Until we started working on this project, all that was known about vector transmission of Grapevine leafroll-associated viruses (GLRaV) was that mealybugs and scales were capable of transmitting this group of grape pathogens. We did not know how transmission occurred, if different mealybugs were capable of transmitting different viruses, if plant tissue affected transmission rates, etc. Our goal was to tackle some of these questions. We showed that transmission of GLRaV appears to lack vector specificity, in other words, different mealybugs may be capable of transmitting different GLRaV. We caution that this conclusion is based on limited data. We also showed, for the vine mealybug and GLRaV-3, that transmission occurs in a semi-persistent manner. That means insects can acquire and inoculate the pathogen within 1 hour of plant access, and that they loose infectivity over a few days. In addition, first instars seem to be better vectors than adults. Lastly, we analyzed the role of plant tissue on transmission rates. Under greenhouse conditions it seems that plant tissue does not affect transmission, but virus populations in the field vary dramatically and may affect disease spread rates.
Our work by no means fills all the gaps in knowledge emergently needed to develop management strategies for leafroll diseases, but it goes beyond the identification of vectors, adding information on i) what leafroll viruses may be mealybug-transmitted, ii) leafroll-mealybug specificity in relation to transmission, iii) conclusively demonstrating that transmission of at least one virus/vector combination in this system occurs in a semi-persistent manner, and iv) generating new methods and protocols, and using them, to study the ecology of leafroll diseases in relation to seasonality, virus strain, host variety, mealybug species, feeding preference. The last item will be an exciting and certainly fruitful research venue.