Partial Characterization of a Carla-Like Virus Infecting Yam (Dioscorea spp.) from China.

Dioscorea opposita (yam) from China was tested for viruses during post-entry quarantine in New Zealand during 2004. No obvious symptoms or virus particles were observed from yam. Mechanically inoculated Nicotiana occidentalis cvs. 37B and P1 produced systemic chlorosis, leaf reduction, and stunting, whereas no symptoms were observed on other tested herbaceous plants (Chenopodium amaranticolor, C. quinoa, Cucumis sativum, Gomphrena globosa, N. benthamiana, N. clevelandii, N. glutinosa, N. sylvestris, and N. tabacum cv. White Burley). Numerous filamentous particles (approximately 600 nm long) were observed by using electron microscopy from symptomatic N. occidentalis. Total RNA was extracted from yam and symptomatic N. occidentalis leaf samples using the Qiagen Plant RNeasy kit (Qiagen, Valencia, CA). Reverse transcription (RT) was carried out using random hexamer primers and SuperScript II RNase H¯ reverse transcriptase (Invitrogen, Carlsbad, CA) followed by polymerase chain reaction (PCR) with different primer pairs. Samples tested negative for Chinese yam necrotic mosaic virus (ChYNMV; genus Macluravirus) with specific primers (supplied by T. Kondo, Aomori Green BioCenter, Aomori, Japan). Negative results were also obtained for the genera Potyvirus, Potexvirus, Capillovirus, Trichovirus, and Foveavirus using RT-PCR with broad detection primers (1,2,4). A positive RT-PCR amplification was obtained from the yam and N. occidentalis samples with universal primers for the genus Carlavirus (Agdia Inc., Elkhart, IN). The 275-bp amplified products from the viral replicase were cloned and sequenced. The yam virus shows a high amino acid similarity with Hop latent virus (87.9%), Aconitum latent virus (86.8%) and Potato virus M (86.8%). Filamentous virus particles belonging to the genera Macluravirus, Potyvirus, and Potexvirus have been reported in yam (3). These virus species are not associated with the carlavirus infection since the virus found in D. opposita tested negative using RT-PCR with primers for these genera. There are no carlaviruses reported to be infecting yams, therefore, it may be considered as a new host-virus association. References: (1) X. Foissac et al. Acta Hortic. 550:37, 2001. (2) S. A. Langeveld et al. J. Gen. Virol. 72:1531, 1991. (3) B. S. M. Lebas. Ph.D. thesis. Greenwich University, Chatham Maritime, UK, 2002. (4) R. A. A. Van der vlugt and M. Berendsen. Eur. J. Plant Pathol. 108:367, 2002.