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 wa s carried out for 3 years in the Rhone valley (France). This yellows is cau sed 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 STOL11 f2/r1. In addition, a nested polymerase chain reaction (PCR) was used with primers P1/P7 and fU5/rU3 for detection in stolbur-infected plants with low titre. Several thousand insects were captured and species of Hemiptera were liste d. Fourteen wild or reared Hemiptera species were used in transmission tria ls. Thirty-four wild species were monitored for phytoplasma DNA by PCR. A p lanthopper, Hyalesthes obsoletus Sign., tested positive for stolbur at a le vel of 28% (98/343) in 1995 and 38% (205/529) in 1996. In 1995, two leafhop pers, Mocydia crocea (1/78) and Euscelis lineolatus (2/309), were infected at a much lower ratio. Successful experimental transmission to grapevine, p eriwinkle and thorn apple was only obtained with H. obsoletus. Among wild p lant species, hoary cress (Car dar in draba L.), bindweed (Convolvulus arve nsis L.), sweet cherry (Prunus sp.), plum (Prunus domestica L.), lilac (Syr inga vulgaris L.), fig tree (Ficus carica L.) and elm (Ulmus sp.) were show n 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 vineyar ds as sites where the inoculum of stolbur phytoplasma was maintained in ins ect vector and reservoir plants, is discussed. The natural inoculation peri od to grapevine was shown to extend from June to August, corresponding to t he adult activity of H. obsoletus. Together with the close relationships of the phytoplasmas involved in vergilbungskrankheit, a German grapevine dise ase, and bois noir, The results of this study suggest that the two yellows are very close.