Studies of grapevine leafroll viruses vectored by scale insects

Grapevine leafroll disease is one of the most severe viral diseases of grapevine worldwide. It is associated to a range of closterovirid species named Grapevine leafroll-associated virus (GLRaV) -1, -2, etc. In France, GLRaV-1 and -3 (genus Ampelovirus) are highly prevalent in vineyards and are disseminated along with exchange of infected material, as well as by natural vectors, the scale insects (Hemiptera, Pseudococcidae and Coccidae). Deeper knowledge on the epidemiology and the transmission of leafroll (Herrbach et al., APS book, in press) will help to improve the protection of grapevine against this severe syndrome. Virus–vector specificity studies revealed that the pseudococcid Phenacoccus aceris is able to vector the ampeloviruses GLRaV-1, -3, -4, -5, -6, and -9, as well as the vitiviruses Grapevine virus A (GVA) and GVB (Le Maguet et al., 2012, EJPP). This emphasizes the attention to be paid to this Holarctic widespread species. In addition, the coccid Neopulvinaria innumerabilis was shown to transmit GLRaV-1 and GVA (Le Maguet, 2012, PhD thesis). Finally, the retention over two weeks of the RNA of GLRaV-1, -3 and GVA in the viruliferous vector was assessed by RT-PCR in the pseudococcid Heliococcus bohemicus and the coccid Parthenolecanium corni. The spatio-temporal dispersal of leafroll in a young vineyard in Burgundy has been surveyed over 8 years (Le Maguet et al., 2013, Phytopathology). Leafroll symptoms spread rapidly from the edges to virtually the entire plot. Biostatistical and molecular analyses showed that the vectors and the ampelovirus originated from the adjacent plots, heavily Ph. aceris–infested and GLRaV-1– infected, highlighting the risk generated by leafroll-affected neighboring plots to newly healthyplanted plots. Field experiments were set up to estimate the possible natural spread by the wind of Pa. corni L1 and L2 larvae from infested vineyards to a newly planted plot, by using sticky traps (Hommay et al., 2012, ICVG). Results reveal that L1 larvae ('crawlers') were commonly trapped and that some of them were viruliferous. We are most grateful to three French regional viticulture committees ('Comité Interprofessionnel du Vin de Champagne' CIVC, 'Comité Interprofessionnel des Vins d’Alsace' CIVA, 'Bureau Interprofessionnel des Vins de Bourgogne' BIVB) and to the FranceAgriMer agency for financial support. Thanks are also due to winegrowers and viticultural advisors, as well as to Dr Elisabeth BOUDON-PADIEU who initiated leafroll studies in France