Incidence and spread of viruses in white-clover pastures of the South Island, New Zealand

Sixty-two white-clover pastures were tested by enzyme linked immunosorbent assay (ELISA) for five viruses ( White clover mosaic virus (WClMV), Alfalfa mosaic virus (AMV), Red clover necrotic mosaic virus (RCNMV), Soybean dwarf virus (SbDV), beet western yellows virus (BWYV)) and also for members of the potyvirus group. Only one pasture was found to be virus-free. WClMV and AMV occurred, often at high incidences, on farms in every region. RCNMV and SbDV occurred in more than half the pastures whereas BWYV and potyvirus infection was less common. There was a general reduction in virus diversity with increasing latitude, with the most northern farms having greater species richness (all 6 viruses present) than those in Southland (1–3 viruses present). There was a significant relationship between the presence of AMV, BWYV, RCNMV, SbDV and irrigation. Dairy farming also had a positive relationship with BWYV, RCNMV and SbDV. WClMV increase was monitored in six pastures and incidence was observed to increase geometrically in young pastures. The relatively high incidence of RCNMV is in contrast to the findings of previous studies on white-clover pastures and indicates that assessing this virus’s effects on white clover is of high priority.