Charles, J.G., Walker, J.T.S., & Bell, V.A. 2008 Spread of grapevine leafroll disease by mealybugs in New Zealand vineyards.. Proceedings of the XI International Symposium on Scale Insect Studies, Oeiras, Portugal, 24-27 September 2007. ISA Press Lisbon, Portugal 322 pp. Lisbon, Portugal 322 pp.

Notes: Grapevine leafroll disease has been present in New Zealand for over 100 years, but its economic impact has only become of concern in the last two decades as red varieties such as Pinot Noir and Merlot have become economically important. Several studies over the past 15 years have documented the very rapid spread of the disease within some vineyards, with increases in vine infection from <10% to >90% within 5-6 years being repeatedly measured. Recently, some vineyards have been replanted because the disease incidence means that they can no longer produce the premium quality grapes required by the winemaker. The key vectors of GLRa V -3, the ampelovirus that is recognised as the causal agent of grapevine leafroll disease, are mealybugs. Three species have been identified as vectors in New Zealand: long-tailed mealybug Pseudococcus longispinus, citrophilus mealybug P. calceolariae, and obscure mealybug P. viburni. Recent work has attempted to quantify the relationship between mealybug numbers in diseased vines and the rate of spread of the disease into and within a new block. Over a six-year period, the two years in which mealybug numbers were highest were strongly associated with the two years in which the largest increases in numbers of infected vines were recorded. However, it was not possible to relate quantitatively at a fine scale the occurrence of new point sources of the disease to neighbouring patches of mealybug infested vines. The data implied that GLRa V -3 infected mealybugs may infest healthy vines in one of three ways: by crawling from vine to vine; by 'hitch-hiking' on tractors or other vineyard machinery; or by aerial dispersal, possibly over significant distances. The data also implied that differing vine tolerance to virus and or mealybugs, especially between cultivars, might also be a factor in disease transmission. A multi-disciplinary team of entomologists, plant virologists and plant physiologists is developing this research, to enable the New Zealand wine industry to continue producing high quality wine for the foreseeable future.