Study of phytoplasma-associated grapevine yellows diseases in Georgia

Flavescence dorée (FD) and Bois noir (BN) are two phytoplasma-associated diseases, belonging to the grapevine yellows (GY) complex, responsible for serious crop losses in the Euro-Mediterranean area and in other continents. Even their symptoms are undistinguishable, FD and BN are associated with phytoplasmas distinct at both taxonomic/genetic and ecological/epidemiological level. FD phytoplasmas are efficiently transmitted to grapevine by the Scaphoideus titanus, a leafhopper sustaining its whole life cycle on Vitis spp. Consequently, geographic areas hosting large vector populations and FD phytoplasmas can be damaged by strong FD epidemics. Due to this aspect, FD phytoplasmas are quarantine pathogens to be controlled through mandatory measures. On the other hand, BN phytoplasmas are occasionally transmitted to grapevine by Hyalesthes obsoletus, a polyphagous vector living preferentially on nettle, bindweed, and chaste tree. Due to its complex epidemiological cycle, it is difficult to design efficient control strategies against BN. Preliminary survey highlighted that BN affects grapevine varieties in Georgia, while FD was not reported. Further research was carried out to investigate the BN symptom severity in international and Georgian native varieties, and its epidemiology. Identification and characterization of BN phytoplasma was performed by analysis of multiple gene nucleotide sequences. During field surveys, moderate/mild and severe symptoms were observed on Georgian grapevine varieties and international cultivars, respectively. Molecular characterization revealed the presence of several genetically distinct BN phytoplasma types described here for the first time. Molecular detection, supported by phylogenetic analyses, indicated that BN phytoplasma strains in Georgia are associated mainly with the bindweed-related host system. Moreover, the presence of the same phytpoplasma strains in grapevine cultivars showing a range of symptom intensity suggested a different susceptibility of Georgian local varieties to BN. To prevent the spread of GY diseases, further studies are needed to survey BN and FD phytoplasmas in Georgian vineyards and nurseries.