Flavescence dorée phytoplasma titre in field‐infected B arbera and N ebbiolo grapevines

Flavescence dorée phytoplasma ( FDP ) titre in two red grapevine cultivars, B arbera and N ebbiolo, was measured over the vegetative seasons of two consecutive years in two vineyards of the P iemonte R egion (northwestern I taly), with a double absolute quantification of FDP cells and grapevine DNA in real‐time PCR . The relationships of pathogen concentration to cultivar susceptibility and symptom severity were investigated. FDP titre was always higher in cv. Barbera than in cv. N ebbiolo infected vines, and this difference was significant at early and late summer samplings of 2008 and at early summer sampling of 2009. A seasonal trend in FDP concentration (low in spring, high in early summer and intermediate in late summer) was conserved for cvs B arbera and N ebbiolo in both years and vineyards. Considering both cultivars and years from both vineyards, a significant positive correlation between FDP concentration and symptom severity was found in the spring samples. Regarding the FDP strains (‐ C or ‐ D ), no differences in pathogen titres were detected for either cultivar. Similarly, the presence of another grapevine yellows phytoplasma, bois noir, a subgroup 16Sr XII ‐ A phytoplasma, in mixed infection with FDP strains had no effect on FDP concentration. These results demonstrate for the first time that grapevine cultivars with different susceptibility to FDP support different pathogen titres.