The agronomic impact of white clover mosaic potexvirus (WC1MV) was determined using yield trials and a survey of virus incidence. The virus was shown to occur at high levels in white clover plants in pastures throughcut the North Island. The virus was detected in 69% of white clover plants from 42 mature pastures examined (>4 years old) and in 52% of white clover plants examined on roadsides. The virus was also detected in 15 pastures less than 4 years old, but at a lower level (18% infection). The effect of WC1MV on plant growth was determined using clonal and non-clonal lines of the modern white clover cv. Grasslands Tahora in a replicated spaced plant field trial. Uninoculated plants were shown to out-produce inoculated plants at each of three harvest dates over one year. Infected plants produced, in total, 36.5% less dry matter than uninoculated plants. The major effect of WC1MV A 97056Received 11 August 1997; accepted 4 December 1997 infection was on stolon elongation, with lesser effects on leaf length and petiole height. These results indicate that substantial gains can be expected from cultivars resistant to this pathogen.