In 2009, some commercially grown tomato (Solanum lycopersicum) plants in Chiba Prefecture, Japan, exhibited mosaic symptoms. Ten plants from a total of about 72,000 cultivated plants in the greenhouses showed such symptoms. To identify the causal agent, sap from leaves of the diseased plants was inoculated into Chenopodium quinoa and Nicotiana benthamiana plants. Local necrotic lesions appeared on inoculated leaves of C. quinoa, but no systemic infection was observed. Systemic mosaic symptoms were observed on the N. benthamiana plants inoculated. Single local lesion isolation was performed three times using C. quinoa to obtain a reference isolate for further characterization. N. benthamiana was used for propagation of the isolate. Sap from infected leaves of N. benthamiana was mechanically inoculated into three individual S. lycopersicum cv. Momotaro. Symptoms appearing on inoculated tomatoes were indistinguishable from those of diseased tomato plants found initially in the greenhouse. Flexuous, filamentous particles, ~750 nm long, were observed by electron microscopy in the sap of the tomato plants inoculated with the isolate, indicating that the infecting virus may belong to the family Potyviridae. To determine genomic sequence of the virus, RT-PCR was performed. Total RNA was extracted from the tomato leaves experimentally infected with the isolate using an RNeasy Plant Mini kit (QIAGEN, Hilden, Germany). RT-PCR was performed by using a set of universal, degenerate primers for Potyviruses as previously reported (2). Amplicons (~1,500 bp) generated by RT-PCR were extracted from the gels using the QIAquick Gel Extraction kit (QIAGEN) and cloned into pCR-BluntII TOPO (Invitrogen, San Diego, CA). DNA sequences of three individual clones were determined using a combination of plasmid and virus-specific primers, showing that identity among three clones was 99.8%. A consensus nucleotide sequence of the isolate was deposited in GenBank (AB823816). BLASTn analysis of the nucleotide sequence determined showed 99% identity with a partial sequence in the NIb/coat protein (CP) region of Colombian datura virus (CDV) tobacco isolate (JQ801448). Comparison of the amino acid sequence predicted for the CP with previously reported sequences for CDV (AY621656, AJ237923, EU571230, AM113759, AM113754, and AM113761) showed 97 to 100% identity range. Subsequently, CDV infection in both the original and experimentally inoculated plants was confirmed by RT-PCR using CDV-specific primers (CDVv and CDVvc; ), and, hence, the causal agent of the tomato disease observed in greenhouse tomatoes was proved to be CDV. The first case of CDV on tomato was reported in Netherlands (3), indicating that CDV was transmitted by aphids from CDV-infected Brugmansia plants cultivated in the same greenhouse. We carefully investigated whether Brugmansia plants naturally grew around the greenhouses, but we could not find them inside or in proximity to the greenhouses. Therefore, sources of CDV inoculum in Japan are still unclear. This is the first report of a mosaic disease caused by CDV on commercially cultivated S. lycopersicum in Japan.
References: (1) D. O. Chellemi et al. Plant Dis. 95:755, 2011. (2) J. Chen et al. Arch. Virol. 146:757, 2001. (3) J. Th. J. Verhoeven et al. Eur. J. Plant. Pathol. 102:895, 1996.