Molecular typing of 'bois noir' phytoplasma strains in the Chianti Classico area (Tuscany, central Italy) and their association with symptom severity in Vitis vinifera L. cv. Sangiovese

Bois Noir (BN) is the most widespread disease of the grapevine yellows complex in the Euro-Mediterranean area. BN is caused by 'Candidatus Phytoplasma solani' (BNp), transmitted from herbaceous plants to grapevine by polyphagous insect vectors. In this study, genetic diversity among BNp strains, their prevalence and possible association with grapevine symptom severity were investigated in a Sangiovese clone organic vineyard, in the Chianti Classico area (Tuscany). Field surveys over two years revealed a range of symptom severity on grapevine and an increase of BN incidence. TaqMan allelic discrimination assay detected only tufB-type b among BNp strains, suggesting the prevalence of the bindweed-related ecology. Nucleotide sequence analyses of vmp1 and stamp genes identified 12 vmp1 and 16 stamp sequence variants, showing an overall positive selection for such genes. The prevalent genotype was Vm43/St10, reported for the first time in this study and closely related to strains identified only in the French Eastern Pyrenees. BNp strains, identified in the examined vineyard and mostly grouped in separate bindweed-related phylogenetic clusters, showed statistically significant differences in their distribution in grapevines exhibiting distinct symptom severity. These results suggest the possible occurrence of a range of virulence within BNp strain populations in the Chianti Classico area.