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.

In 2011, an outbreak of a yellowing disease causing chlorosis and Interveinal chlorotic spots on lower leaves was observed in cucumber (Cucumis sativus) and melon (C. melo) plants in two greenhouses on the island of Rhodes, Greece. Similar symptoms were observed in 2012 in open field watermelon (Citrullus lanatus) plants in Rhodes and in November 2013 in a cucumber greenhouse in Tympaki, Crete. Disease incidence ranged from 10 to 40%. The observed symptoms were similar to those caused by whitefly transmitted criniviruses (family Closteroviridae) Cucurbit yellow stunting disorder virus (CYSDV) and Beet pseudo-yellows virus (BPYV), as well as Cucurbit chlorotic yellows virus (CCYV), a recently described crinivirus that infects cucurbits in Japan (4) and by the aphid transmitted polerovirus (family Luteoviridae) Cucurbit aphid-borne yellows virus (CABYV). Dense populations of whiteflies were present in all the affected crops. Leaf samples from cucumber (10 from Rhodes and 10 from Crete), melon (10), and watermelon (10) were collected and tested for the presence of the above viruses. Total RNA was extracted from the samples (2) and detection of BPYV, CYSDV, and CABYV was done as previously described (1,3) whereas detection of CCYV was conducted by herein developed two-step RT-PCR assays. Two new pairs of primers, ‘CC-HSP-up’ (5′-GAAGAGATGGGTTGGTGTAGATAAA-3′)/‘CC-HSP-do’ (5′-CACACCGATTTCATAAACATCCTTT-3′) and ‘CC-RdRp-up’ (5′-CCTAATATTGGAGCTTATGAGTACA-3′)/‘CC-RdRp-do’ (5′-CATACACTTTAAACACAACCCC-3′) were designed based on GenBank deposited sequences of CCYV for the amplification of two regions partially covering the heat shock protein 70 homologue (HSP70h) (226 bp) and the RNA dependent RNA polymerase (RdRp) genes (709 bp). Interestingly, CCYV was detected in all samples tested, while CYSDV was detected in 18 cucumbers (10 from Rhodes and 8 from Crete), 1 melon, and 3 watermelon plants. Neither BPYV nor CABYV were detected. In order to verify the presence of CCYV, the partial HSP70h and RdRp regions of a cucumber isolate from Crete were directly sequenced using the primers ‘CC-HSP-up’/‘CC-HSP-do’ and ‘CC-RdRp-up’/‘CC-RdRp-do’. BLAST analysis of the obtained sequences (HG939521 and 22) showed 99% and 100% identities with the HSP70h and RdRp of cucumber CCYV isolates from Lebanon, respectively (KC990511 and 22). Also, the partial HSP70h sequence of a watermelon CCYV isolate from Rhodes showed 99% identity with the cucumber isolate from Crete. Whitefly transmission of CCYV was also carried out by using an infected cucumber from Crete as virus source. Four groups of 30 whitefly adults of Bemisia tabaci biotype Q were given an acquisition and inoculation access time of 48 and 72 h, respectively. Each whitefly group was transferred to a healthy cucumber plant (hybrid Galeon). Two weeks post inoculation, the plants, which have already been showing mild interveinal chlorosis, were tested for virus presence by RT-PCR. CCYV was successfully transmitted in three of four inoculated cucumbers, which was further confirmed by sequencing. In Greece, cucurbit yellowing disease has occurred since the 1990s, with CYSDV, BPYV, and CABYV as causal agents. To our knowledge, this is the first report of CCYV infecting cucurbits in Greece; therefore, our finding supports the notion that the virus is spreading in the Mediterranean basin and is an important pathogen in cucurbit crops. References: (1) I. N. Boubourakas et al. Plant Pathol. 55:276, 2006. (2) E. Chatzinasiou et al. J. Virol. Methods 169:305, 2010. (3) L. Lotos et al. J. Virol. Methods 198:1, 2014. (4) M. Okuda et al. Phytopathology 100:560, 2010.

Tomato chlorosis virus (ToCV) is a serious, emerging tomato pathogen that has a significant impact on the quality and quantity of tomato production worldwide. Detecting ToCV via means of spectral measurements in an early pre-symptomatic stage offers an alternative to the existing laboratory methods, leading to better disease management in the field. In this study, leaf spectra from healthy and diseased leaves were measured with a spectrometer. The diseased leaves were subjected to RT-qPCR for the detection and quantification of the titer of ToCV. Neighborhood component analysis (NCA) algorithm was employed for the feature selection of the effective wavelengths and the most important vegetation indices out of the 24 that were tested. Two machine learning methods, namely XY-fusion network (XY-F) and multilayer perceptron with automated relevance determination (MLP–ARD), were employed for the estimation of the disease existence and viral load in the tomato leaves. The results showed that before outlier elimination, the MLP–ARD classifier generally outperformed the XY-F network with an overall accuracy of 92.1% against 88.3% for the XY-F. Outlier elimination contributed to the performance of the classifiers as the overall accuracy for both XY-F and MLP–ARD reached 100%.

The study of an emerging yellows disease of pepper crops (pepper yellows disease [PYD]) in Greece led to the identification of a polerovirus closely related to Pepper vein yellows virus (PeVYV). Recovery of its full genome sequence by next-generation sequencing of small interfering RNAs allowed its characterization as a new poleroviruses, which was provisionally named Pepper yellows virus (PeYV). Transmission experiments revealed its association with the disease. Sequence similarity and phylogenetic analysis highlighted the common ancestry of the three poleroviruses (PeVYV, PeYV, and Pepper yellow leaf curl virus [PYLCV]) currently reported to be associated with PYD, even though significant genetic differences were identified among them, especially in the C-terminal region of P5 and the 3′ noncoding region. Most of the differences observed can be attributed to a modular type of evolution, which produces mosaic-like variants giving rise to these different poleroviruses Overall, similar to other polerovirus-related diseases, PYD is caused by at least three species (PeVYV, PeYV, and PYLCV) belonging to this group of closely related pepper-infecting viruses.