Biological Control of Tomato Bacterial Leaf Spots and Its Impact on Some Antioxidant Enzymes, Phenolic Compounds, and Pigment Content

Abstract

Tomato bacterial spots, caused by <i>Xanthomonas campestris</i> pv. <i>vesicatoria</i> (<i>Xcv1</i>) and <i>X. euvesicatoria</i> (<i>Xe2</i>), as well as bacterial specks, caused by two strains of <i>Pseudomonas syringae</i> pv. <i>tomato</i> (<i>Pst1</i> and <i>Pst2</i>), represent significant threats to tomato production in the El-Sharkia governorate, often resulting in substantial yield losses. The objective of this study was to evaluate the efficacy of various biocontrol culture filtrates, including bacteria and fungi agents, in managing the occurrence and severity of these diseases, while also monitoring physiological changes in tomato leaves, including antioxidant enzymes, phenolics, and pigment content. The culture filtrates from examined <i>Trichoderma</i> species (<i>T. viride, T. harzianum</i>, and <i>T. album</i>), as well as the tested bacteria (<i>Bacillus subtilis</i>, <i>Pseudomonas fluorescens</i>, and <i>Serratia marcescens</i>) at concentrations of 25%, 50%, and 100%, significantly inhibited the proliferation of pathogenic bacteria In vitro. For the In vivo experiments, we used specific doses of 5 mL of spore suspension per plant for the fungal bioagents at a concentration of 2.5 × 10⁷ spores/mL. The bacterial bioagents were applied as a 10 mL suspension per plant at a concentration of 1 × 10⁸ CFU/mL. Spraying the culture filtrates of the tested bioagents two days before infection In vivo significantly reduced disease incidence and severity. <i>Trichoderma viride</i> exhibited the highest efficacy among the fungal bioagents, followed by <i>T. harzianum</i> and <i>T. album</i>. Meanwhile, the culture filtrate of <i>B. subtilis</i> emerged as the most potent among the bacterial bioagents, followed by <i>P. fluorescens</i>. Furthermore, applying these culture filtrates resulted in elevated levels of chitinase, peroxidase, and polyphenol oxidase activity. This effect extended to increased phenol contents, as well as chlorophyll a, chlorophyll b, and carotenoids in sprayed tomato plants compared to the control treatment. Overall, these findings underscore the potential of these biocontrol strategies to effectively mitigate disease incidence and severity while enhancing plant defense mechanisms and physiological parameters, thus offering promising avenues for sustainable disease management in tomato production.