Grapevine leafroll-associated viruses (GLRaV) are a complex of viruses that cause leaf chlorosis and leaf margins to “roll” downward. GLRaVs can reduce yields, delay fruit maturity, and impede fruit pigmentation. Our work concerns GLRaV field epidemiology with respect to its insect vectors. In field studies, we continued evaluating seasonal densities of grape mealybugs and leafroll virus in five vineyards with varying levels of leafroll infections and found 43%of grape mealybugs collected on leafroll-infected vines tested positive for GLRaV-3. The proportion of leafroll-infective mealybugs was related to the number of leafroll-positive vines in the vineyards surveyed. Vineyards were mapped for leafroll virus symptoms in fall 2012 but the presence of Red Blotch Virus at two of the sites made accurate mapping difficult.
We continued a five-year field trial testing the impacts of “zero tolerance” for mealybugs on GLRaV infection establishment and spread. A 20 acre vineyard planted in 2008 from certified virus-free scion, and bordered by blocks with GLRaV-infected vines and mealybugs, received pesticide treatments in 2009, 2010, 2011 and 2012. No mealybugs were found in harvest counts; however, pheromone traps showed the presence of male grape mealybugs. All vines were inspected annually for GLRaV symptoms. In 2009 and 2010, there was one new infected vine, with strong leafroll symptoms and positive PCR test, each year in 2009 and 2010, six new vines in 2011, and 2 in 2012. GLRaV-3 variants in the spray trial block (-3a, -3c, and -3d) were different than the GLRaV-3 variants in adjacent virus-infected blocks to the north and west (-3b). The leafroll-infected vines were randomly distributed inside the mapped plot, with similar numbers in insecticide and untreated rows. Our hypothesis is that virus-carrying grape mealybug crawlers were “blown” into the plot, and infecting previously-healthy vines during feeding.
We conducted mealybug behavior studies in the greenhouse to determine if mealybugs prefer vines without GLRaV; this would result in movement of vectors away from infected vines to healthy vines. In non-choice tests, mealybugs survived equally well on infected and uninfected vines, while in choice tests, equal numbers of mealybugs were chose infected and healthy vines, indicating no preference for one over another.
In side-by-side comparisons, the five major GLRaV vector species found in California, EFLS, OMB, LTMB, GMB, VMB and GiMB all transmitted GLRaV-3 but at different rates. We showed that GLRaV-3 can be detected 3-4 weeks after first inoculation in any part of the inoculated vines using crude extraction of RNA and RT-PCR. The latency period studies (collected thus far) further signify the fact that VMB crawlers can acquire leafroll virus (GLRaV3) from newly inoculated vines as soon as two weeks after first inoculation and then successfully transmit it to the healthy grapevines. Ongoing studies are investigating the effectiveness of different insecticides to not simply kill mealybugs but reduce transmission rates.