New 'Candidatus Phytoplasma solani' strains associated with Bois noir disease in Vitis vinifera L. cultivars in Georgia

Preliminary survey highlighted that bois noir (BN) disease, associated with 'Candidatus Phytoplasma (‘Ca. P.’) solani', affects grapevine varieties in Georgia (South Caucasus). In this study, further research was carried out to investigate the BN symptom severity in international and Georgian native varieties. Identification and characterization of 'Ca. P. solani' was performed by analysis of 16S rDNA, vmp1 and stamp gene nucleotide sequences. During field surveys, moderate/mild and severe symptoms were observed on Georgian grapevine varieties and international cultivars, respectively. Molecular characterization of 'Ca. P. solani' identified in grapevines revealed the presence of 11 genetically distinct phytoplasma types. Ten of such 'Ca. P. solani' types were described here for the first time; only one type (VmGe12/StGe7) was identical to a strain previously reported in periwinkle in Lebanon. Phylogenetic analyses of vmp1 and stamp gene concatenated nucleotide sequences indicated that 'Ca. P. solani' strains in Georgia are associated mainly with the bindweed-related BN host system. Moreover, the presence of the same 'Ca. P. solani' strains in grapevine cultivars showing a range of symptom intensity suggested a different susceptibility of Georgian local varieties to BN.