Common Bacillus mitigate tomato verticillium wilt and bacterial specks when combined with an essential oil extract

This study was conducted to evaluate the effect of growth promoting rhizobacteria (PGPR) alone and in combination with an essential oil (EO) extracted from Tetraclinis articulata (Vahl) Masters in protecting tomato plants against verticillium wilt and bacterial speck diseases. Three strains, identified as Bacillus toyonensis EI, Bacillus thuringiensis ES and Bacillus thuringiensis RA, possess in vitro PGPR activity such as the production of indole acetic acid and ammonia, and the solubilization of phosphates. In planta, they significantly increased the stem length, number of leaflets, leaf area, and root biomass. EO and the three strains, alone or in combination, were able to enhance photosynthetic parameters such as net photosynthesis and transpiration rate. They also significantly reduced leaf alteration (LA) index, stunting index (SI) and browning index (BI) caused by Verticillium dahliae kleb. In tomato. Reduction of LAI ranged from 25% to 37%, and it enhanced to more than 60% when plants were treated with EO combined with B. thuringiensis SE or with B. toyonensis EI, indicating a synergistic effect. Moreover, EO as well as the three Bacillus strains were also able to enhance tomato resistance against bacterial speck disease caused by Pseudomonas syringae pv. tomato. The average number of lesions was significantly reduced by the application of EO and the three strains, reaching 94% with combination of EO with B. thuringiensis RA. Bacterial population analyses showed that plants with reduced symptoms exhibited significantly lower bacterial concentrations. In tomato plants inoculated with V. dahliae results showed that EO as well as three strains were able to enhance H₂O₂ accumulation, at the late stage of infection, which might lead to reduce disease. However, within the pathosystem tomato/P. syringe pv. tomato, H₂O₂ accumulation was highly enhanced by the pathogen and inhibited after combined treatment.