Using certain endophytic bacteria as biocontrol agents against bacterial speck disease of tomato

Bacterial speck, caused by Pseudomonas syringae pv. tomato (Pst), is a major constraint to tomato production worldwide and the limited efficacy of chemical control indicates that there are sustainable alternatives. This study aims to evaluate the antagonistic potential of endophytic bacteria isolated from healthy tomato tissues as biocontrol agents against Pst in order to identify effective candidates for sustainable disease management. In vitro screening of 15 isolates revealed that strains KAU7 and KAU11 exhibited the strongest antibacterial activity, producing inhibition zones of approximately 1.33 and 1.23 cm, comparable to streptomycin. Biochemical characterization showed that both isolates produced siderophores, solubilized phosphate and synthesized indole-3-acetic acid (IAA), indicating their ability to promote plant growth and suppress pathogens. Molecular identification confirmed that KAU7 and KAU11 were Bacillus halotolerans and Bacillus paralicheniformis, respectively. In greenhouse trials, both strains significantly reduced disease severity and B. paralicheniformis achieved the highest reduction (32–45%), depending on application method. Treatments also enhanced plant growth and increased defense-related enzyme activity, specifically peroxidase and polyphenol oxidase. The present study demonstrates that B. halotolerans and B. paralicheniformis effectively suppress bacterial speck disease in tomato plant growth and inducing defense responses. These strains represent promising alternatives to chemical bactericides in integrated disease management strategies.