At this time there is no accurate information on the epidemiology of Grapevine Red Blotch associated Virus (GRBaV) in vineyards, including whether GRBaV is transmitted by insects ordispersed only with the movement of infected nursery material. Our goal is to provide concreteevidence of whether common insect species can or cannot move GRBaV among vines, and to then determine the transmission efficiency of any discovered insect vector. We prioritized the screening of leafhoppers and whiteflies as the most likely potential vectors based upon current knowledge of GRBaV, and also mealybugs due to their prevalence. In 2013 we completed most of the replicates for inoculation trials for the Western grape leafhopper, variegated leafhopper, Virginia creeper leafhopper, vine mealybug, blue-green sharpshooter, and grape whitefly.
Thus far, we have tested the most mobile life stages of each species. Petiole samples from inoculated plants were tested for the presence of GRBaV at 4-6 weeks after initial inoculation and then at 4 month intervals thereafter. To date, none of the inoculated plants show symptoms of GRBaV and all petioles have tested negative. Subsamples of insects that were used in experiments have also tested negative. Plants will continue to be tested quarterly, for a period of 2 years, as it may take a year or longer for viral populations to reach detectable levels in inoculated vines. We also sampled field populations of western grape leafhopper from symptomatic vines at a vineyard known to be infected by GRBaV, to examine whether this species could acquire GRBaV from infected vines. Out of the 15 vines sampled, 14 tested positive for GRBaV, and leafhoppers from 50%(7 out of 14) of the GRBaV-positive vines tested positive for GRBaV. These results indicate that leafhoppers can acquire the virus by feeding on infected vines, but does not provide conclusive evidence that they can transmit GRBaV.
To better understand field epidemiology, we monitored progression of GRBaV in an established 20 ha vineyard where the spread of Grapevine leafroll-associated viruses (GLRaV) was mapped from 2009-2012. Not all symptomatic vines from previous years tested positive for GLRaV, and in 2013 we recorded possible GRBaV-symptomatic vines in addition to mapping GLRaV spread. We also tested all vines for GRBaV that were visually identified as GLRaV-infected from 2009-2012, but that tested negative for in PCR analysis. Out of the 25 vines from 2009-2012 that showed “red leaf” symptoms but tested negative for GLRaV, 17 (68%) were positive for GRBaV. All vines testing positive were recorded as symptomatic in 2011 or 2012, indicating that GRBaV was present in the plot at least by 2011. Infected vines were randomly distributed within the plot, indicating that infection had not spread out from previously infected vines.