Characterization of Pseudomonas mediterranea and Pseudomonas viridiflava strains associated with tomato pith necrosis in the Al Taif region, Saudi Arabia

Tomato pith necrosis is a damaging disease affecting greenhouse and field-grown tomato production globally, often associated with environmental stress and excessive nitrogen fertilization. This study investigates the bacterial etiology and the (inter)national context of disease outbreaks during the 2023-2024 growing seasons in Al Taif, Saudi Arabia (SA). Sixteen bacterial isolates were recovered from symptomatic plants, and phenotypic and molecular analyses identified Pseudomonas mediterranea (n = 12) and Pseudomonas viridiflava (n = 4) as the causal agents, although Pseudomonas corrugata and Pseudomonas fluorescens biotype I have also been reported from SA in the past. P. mediterranea was the most prevalent species, consistent with earlier findings in southern Europe. Genetic characterization using multilocus sequence typing and minimum spanning tree analysis revealed high genetic diversity in P. viridiflava, including a novel singleton lineage, indicating potential local adaptation, and all SA strains belonged to Pseudomonas syringae (sensu latu) phylogroup 7a. In contrast, P. mediterranea isolates exhibited lower genetic diversity, but six novel sequence types (ST8-ST13) were identified exclusively in SA strains, forming a distinct phylogenetic clade. Pathogenicity assays showed that P. viridiflava caused severe necrosis, while P. mediterranea caused internal browning without wilting, suggesting a difference in virulence. The co-occurrence of both species in the same greenhouses highlights the complexity of disease dynamics and potential interactions between these pathogens. Given the global relevance of P. syringae phylogroup 7a, mainly formed by P. viridiflava strains, and the risk of pathogen spread via plant trade, this study underscores the importance of stringent phytosanitary controls and accurate diagnostics tailored to regional pathogen variants.