Production of phytotoxic metabolites by pseudomonas syringae pv. Actinidiae, the causal agent of bacterial canker of kiwifruit

Pseudomonas syringae pv. actinidiae, the causal agent of bacterial canker of Actinidia chinensis and A. deliciosa, is currently causing severe economic losses worldwide. A study was conducted to verify if a highly virulent Psa strain, solated during the current outbreaks of bacterial canker of kiwifruit in Italy, produces phytotoxic metabolites in vitro. Culture filtrate, obtained from 14-day-old cells grown in Pseudomonas minimal medium, induced an evident hypersensitivity-like reaction to both tobacco and kiwifruit leaves. From culture filtrates, extracts were obtained using different solvents and pH values. The extracts and their corresponding aqueous phases, were further tested for phytotoxicity. Basic, hydrophilic, low- molecular weight and hydrophilic, high-molecular weight compounds belonging to exopolysaccharides were isolated and analyzed. These compounds proved highly phytotoxic to kiwifruit, tobacco leaves and lemon fruits. Gas-chromatography mass-spectrometry analysis carried out on crude exopolysaccharides showed glucose as the main monosaccharide constituent. These results suggest that phytotoxic metabolites, other than the antimetabolite phaseolotoxin, could be involved in the virulence of the pathogen to kiwifruit species.