We tested for host-associated genetic differentiation in 22 populations of Thrips tabaci collected from tobacco and leek, respectively. Clustering analyses and haplotype networks based on sequence variation at a fragment of the mitochondrial cytochrome oxidase I gene yielded three major evolutionary lineages; two were clearly associated with leek and the third with tobacco. These genetic findings corroborated recent experimental observations on the heterogeneity of T. tabaci populations with regard to host-plant preference and their capacity to be vectors for tomato spotted wilt virus.Estimated divergence times suggested an ancient divergence of these lineages dating back to the Miocene 28-21 million years ago. F ST values between these lineages ranged between 0.824 and 0.954 (Po0.001 for all comparisons), and sequence divergences ranged between 4 and 11%. Given these findings and by the standards of genetic and ecological differentiation in other published species groups, T. tabaci must be considered a complex of cryptic (sub)species.

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.

Perennial crops, such as fruit trees, are infected by many viruses, which are transmitted through vegetative propagation and grafting of infected plant material. Some of these pathogens cause severe crop losses and often reduce the productive life of the orchards. Detection and characterization of these agents in fruit trees is challenging, however, during the last years, the wide application of high-throughput sequencing (HTS) technologies has significantly facilitated this task. In this review, we present recent advances in the discovery, detection, and characterization of fruit tree viruses and virus-like agents accomplished by HTS approaches. A high number of new viruses have been described in the last 5 years, some of them exhibiting novel genomic features that have led to the proposal of the creation of new genera, and the revision of the current virus taxonomy status. Interestingly, several of the newly identified viruses belong to virus genera previously unknown to infect fruit tree species (e.g., Fabavirus, Luteovirus) a fact that challenges our perspective of plant viruses in general. Finally, applied methodologies, including the use of different molecules as templates, as well as advantages and disadvantages and future directions of HTS in fruit tree virology are discussed.

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Arrhenotokous and thelytokous populations of Thrips tabaci from tobacco or leek plants were evaluated for their ability to transmit Tomato spotted wilt virus (TSWV) and for their host preference. Transmission efficiencies were comparatively studied using leaf disks of Petunia hybrida, Datura stramonium, and Nicotiana tabacum cv. Basmas. Adults of arrhenotokous populations collected on infected tobacco plants in the field were efficient transmitters (up to 48.5% transmission) and remained so when maintained on tobacco for several generations. Arrhenotokous T. tabacipopulations from leek plants were poor transmitters (up to 3.1% transmission), whereas no transmission was obtained with thelytokous populations from leek. All populations could infest leek, however none of the arrhenotokous and thelytokous populations from leek plants was able to infest tobacco. TSWV could be acquired by both first and second larval instars of a T. tabacipopulation from tobacco. However, the transmission by adults decreased with the age at which the virus was acquired by larvae. The highest efficiencies (61% of males and 51% of females transmitted) were obtained when newborn (0- to 24-h old) larvae acquired the virus. The majority of thrips started to transmit after becoming adult and rates were positively correlated with the temperature at which the thrips were kept. The median latent period values found for adults decreased with increasing temperature. The median acquisition access period (AAP50) of the population was 41 min, whereas the AAP(50) was 65 min for males and 35 min for females. The median inoculation access period of males was 246 and 365 min on tobacco and petunia, respectively, and 96 and 345 min for females. The results show that T. tabaci forms a complex in terms of host preference, reproductive strategy, and ability to transmit TSWV. The transmission parameters show that the thrips of arrhenotokous populations infesting tobacco are highly efficient vectors.

Plant viruses have important global impacts on crops, and identifying their centre and date of emergence is important for planning control measures. Turnip mosaic virus (TuMV) is a member of the genus Potyvirus in the family Potyviridae and is a major worldwide pathogen of brassica crops. For two decades, we have collected TuMV isolates, mostly from brassicas, in Turkey and neighbouring countries. This region is thought to be the centre of emergence of this virus. We determined the genomic sequences of 179 of these isolates and used these to estimate the timescale of the spread of this virus. Our Bayesian coalescent analyses used synonymous sites from a total of 417 novel and published whole-genome sequences. We conclude that TuMV probably originated from a virus of wild orchids in Germany and, while adapting to wild and domestic brassicas, spread via Southern Europe to Asia Minor no more than 700 years ago. The population of basal-B group TuMVs in Asia Minor is older than all other populations of this virus, including a newly discovered population in Iran. The timescale of the spread of TuMV correlates well with the establishment of agriculture in these countries.

This chapter reviews the transmission of plant viruses by aphids, with particular emphasis on the virus-vector interactions, the attributes that make aphids such well-adapted vectors, the different modes of transmission, the determinants of transmissibility, the factors affecting virus acquisition and inoculation, control methods (e.g. use of virus-resistant or aphid-resistant genotypes, elimination of virus sources, and eradication of weeds, infected plants and volunteer plants), and prevention or reduction of virus spread by plant spacing and chemical, biological, cultural and integrated control of vectors among others.

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.