Mealybugs and the spread of grapevine leafroll-associated virus 3 (GLRaV-3) in a New Zealand vineyard

The spread of grapevine leafroll-associated virus 3 (GLRaV-3) through a newly planted block of Merlot and Chardonnay grapevines in a commercial vineyard was measured between 1998 and 2003. The population changes of the mealybug vector (Pseudococcus longispinus) were recorded over the same period in 12-17-year-old GLRaV-3-infected grapevines immediately adjacent to two sides of the new block. Relationships between the spread of GLRaV-3 and mealybug numbers were examined. Mealybugs developed through three generations a year and populations were low for 4 of the 6 years. Numbers were usually higher in the 12-17-year-old Sauvignon Blanc vines and adjacent young 1-6-year-old Merlot vines than in the 12-17-year-old Breidecker and adjacent young 1-6-year-old Chardonnay vines on the other side of the block. However, GLRaV-3 spread more rapidly in young Chardonnay vines than in young Merlot vines. New infections in both Merlot and Chardonnay initially occurred at random, but newly infected vines were more often clustered from 2002 to 2003, and especially in Merlot. Most newly infected Chardonnay were recorded in the years following the highest numbers of mealybugs, in 1997-98 and 2000-01, but the disease spread more gradually in the Merlot. The data indicated that GLRaV-3 did not spread into the new block when third generation mealybug populations in adjacent infected vines were lower than similar to 5 mealybugs/leaf. Three methods for virus transmission by mealybug were proposed: one method was from natural, crawling dispersal of infected mealybugs, leading predominantly to within-row, vine-to-vine transmission; another method was from human-assisted movement of mealybug crawlers (such as on vineyard machinery) leading to random but localised infection on a within-or between-block scale; and a third method was through aerial dispersal of infectious mealybugs, resulting in random infections of vines and particularly operating on a between-block or between-vineyard scale. Infection in any vineyard probably occurs by all three methods, mediated by the poorly understood, and possibly site-and grape variety-specific, transmission ecology that underpins the mealybug-virus-vine relationships.