Biotechnology Laboratory, Sumitomo Chemical Co. Ltd., 4-2-1 Takatsukasa, Takarazuka, Hyogo 6658555, Japan
A specific and highly efficient indexing method for four major garlic viruses, GV1 carlavirus, GV2 potyvirus, onion yellow dwarf virus (OYDV), and mite-borne mosaic virus, was developed using the reverse transcription-polymerase chain reaction method (RT-PCR). When specific primers were synthesized to amplify the coat protein genes of these viruses, the amplified PCR bands had the same expected sizes. This indexing method can be performed using an extremely small amount of leaf tissue (50 mg) to obtain reproducible data. Diseased garlic plants collected from five fields in northern Japan were tested using this method. As GV2 was detected in all the fields, and high frequency of disease symptoms was obtained in the case of heavy infection, GV2 may well be a major garlic virus in this region. By contrast, GV1 was detected in only one of the five fields. OYDV and mite-borne mosaic viruses were also detected in various fields and at different frequencies. Judging from the virus indexing data and the disease symptoms, these two viruses appeared to cause severe symptoms in the case of a mixed infection with GV2. The frequency of virus re-infection to virus-free clones was low in isolated fields, and GV2 was the major virus that re-infected virus-free clones.