Strawberry mottle virus detected on strawberry in New Zealand

Strawberry mottle virus (SMoV) is a serious pathogen of strawberries (Fragaria ´ ananassa) and is transmitted by aphids in a semi-persistent manner. Severe strains of SMoV may reduce yield by up to 30% and losses can be up to 80% in mixed infections with other viruses (Thompson & Jelkmann, 2003; Martin & Tzanetakis, 2006). In May 2007, leaf mottling was observed on indicator plants of Fragaria vesca 'EMK' (Fig. 1) and 'UC4', and F. virginiana 'UC12' held in an open-shade house near Christchurch, New Zealand. No virus particles were seen using electron microscopy on crude sap preparations from symptomatic leaves, nor were symptoms observed when healthy plants of Chenopodium amaranticolor, C. quinoa, Cucumis sativus, Gomphrena globosa, Nicotiana benthamiana, N. clevelandii, N. occidentalis and N. sylvestris were mechanically inoculated with plant sap. RNA was extracted from symptomatic strawberry plants and tested by RT-PCR using published SMoV-specific primers and cycling conditions (Thompson et al., 2003). PCR amplicons of the expected size (219 bp) were obtained and an amplicon from F. vesca was sequenced (GenBank Acc. No. EU440731). A BLAST search in GenBank confirmed SMoV (98% nt identity with AJ311876). The plants also tested positive for SMoV using real-time RT-PCR (Ratti et al., 2006).

In July 2007, 35 plants of commercial cultivars of F. ananassa were collected at random from 10 locations throughout New Zealand and the crowns and leaves were tested for SMoV using conventional and real-time RT-PCR. Indicator plants of F. virginiana 'UC11' and 'UC12' held in a glasshouse in Auckland were also tested. All commercial cultivars were uninfected but the indicator plants from Auckland tested positive for SMoV with both PCR methods. All plants were also tested by RT-PCR for Raspberry ringspot virus, Strawberry latent ringspot virus, Strawberry vein banding virus, Tomato ringspot virus and Tomato black ring virus and were found to be uninfected.

Strawberry mottle virus (SMoV) occurs in many areas where strawberries are grown; this is the first report of the virus in New Zealand. The SMoV-infected indicator plants have been grown in New Zealand for at least 10 years. Despite the indicators in Christchurch being held in an open shade house, and the presence of at least two aphid species that can vector the virus in New Zealand (Aphis gossypii and Chaetosiphon fragaefolii), the virus does not appear to have become widely distributed within commercial strawberry cultivars. However, it has been decided not to impose regulatory controls to eradicate or manage the disease and therefore the disease is likely to spread in the future.