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Several viruses, including the thrips-borne tospovirus Watermelon silver mottle virus (WSMoV), cause severe damage to cucurbits in Taiwan. In June 2006, a survey for WSMoV on watermelon (Citrullus lanatus) was conducted in Miaoli County central Taiwan by indirect ELISA using the antiserum against the nucleocapsid (N) protein of Capsicum chlorosis virus, which is serologically related to WSMoV. Extracts from diseased samples with a positive reaction were used as inocula for bioassay on watermelon and Chenopodium quinoa. One of these gave symptoms on watermelon and C. quinoa different from those caused by WSMoV.

After three passages of single lesion isolation, this new isolate reacted with the monoclonal antibody (MAb) against the nonstructural WSMoV NSs protein (Chen et al., 2006), but not with the MAb to WSMoV N protein. This suggested that this tospovirus was only distantly related toWSMoV. RT-PCR with the degenerate primers, WScon1p (5′-GTCAACTTTCTCAATTTGTTCATGCATA-3′) and WScon5 m (5′-CTCTTTATCATACAT(T/C)TTGAACACAAT(C/G)AC-3′), designed from the NSs genes of members of WSMoV serogroup, amplified a DNA fragment from the host tissues infected by the newly isolated tospovirus. The sequence of this fragment was found to share 97% identity with that of the NSs gene of Melon yellow spot virus (MYSV), a virus previously reported from Japan (Kato et al., 2000) and distinct from WSMoV.

When the complete sequences of N and NSs genes of the newly isolated tospovirus were determined from the DNA fragments amplified by the specific primers designed from the S RNA of MYSV, the results showed 98·9 and 97·9% amino acid identities, respectively, to those of MYSV. Over all, the data indicate that the new virus is an isolate of MYSV, denoted as MYSV-TW and is a new record for Taiwan.

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