Prevalence of a ‘Candidatus Phytoplasma solani’ strain, so far associated only with other hosts, in Bois Noir-affected grapevines within Tuscan vineyards

Due to its complex epidemiological cycle, including several polyphagous insect vectors and host plants, and the absence of efficient control strategies, Bois Noir (BN) disease of grapevine is encroaching wider territories in the main viticultural areas worldwide. Molecular approaches allowed to increase the knowledge about its etiological agent (Bois Noir phytoplasma, BNp; ‘Candidatus Phytoplasma solani’ species), revealing interesting features concerning BNp population structure and dynamics and transmission routes in vineyard agro-ecosystems. In the present study, a multilocus sequence typing approach (vmp1 and stamp genes) was utilised for describing the genetic diversity among BNp strain populations in 17 vineyards localised in two distinct geographic areas in Tuscany (central Italy). The results confirmed that BNp ecology in Tuscan vineyards is mainly associated to the bindweed-related host system, and allowed the identification of 14 BNp vmp1/stamp genotypes. Interestingly, the prevalent genotype (Vm43/St10) was never found in grapevines outside of Tuscany. Moreover, statistical analyses showed significant differences between the composition of BNp strain populations identified in grapevines from north-western and central-eastern Tuscany. These results reinforce the hypothesis that distinct geographic areas, probably associated with different ecological niches, can drive the selection of BNp strains, also favouring the entrance of unusual ‘Ca. Phytoplasma solani’ genotypes in vineyards.