Determining the effectiveness of grazing and trampling by livestock in transmitting white clover mosaic and subterranean clover mottle viruses

摘要

Glasshouse and mini-sward experiments were conducted at the Agriculture Western Australia site at South Perth, Australia, to determine the relative roles of grazing and trampling by livestock in transmitting white clover mosaic potexvirus (WClMV) and subterranean clover mottle (SCMoV) sobemovirus between clover plants in pastures. Wounding due to grazing was simulated by repeatedly cutting plants with serrated scissors (glasshouse) or mowing (mini-swards), while wounding due to trampling was simulated by repeatedly bashing plants with the flat end of a wooden hammer handle (glasshouse) or rolling (mini-swards). In glasshouse experiments, cutting was more effective than bashing in transmitting WClMV to white clover (Trifolium repens) plants but cutting and bashing transmitted it to subterranean clover (T. subterraneum) plants at similar rates. In an experiment with white clover mini-swards, mowing was more effective than rolling in transmitting WClMV, and when both were combined, initially spreadexceeded that obtained when the spread from mowing and rolling alone was added together. In glasshouse experiments, bashing was more effective than cutting in transmitting SCMoV to subterranean clover plants. In 1 experiment, neither mowing nor rolling spread SCMoV in mini-swards of subterranean clover. When transmission to subterranean clover cultivars which were 'susceptible' or 'moderately susceptible' to SCMoV was compared in glasshouse experiments, repeated bashing spread the virus more slowly to the 'moderately susceptible' cultivars. When mixed with ruminant saliva, infective sap containing WClMV or SCMoV was still infective to clover plants after 4 weeks storage at room temperature. When infective sap was allowed to dry naturally on a metal surface, SCMoV still infected clover plants when the dried sap was taken up in tap water after 4 but not 14 days, while WClMV was infective after 24 h but not 4 days. These results suggest that grazing and mowing are more effective than trampling at transmitting WClMV to white clover plants in pastures, while trampling is more effective at spreading SCMoV to subterranean clover. However, both transmitted WClMV to subterranean clover at similar rates. Possible reasons for these differences are discussed inrelation to differences in clover plant morphology and virus-specific factors.