Impact of host plant tissue and growing season on the vector transmission of grapevine leafroll-associated virus 3

Grapevine leafroll disease is caused by grapevine leafroll-associated viruses (GLRaVs). Within this virus complex, GLRaV-3 is probably the predominant species in the world. GLRaV-3 has been shown to be transmitted from vine to vine by many mealybugs and soft scales. The introduction of the vine mealybug (Planococcus ficus) in California and other regions of the world may result in increasing disease incidence of established GLRaVs. We examined the tissue specificity for GLRaV-3 transmission by P. ficus under greenhouse conditions and monitored the seasonal progress of GLRaV-3 in chronically infected vines in a vineyard using quantitative RT-PCR. Our results indicate that there were no significant differences in transmission rate whether mealybugs acquired or inoculated the virus from different tissues. GLRaV-3 was detected in chronically infected vines during early growing season. In spite of high infection rate at all sampling dates, virus populations within plants increased during the summer. The results suggest that GLRaV-3 may be acquired from vines more efficiently during the summer, independent of vector host tissue preference, although further research is necessary to confirm this hypothesis. In addition, the invasion of P. ficus in California and other regions posts a threat to increase incidence of grapevine leafroll disease.