leafroll virus, virus disease that is widespread in all countries where grapes are grown. The disease is now thought to be due to a complex of ten different filamentous viruses referred to as grapevine leafroll associated viruses (GLRaVs). Of all the virus diseases of vines, it can have the most serious effects on wine quality. These dramatic effects are not understood by the many appreciative tourists in wine regions who marvel at the attractive autumnal red colours of vineyards. Few realize that these colours often indicate the presence of a serious disease, although other factors may contribute to autumnal colours. Leafroll virus causes yield to be reduced by as much as 50%. Wine quality is also affected because of delayed ripening. Thus wines from infected vines are lower in alcohol, colour, flavour, and body. The disease does not kill vines, so they are infrequently removed. Yet removal is the only known treatment to overcome the effects of the virus.