It’s the often-invisible virus that lowers vineyard yields and affects wine quality, making it arguably the most economically damaging threat to the New Zealand wine industry. Now, a world first indicator grafting technique developed by Mission Estate viticulturist Caine Thompson and Professor Gerhard Pietersen from the University of Pretoria, South Africa could identify leaf roll virus in white varieties before it takes hold.
Grapevine leaf roll associated virus 3 can be seen in red grape varieties, as the leaves turn red when infected. But with white varieties, it’s impossible to detect visually as there are no obvious symptoms – meaning growers often don't realise there’s a problem until far too late. There is no cure for leaf roll virus and the only remedy is to remove the infected vines and replant.
Professor Pietersen’s solution is compellingly simple: take the bud of a red variety and graft it onto the trunk of a white variety. If the vine becomes infected with virus then the indicator shoot, being a red variety, will express visual symptoms – an obvious alarm bell.
Last year Thompson established the first field trial in Mission Estate’s Hawke’s Bay Greenmeadows vineyard. He used Pinot Noir as the indicator shoot, grafting onto Sauvignon Blanc, Chardonnay and Pinot Gris vines.
The cost for installation of the indicator shoot is approximately 90 cents per vine (a one off cost), with annual running costs of about 13 cents per vine in terms of managing the shoot – compared to a single diagnostic ELISA test of $15 per vine (annual cost).
“Grafting has been used for centuries in viticulture, just never for this application and use,” says Thompson. “This technique has the potential to be a real game changer for virus management in white wine varieties in New Zealand and across the world. It’s a highly cost-effective method for protecting white varieties from virus so that wine quality continues to improve as vines age.”
Further research is being carried out in collaboration with New Zealand Winegrowers to fine-tune the technique for wider adoption.