Viruses in the Vineyard Flashcards
Explain Fanleaf Virus and its effect on the vine
Also called Fanleaf degeneration, early shoot growth is stunted, canes can grow in distorted ways and leaves are very pale, malformed and can look like a fan. The effects vary widely from little effect to losing most of the crop in susceptible varieties such as Cabernet Sauvignon. Was spread enormously by the move to grafted vines following phylloxera and the inadvertent use of infected plant material. Otherwise, the disease is spread slowly by the dagger nematode.
How can Fanleaf Virus be managed?
There is no cure for the disease and eventually affected vines will have to be removed, adding to cost.
Before vineyards are replanted, soil tests should be done to check for the presence of dagger nematodes and only virus-tested, clean planting material should be used.
Explain Leafroll Virus and its effect on the vineyard
The virus was spread by grafting and by mealy bugs, a key pest in South Africa, the Mediterranean, Argentina and some parts of California. It does not kill vines, but can reduce yield by up to half and affect quality negatively.
It slows down the growth of roots and shoots. Surviving fruit may take several additional weeks to ripen and have more acidity, less colour and lower sugar levels. The overall health of the vine is also affected as it stores less carbohydrate. The typical downward rolling of the leaf usually occurs in autumn. The leaves change colour in the autumn to red for black grape varieties and to yellow for white grape varieties.
How can Leafroll virus be managed?
There is no cure for leafroll virus and therefore the only solution is to remove unproductive vines and replant with virus-free stock.
Nurseries can screen vines for virus infections.
Mealy bugs favour humid environments and therefore open canopies help to reduce the pest. Control by spraying is difficult because of the mealy bug’s waxy coating. Steps can be taken to encourage the mealy bug’s natural predators: ladybugs, lacewings and others.
