New insights into Pseudomonas syringae species complex causing leaf spots and bunch necrosis on table grape in Puglia

Pseudomonas syringae is described as a species complex including different pathovars and the related species and acting as plant pathogens and beneficials. In 2025, fields surveys were conducted in 22 vineyards located in Bari province of cvs. Allison (1), Autumn Crisp (3), Italia (3), Ivory (1), Jack's Salute (1), Red Globe (6), Regal (1), Ruby Rush (1), Scarlotta (1), Sweet Globe (3), and Superior (1) at the phenological stages from BBCH55 (Inflorescences swelling, flowers closely pressed together) to BBCH75 (Berries pea-sized, bunches hang). Symptomatic shoots, leaves and bunches showing respectively black streak, leaf spots and bunch necrosis were sampled and bacteria were isolated on nutrient agar medium. Bacterial colonies were obtained from 'Red Globe' and 'Sweet Globe' showing leaf spots and bunch necrosis. The colonies were transferred to King B medium, and the fluorescent ones were transferred to hypersucrose medium, where two main groups of isolates were observed. One showed levaniform colonies, addressing P. syringae pv. syringae (Pss), and the other convex colonies with yellow mucoid material, addressing Pseudomonas viridiflava. All were negative to oxidase test and positive to the tobacco hypersensitivity test. Conversely to Pss, P. viridiflava isolates were also positive to potato soft rot test. All isolates were inoculated on grape leaves and wide necrotic area appeared after 48 h. 16SrDNA-BLASTn analysis confirmed the morphological and biochemical identification. In conclusion, we report that Pss and P. viridiflava are associated with leaf spots and bunch necrosis on table grapes, and their management should be considered in crop protection strategies.