virus diseases, group of vine diseases caused by very small and simple organisms, consisting of ribonucleic acid (RNA) wrapped in a protein sheath. Some virus diseases can seriously affect grapevine yield and wine quality, and since they are mainly spread by propagation in cuttings, there has been an emphasis on vine improvement and clonal selection to prevent their spread. Virus diseases began to affect European vines from about 1890, when rootstocks were used in France to control phylloxera. grafting doubles the risk of virus spread and, unlike many fruiting varieties, rootstocks do not always show virus symptoms.
