At this time there is no accurate information on the epidemiology of grapevine red blotch-associated virus (GRBaV) – is it transmitted by insects or dispersed with the movement of infected planting material? Our goal is to screen possible vectors to determine if they can or cannot acquire GRBaV from infected vines and transmit GRBaV to clean vines. In 2013, replicated groupings of 30-50 western grape leafhopper, variegated leafhopper, Virginia creeper leafhopper, vine mealybug, blue-green sharpshooter, and grape whitefly were tested. Petiole samples from inoculated test plants were tested for the presence of GRBaV and, 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. Based on the negative results of these studies, in 2014 trials we used more insects and allowed them to feed for longer acquisition and inoculation periods. Infected and uninfected vines were placed together in cages and adult insects were allowed to move between plants at will for 1-6 weeks, with replicated groupings of 600 western grape leafhopper and Virginia creeper leafhopper per cage, 30 blue-green sharpshooter per cage and 1500 grape whitefly per cage. The more sedentary vine mealybug crawlers were added to known infected plants at the rate of 1000 per plant for a 1 week acquisition period and were then moved to uninfected plants for a 1 week inoculation period.
To date, all recipient plants and exposed insects have tested negative for GRBaV. Plants from 2013 and 2014 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. Field epidemiology was monitored at two sites. In a 20 ha block planted in 2008 the spread of grapevine leafroll-associated viruses (GLRaV) was mapped from 2009-2012, also recording ‘red leaf’ symptomatic vines’ that tested (PCR) negative for GLRaV. In both 2013 and 2014, there were about 150 ‘symptomatic vine’ that tested negative for GLRaV. In 2014 we surveyed and tested 156 of these suspect vines using new more complete primers for leafroll and primers for red blotch. Of these 156 vines, 136 tested positive for red blotch, 9 tested positive for leafroll and 11 tested positive for both red blotch and leafroll. The red blotch infected vines were randomly distributed within the plot, indicating that infection did not spread from previously infected vines, which is often indicating of vector movement. During field surveys we collected and tested western grape leafhoppers from red blotch infected vines Batches of 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 evidence that they can transmit GRBaV. We repeat for emphasis that acquisition does not imply transmission, basically the virus is in the bug after feeding on the vine.