First results on the trials in progress to identify the vector of the agent of a grapevine yellows in Italy.
In Jun. 1990, young Scaphoideus titanus captured in the vineyards of the Friuli-Venezia Giulia (F-VG) region were placed on potted grapevines of 3 different varieties (Chardonnay, Gray Pinot and Perera) grown in a screenhouse, which had shown clear symptoms of yellows (GYD) in previous years and had incipient symptoms on new growth. After 35 d, 3 groups of leafhoppers, collected from 3 sources of inoculum, were transferred for the transmission phase, to separate outdoor cages, each containing 3 healthy grapevines of the 2 indicators, Chardonnay and Perera. After an additional 40 d, 128 surviving S. titanus were placed in a new cage containing 1 Chardonnay grapevine and 3 young periwinkle (Catharanthus roseus) plants. Fifteen days later, 38 surviving leafhoppers were collected and used for serological and Western-blot tests with antibodies obtained from a French isolate of flavescence dorée (FD 70). In indirect DAS-ELISA, 11 of the 38 S. titanus adults from Udine, individually tested, reacted with the polyclonal antiserum against the mycoplasma-like organism (MLO) isolate FD 70, confirming the presence in the leafhopper extracts of antigens closely related to those of the French isolate. The Western-blot patterns of antigens, detected with rabbit anti FD 70 MLO antibodies, in the extracts of FD 70 and FD 88-infected leafhoppers were characteristic; in contrast, the ELISA positive leafhoppers from Udine exhibited a mixed pattern of 2 French MLO reference isolates. The number and intensity of the bands detectable in each individual extract depended on the level of the corresponding ELISA response, which in turn was related to the level of infection of the leafhopper. The results indicated a relationship between FD-sensu stricto and the form of GYD present in the F-VG region. However, although 29% of S. titanus surviving the transmission trials reacted positively in the serological assays, only 1 of 19 indicator grapevines showed GYD symptoms in 1991, and none of the 81 grapevines used in the transmission trials carried out during the previous 3 years reacted positively. In spite of the fact that S. titanus captured in GYD affected vineyards and forced on grapevines showing typical disease symptoms contained a MLO closely related to the FD agent, the question remains if this leafhopper species can be the main vector of the agent of GYD.