Molecular identification and characterization of phytoplasmas infecting tomato in Oltrepò pavese (northern Italy)

Tomato is affected by economically important phytoplasma diseases, such as stolbur, tomato big bud, tomato yellows, tomato witches’-broom, tomato little leaf, and hojia de perejil. Such diseases, characterized by different geographic distribution patterns, have been associated with eight distinct ‘Candidatus Phytoplasma’ species in at least 30 countries throughout the world. In Italy, phytoplasma diseases of tomato were associated with infections by ‘Ca. Phytoplasma solani’, ‘Ca. Phytoplasma asteris’ and phytoplasmas of taxonomic groups 16SrIII and 16SrV. The present work aimed to investigate the aetiology of a disease affecting tomato in Oltrepò pavese (Lombardy, North Italy). During field surveys, carried out in a tomato plantation in August 2015, symptoms typically associated with phytoplasma infection (leaf purpling, witches’-broom, flower and fruit alterations, and stunting) were observed on tomato plants. Based on 16S rRNA gene amplification and nucleotide sequence analyses, ‘Ca. P. solani’ was detected in 82% of symptomatic tomato plants and in bindweed plants. Molecular characterization based on tufB and stamp gene nucleotide sequence analyses revealed the presence of two 'Ca. P. solani' strain types: the type tufB-b/St5, identified respectively in 83% and 100% of tomato and bindweed infected plants, and the type tufB-a/St18 identified in 17% of tomato infected plants. These results suggest that ‘Ca. P. solani’ ecology in the examined agro-ecosystem is associated mainly with the bindweed-related host system. Based on such evidences, it should be interesting to evaluate the incidence dynamics of the disease and to investigate its epidemiology by tracing the movements of ‘Ca. P. solani’ throughout neighbouring fields.