GRAPEVINE LEAFROLLASSOCIATED VIRUS 3 PERSISTENCE IN VITIS VINIFERA REMNANT ROOTS
V.A. Bell, R.G.E. Bonfiglioli, J.T.S. Walker, P.L. Lo, J.F. Mackay, S.E. McGregor
Grapevine leafroll-associated Virus 3 (GLRaV-3) adversely alters quantitative and qualitative parameters of wine production. New Zealand grape growers respond to this economic threat by replacing virus-infected grapevines with certified virus-tested vines. When vines are removed, most roots remain in situ, potentially acting as long-term reservoirs of GLRaV-3. In New Zealand this disease is vectored by three species of pseudococcid mealybugs (Hemiptera: Pseudococcidae), i.e. Pseudococcus longispinus, P. calceolariae and P. viburni, with the two latter species frequently found on roots of host plants. Viruliferous mealybugs moving from GLRaV-3 infected remnant roots to newly planted vines form a probable pathway explaining the relatively rapid re-appearance of the disease in replanted vineyards. We conducted four field studies to determine the status of GLRaV-3 in remnant roots after applying herbicide and/or leaving ground fallow for variable intervals following vine removal. In vineyard A, one of three herbicides (glyphosate, triclopyr, or metasulfuron) was applied to freshly cut vine stumps. One year after treatment, roots and mealybugs found tested positive for GLRaV-3. In vineyard B, no herbicide was applied to cut vines, which were removed and the ground left fallow for 12 months. Twenty-six weeks after vine removal, mealybugs found on remnant roots tested positive for GLRaV-3 and after 12 months, virus was detected in 97% of the roots. The absence of any real decline in the proportion of roots with GLRaV-3 prompted testing at vineyard C, where 4 years earlier vines were cut and the stumps swabbed with glyphosate. Despite this fallow period, two thirds of root samples tested positive for GLRaV-3. Before vine removal in vineyard D, glyphosate was applied to leaves. Six months later 87% of root samples tested positive for GLRaV-3. The implications of these results for the management of leafroll virus are discussed.