Influence of Different Application Methods of Crude Lipopolysaccharides on Plant Growth, Productivity and on the Control of Tomato Bacterial Leaf Spot and Powdery Mildew

Lipopolysaccharides (LPS) are structural components of gram-negative bacteria that potentially induce innate immune responses in plants. A previous study demonstrated that an LPS acts as an elicitor, triggering systemic resistance against bacterial spot in tomato caused by Xanthomonas euvesicatoria pv. perforans. However, its long-term impact on plant growth, disease control and productivity remained unclear. The first experiment determined the optimal concentration of the crude LPS for effective bacterial spot control without deleterious effects on plants. The second experiment evaluated LPS application methods and timings that provide balanced benefits for plant growth, productivity and control of bacterial spot and powdery mildew. Additionally, the involvement of polyphenoloxidase and phenylalanine ammonia-lyase (PAL) in disease resistance was also investigated through gene expression assays. Results estimated 7.3 μg/mL as the optimal dose for minimising disease severity without compromising chlorophyll content. At least one application in the vegetative stage, using either electrostatic spraying or soil dispensing, yielded the most balanced outcomes for plant growth, productivity and disease control. Electrostatic spraying provided an average reduction in severity of 60% for bacterial spot and 32% for powdery mildew, while soil dispensing provided 58% and 28%, respectively. These treatments also promoted increased PAL gene expression, suggesting that this enzyme is involved in the resistance response.