Leafroll virus is generally considered the biggest limitation to further quality advancements in the South African wine industry, the most important effect of the disease being a reduction in the yield and quality of fruit from infected vines.
Symptoms of leafroll virus appear between the time of harvest and leaf fall. On affected vines, the margins of the leaf blades roll downward and the areas between the major veins turn yellow or red, depending on whether the variety produces white- or red-coloured fruit.
This image was captured on 14 May in Stellenbosch. The vineyard in the middle distance is clearly infected with leafroll virus.