The Role of Hyalesthes obsoletus (Hemiptera: Cixiidae) in the Occurrence of Bois noir of Grapevines in France

An epidemiological study on a grapevine yellows disease called bois noir was carried out for 3 years in the Rhone valley (France). This yellows is caused by a stolbur type phytoplasma. Vectors and alternative host plants were searched, and the inoculation period was determined. Detection of stolbur phytoplasma in insects and plants was obtained using primers STOLl 1 f2,rl. In addition, a nested polymerase chain reaction (PCR) was used with primers P1/P7 and fU5/rU3 for detection in stolburinfected plants with low titre. Several thousand insects were captured and species of Hemiptera were listed. Fourteen wild or reared Hemiptera species were used in transmission trials. Thirty-four wild species were monitored for phytoplasma DNA by PCR. A planthopper. Hvalesthes obsoletus Sign., tested positive for stolbur at a level of 28% (98,/343) in 1995 and 38% (205,529) in 1996. In 1995, two leafhoppers, Mocydia crocea (1 78) and Euscelis tineolatus (2/309), were infected at a much lower ratio. Successful experimental transmission to grapevine, periwinkle and thorn apple was only obtained with H. obsoletus. Among wild plant species, hoary cress (Cardaria draba L.), bindweed (Convolvulus arvensis L.), sweet cherry {Prunus sp,), plum (Prunus domestica L), lilac (Syringa vulgaris L.), fig tree [Ficus carica L.) and elm {Ulmus sp.) were shown to be stolbur infected and hoary cress was identified as a new host plant for H. obsoletus in France. The role of some fallow lands close to vineyards as sites where the inoculum of stolbur phytoplasma was maintained in insect vector and reservoir plants, is discussed. The natural inoculation period to grapevine was shown to extend from June to August, corresponding to the adult activity of H. obsoletus. Together with the close relationships of the phytoplasmas involved in vergilbungskr ankheit, a German grapevine disease, and bois noir. The results of this study suggest that the two yellows are very close.