Faccincani, Monica

AbstractBois noir, a disease of the grapevine yellows complex, is associated with ‘CandidatusPhytoplasma solani’ and transmitted to grapevines in open fields by the cixiidsHyalesthes obsoletusandReptalus panzeri. In vine-growing areas where the population density of these vectors is low within the vineyard, the occurrence of bois noir implies the existence of alternative vectors. The aim of this study was to identify alternative vectors through screening of the Auchenorrhyncha community, phytoplasma typing bystampgene sequence analyses, and transmission trials. During field activities, conducted in Northern Italy in a vineyard where the bois noir incidence was extremely high, nine potential alternative insect vectors were identified according to high abundance in the vineyard agro-ecosystem, high infection rate, and harbouring phytoplasma strains characterized bystampgene sequence variants found also in symptomatic grapevines. Transmission trials coupled with molecular analyses showed that at least eight species (Aphrodes makarovi,Dicranotropis hamata,Dictyophara europaea,Euscelis incisus,Euscelidius variegatus,Laodelphax striatella,Philaenus spumarius, andPsammotettix alienus/confinis) are alternative vectors of ‘CandidatusPhytoplasma solani’ to grapevines. These novel findings highlight that bois noir epidemiology in vineyard agro-ecosystems is more complex than previously known, opening up new perspectives in the disease management.

AbstractBois noir (BN), the most prevalent disease of the grapevine yellows complex, causes considerable yield loss in vineyards. BN is associated with phytoplasma strains of the species ‘Candidatus Phytoplasma solani’ (taxonomic subgroup 16SrXII‐A). In Europe, the BN phytoplasma is transmitted to grapevine mainly by Hyalesthes obsoletus, a polyphagous cixiid completing its life cycle on stinging nettle and field bindweed. As a result of the complexity of BN epidemiology, no effective control strategies have been developed. In previous studies conducted in the eastern Mediterranean coast of Israel, chaste tree (Vitex agnus‐castus) was found to be the preferred host plant of H. obsoletus but did not harbour BN phytoplasma. Thus, a ‘push and pull’ strategy was suggested based on the fact that chaste tree plants located at vineyard borders was an effective trap plant for H. obsoletus adults. However, in other studies carried out in the eastern Adriatic coast of Montenegro, chaste tree was found to be a key source plant for BN phytoplasma transmission to grapevine. This study aimed to investigate (i) the interaction between chaste tree and H. obsoletus through survival, attractiveness and oviposition experiments conducted comparing the behaviour of H. obsoletus in chaste tree versus stinging nettle and grapevine and (ii) the capability of chaste tree to harbor ‘Ca. P. solani’ in northern Italy through transmission trials. H. obsoletus adults were found to survive on chaste tree and grapevine over a 1 week period and prefer chaste tree to grapevine. Moreover, H. obsoletus produced eggs and overwintered as nymphs on chaste tree, even if at a lesser extent than on stinging nettle. H. obsoletus originating from nettle was found able to transmit ‘Ca. P. solani’ to chaste tree (2 plants of 16 were found infected by the BN phytoplasma strain St5 identified in H. obsoletus specimens). These results increased our knowledge about the role of Vitex agnus‐castus as host plant of H. obsoletus and BN phytoplasma in northern Italy and do not recommend considering chaste tree as trap plant at vineyard borders.