Synergistic interactions between Pseudomonas spp. and Xanthomonas perforans in enhancing tomato pith necrosis symptoms.

Surveys carried out in soil-grown tomatoes in Sicily ( Italy) showed that Pseudomonas spp. strains were frequently associated with Xanthomonas perforans, causal agent of tomato pith necrosis. In this study, 30 Pseudomonas spp. strains from symptomatic tomato tissues and International Collections with biocontrol properties, were evaluated for pathogenicity on tomato plants. All Pseudomonas spp. strains were able to induce brown vascular discoloration and some of them showed in vitro antagonistic activity against X. perforans. Selected strains were tested for their effects in enhancing tomato pith necrosis caused by X. perforans in co-inoculation experiments. Six strains belonging to Pseudomonas fluorescens, P. putida, P. citronellolis and P. protegens species significantly increased pith necrosis and vascular discoloration symptoms when co-inoculated with X. perforans on tomato plants. In these co-inoculations, X. perforans population density resulted significantly higher than that of X. perforans inoculated individually. The results of this work highlight the need for a careful evaluation of biological control agents during the screening procedures and before commercialization to avoid the risk of introducing strains that could be pathogenic on tomato plants