‘Candidatus Phytoplasma solani’: From infection to all‐round characterisation of distinct tuf‐type strains with different virulence in tomato plants

‘Candidatus Phytoplasma (Ca. P.) solani’ is associated with Bois noir (BN) of grapevine and stolbur of solanaceous plants and is primarily transmitted by Hyalesthes obsoletus Signoret. Four tuf-a and five tuf-b1 ‘Ca. P. solani’ strains were transmitted to tomato plants (cv. Micro-Tom) to set the basis for studying molecular interactions between different strains of the pathogen and host plants. The strains were acquired by using bait-plants and by capturing H. obsoletus adults on bindweed and stinging nettle in vineyards of Friuli Venezia Giulia (northeastern Italy) with a high prevalence of BN. Captured insects were forced to feed on healthy tomato plants to induce infection. All strains obtained from symptomatic plants and confirmed by real-time PCR were maintained on tomato through grafting. Successively, the strains were characterised by macroscopic and microscopic symptoms induced in the host, Multi-Locus Sequence Typing (MLST) based on tuf, secY, stamp, and vmp1 genes, in-planta spread and multiplication patterns. Molecular typing distinguished the strains into five lineages comprised in three clusters: one including strains of tuf-a genotype and two including strains of tuf-b1 genotype. Quite different symptoms were induced on tomatoes by strains belonging to the two tuf genotypes; infection by tuf-a strains resulted in plant decline around 95–100 days after grafting and absence of cauliflower-like inflorescence with symptoms of phyllody and virescence, which were usually associated with tuf-b1 strains. The different symptoms, the outcome of disease, and the ultrastructural observation performed on sieve elements suggested a higher virulence of tuf-a strains in tomato. Overall, our results propose that genomic variability of ‘Ca. P. solani’ strains should be extensively explored to determine possible associations with type of symptoms and strain virulence.