Effect of host plant tissue on the vector transmission of Grapevine leafroll-associated virus 3

Many biotic and abiotic factors affect the transmission efficiency of vector-borne plant pathogens. Insect vector within-plant distribution and host tissue preference are known to affect pathogen acquisition and inoculation rates. In this study, we first investigated whether feeding tissue affects the transmission of Grapevine leafroll-associated virus 3 by Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae) and the effect of mealybug within-plant distribution on virus transmission under greenhouse conditions. Results showed no significant effect on transmission efficiency after insect confinement on leaf blades, petioles or stems of virus source or healthy test plants for either acquisition or inoculation trials. Transmission efficiency of a single mealybug varied from 4 to 25% in those trials. Second, we tested whether leaf position affected transmission efficiency due to potentially variable virus populations within acquisition plant tissues. No significant differences of transmission rate among acquisition leaf position were observed, probably because there were no differences in the virus population within source tissues. Finally, we examined the seasonality of the virus in field-collected samples and found that GLRaV-3 prevalence varied along a growing season, such that GLRaV-3 translocated along expanding shoots to leaves. Similarly, mealybug populations are known to increase in spring, and then mealybugs spread to cordons and leaves. This coordination of spatial and temporal dynamics of the virus and its vector may increase the risk of GLRaV-3 transmission during late spring and early summer. Further integration of information about pathogen populations in plants, vector feeding behavior and vector population seasonality could lead to more effective management practices.