapidly emerging virus which has established itself as one of the most important viral diseases in tomato production worldwide over recent years. Begomovirus species and other whitefly-transmitted viruses are invading into new areas, and several recently described new viruses such as Tomato torrado virus and new Tospovirus species are rapidly spreading over large geographic areas. In this article, emerging viruses of tomato crops are discussed.

Rapid identification of viruses causing sugarcane mosaic by direct sequencing of RT-PCR products from crude extracts: a method for large scale virus surveys

Sugarcane mosaic virus (SCMV) and sorghum mosaic virus (SrMV) diversity studies are important to characterize virus populations in sugarcane producing areas, enabling (i) identification of shifts in predominant strains, (ii) detecting associations of strains with specific varieties, and (iii) possibly exposing the appearance of new strains which may affect the performance of varieties in a region. Recent studies have shown significant sequence variability within SCMV populations around the world, indicating that isolate identification would be best achieved by direct analysis of sequence data. Because virus sequence-based studies that require the characterization of large numbers of isolates may be impractical using standard sample preparation and processing methodology, a simple protocol that yields quality sequence information, requiring neither viral RNA purification nor cloning of RT-PCR products was developed. Rapid virus release extracts are obtained by submerging a portion of leaf tissue into an extraction buffer, followed by a brief incubation at 95 degrees C. An aliquot of the extract is pipetted into an RT-PCR amplification mix for the detection of SCMV and the SrMV coat protein gene fragments. RT-PCR fragments are sequenced directly using oligonucleotide primers similar to the RT-PCR primers, yielding sequence information of an adequate quality. This rapid, cost effective protocol is practical for large scale virus diversity and evolutionary studies.

Co-infection by two criniviruses alters accumulation of each virus in a host-specific manner and influences efficiency of virus transmission

Tomato chlorosis virus (ToCV), and Tomato infectious chlorosis virus (TICV), family Closteroviridae, genus Crinivirus, cause interveinal chlorosis, leaf brittleness, and limited necrotic flecking or bronzing on tomato leaves. Both viruses cause a decline in plant vigor and reduce fruit yield, and are emerging as serious production problems for field and greenhouse tomato growers in many parts of the world. The viruses have been found together in tomato, indicating that infection by one Crinivirus sp. does not prevent infection by a second. Transmission efficiency and virus persistence in the vector varies significantly among the four different whitefly vectors of ToCV; Bemisia tabaci biotypes A and B, Trialeurodes abutilonea, and T. vaporariorum. Only T. vaporariorum can transmit TICV. In order to elucidate the effects of co-infection on Crinivirus sp. accumulation and transmission efficiency, we established Physalis wrightii and Nicotiana benthamiana source plants, containing either TICV or ToCV alone or both viruses together. Vectors were allowed to feed separately on all virus sources, as well as virus-free plants, then were transferred to young plants of both host species. Plants were tested by quantitative reverse-transcription polymerase chain reaction, and results indicated host-specific differences in accumulation by TICV and ToCV and alteration of accumulation patterns during co-infection compared with single infection. In N. benthamiana, TICV titers increased during co-infection compared with levels in single infection, while ToCV titers decreased. However, in P. wrightii, titers of both TICV and ToCV decreased during mixed infection compared with single infection, although to different degrees. Vector transmission efficiency of both viruses corresponded with virus concentration in the host in both single and mixed infections. This illustrates that Crinivirus epidemiology is impacted not only by vector transmission specificity and incidence of hosts but also by interactions between viruses and efficiency of accumulation in host plants.

Synergistic Interaction Between Tomato chlorosis virus and Tomato spotted wilt virus Results in Breakdown of Resistance in Tomato

ABSTRACT Multiple viral infections frequently are found in single plants of cultivated and wild hosts in nature, with unpredictable pathological consequences. Synergistic reactions were observed in mixed infections in tomato plants doubly infected with the positive-sense and phloem-limited single-stranded RNA (ssRNA) crinivirus Tomato chlorosis virus (ToCV) and the negative-sense ssRNA tospovirus Tomato spotted wilt virus (TSWV). Synergism in a tomato cultivar susceptible to both viruses resulted in a rapid death of plants. A pronounced enhancement of ToCV accumulation mediated by TSWV co-infection was observed with no evident egress of ToCV from phloem tissues. No consistent alteration of TSWV accumulation was detected. More remarkable was the synergism observed in tomato cultivars which carry the Sw-5 resistance gene, which are resistant to TSWV. Pre-infection with ToCV resulted in susceptibility to TSWV, whereas co-inoculations did not. This suggested that a threshold level or a time lapse is needed for ToCV to interfere or downregulate the defense response in the TSWV-resistant plants.

Vector Specificity, Host Range, and Genetic Diversity of Tomato chlorosis virus

Tomato chlorosis virus (ToCV), family Closteroviridae, genus Crinivirus, causes interveinal chlorosis, leaf brittleness, and limited necrotic flecking or leaf bronzing on tomato leaves. ToCV can cause a decline in plant vigor and reduce fruit yield. It is emerging as a serious production problem for field and greenhouse tomato growers, and has been increasing in prevalence in many parts of the world. The virus is unique among known whitefly-transmitted viruses, due to its ability to be transmitted by four whitefly vectors from two genera. Studies demonstrated that transmission efficiency and virus persistence in the vector varies significantly among the different whitefly vectors. Trialeurodes abutilonea and Bemisia tabaci biotype B are highly efficient vectors of ToCV. B. tabaci biotype A and T. vaporariorum are less efficient vectors, but are fully capable of transmission. ToCV persists for up to 5 days in T. abutilonea, 2 days in B. tabaci biotype B, and only 1 day in B. tabaci biotype A and T. vaporariorum. ToCV has a moderately wide host range, infecting 24 host plant species in seven families. A portion of the coat protein coding region of five geographically diverse ToCV isolates was compared and found to be highly conserved. This information, coupled with existing information on conservation within the heat shock protein 70 homologue coding region, suggests that many ToCV isolates throughout the world are related very closely, and may have been distributed on plant material.

Complete nucleotide sequence of the RNA2 of the crinivirus tomato chlorosis virus

The complete sequence of genomic RNA2 of Tomato chlorosis virus (ToCV; genus Crinivirus, family Closteroviridae), isolate AT80/99 from Spain, was determined and compared with those from the other members of the genus sequenced to date. RNA2 is 8244 nucleotides (nt) long and putatively encodes nine ORFs that encompass the hallmark gene array of the family Closteroviridae, which includes a heat shock protein 70 family homologue, a 59 kDa protein, the coat protein, and a diverged coat protein. Phylogenetic analysis confirmed assignment of ToCV in the genus Crinivirus, being most similar to sweet potato chlorotic stunt virus and cucurbit yellow stunting disorder virus.

Multiplex Detection of Criniviruses Associated with Epidemics of a Yellowing Disease of Tomato in Greece

Since 1997, a yellowing disease has been observed in greenhouse tomato (Lycopersicon escu-lentum). By 2001, the disease was widespread, including open field tomato crops, and in most cases its incidence was 80 to 90% or even 100%. Epidemics in glasshouses were mainly associated with high populations of the whitefly Trialeurodes vaporariorum and Bemisia tabaci, the major whitefly pests in vegetable crops in Greece. The main leaf symptoms were severe yellowing, rolling, and brittleness. Samples from symptomatic plants were analyzed by polymerase chain reaction (PCR) and shown to be infected with Tomato infectious chlorosis virus (TICV) and Tomato chlorosis virus (ToCV) (family Closteroviridae, genus Crinivirus). TICV was found in 164 of 183 symptomatic samples, while ToCV was less representative (25/183). Sequence comparisons of the amplified 229-bp and 466-bp products revealed 99 and 100% identity with the reported sequences of TICV and ToCV, respectively. A reverse transcription (RT) multiplex PCR assay using a simple sample preparation procedure was developed to allow rapid, specific, and simultaneous detection of both ToCV and TICV sequences in two steps. The method involves a one-tube RT-PCR step in which the combination of primers amplifies part of the heat shock protein to homologue gene of both ToCV and TICV, followed by a multiplex nested PCR amplification. This is the first report of TICV and ToCV in Greece and, as far as we know, the first report of TICV in Europe.