1. Agrivirology Laboratory, Institute of Plant Science and Resources, Okayama University, Kurashiki, Okayama 710-0046, Japan
2. Institute of Agrobiological Sciences, National Agriculture and Food Research Organization, Tsukuba, Ibaraki 305-8634, Japan
3. Faculty of Veterinary Medicine, Okayama University of Science, Imabari, Ehime 794-8555, Japan