Effects of IAA-Producer Phyllosphere Endophytic Bacteria on Plant Growth and Bacterial Speck of Tomato

Context: Bacterial speck disease caused by Pseudomonas syringae pv. tomato (Pst) is one of the main problems in tomato production in Türkiye. Current control methods relying on copper-based bactericides are becoming less effective. Owing to the reduced effectiveness of copper, alternative management strategies are urgently needed. Aims: This study aimed to isolate and identify endophytic bacteria from the phyllosphere and subsequently evaluate the impact of selected indole-3-acetic acid (IAA)-hyperproducing isolates on tomato growth parameters and their potential for the biocontrol of Pst. Methods: Twenty-three endophytic bacteria were isolated from tomatoes and screened for plant growth-promoting traits. Subsequently, three IAA-hyperproducing bacterial isolates (Y25, Y151, and Y189) were identified. The impact of these treatments on tomato growth parameters and their efficacy in controlling Pst were assessed. Key Results: In tissue culture experiments, Pantoea sp. Y25, Flavimonas sp. Y151 and Pseudomonas sp. Y189 promoted emergence, emergence speed, and seedling growth in tomato. In growth chamber experiments, Y189 effectively reduced bacterial speck disease severity by 27% and improved shoot development under disease-free conditions. In contrast, the strains Y25 and Y151 unexpectedly inhibited plant development. Conclusions: Our findings indicate that Pseudomonas sp. Y189 is a promising candidate for biological control, demonstrating a significant 27% reduction in bacterial speck disease severity in tomatoes, alongside enhanced shoot development and overall plant growth. Implications: This study highlights the potential of Pseudomonas sp. Y189 is a sustainable and environmentally friendly alternative for managing bacterial speck disease in tomatoes.