The first identification of two viruses infecting trailing verbena in the UK

Verbena (Verbena spp.) is an ornamental plant, grown widely in the UK and elsewhere. A range of different viruses have been detected in samples of both imported and UK-grown verbenas received at CSL. These include Tomato spotted wilt virus, Impatiens necrotic spot virus, Cucumber mosaic virus, Broad bean wilt virus, Alfalfa mosaic virus and Tobacco mosaic virus (unpublished data).

In April and July 2004, two separate samples of verbena were received at CSL. Both were showing virus-like symptoms; the first (var. 'Aztec White') showed necrotic and chlorotic flecks, while the second (var. 'Aztec Raspberry') exhibited a more severe dark vein and stem necrosis (Fig. 1). Screening by ELISA failed to detect any of the previously recorded viruses in either sample. Sap inoculation tests were performed and two distinct viruses were isolated.

The first sample showed symptoms on a range of indicator plants identical to those previously recorded for Nemesia ring necrosis virus (NeRNV); a proposed tymovirus (Skelton et al., 2004). The presence of NeRNV was confirmed in both indicators and the original verbena sample by ELISA and RT-PCR, using the methods described by Skelton et al. (2004). This is the first record of NeRNV in verbena and of this virus naturally-infecting a host outside of the Scrophulariaceae.

Sample two produced symptoms on Chenopodium quinoa (systemic chlorotic spots) and N. benthamiana (systemic mottle and distortion). Electron microscopy revealed the presence of potexvirus-like particles, measuring 520 nm long. A potexvirus, similar to Clover yellow mosaic virus (ClYMV), has been identified recently in verbena in the USA (Baker et al., 2003). A plate trapped ELISA was performed using ClYMV-specific antiserum (DSMZ, Germany) and both material from indicators and the original sample tested positive. RNA extracted from the original verbena sample was tested by RT-PCR using a degenerate potexvirus ORF3 primer (GAG GCC AXT ACM RXG ACG GCA) and oligo dT. A product of around 1100 bp was obtained and partially sequenced. The 607 bp sequence obtained (Acc. No. AY936202) shared 81% identity with the 3' end of a sequence of ClYMV (Acc. No. D29630); suggesting that the virus isolated from verbena is a strain of ClYMV. ClYMV has subsequently been found in further stocks of trailing verbena. Previously only recorded in North America (Baker et al., 2003; Bos, 1973), this is the first finding of ClYMV in the UK in any host.