More than 51 isolates of bacteria were obtained from leaves, crushed leaves and rhizosphere of lettuce plants. The bacteria isolates were purified and assayed against Cercospora lactucae-sativae on PDA plate by dual culture technique. Four isolates showed zone of inhibition against the pathogens. The antagonistic bacteria isolate LBF02 showed the highest percentage of growth inhibits against C. lactucae-sativae leaf spots with 80.82% inhibition, compared with the control. Based on morphological and biochemical tests, isolate LBF02 was identified as belonging to the Bacillus subtilis group. The LBF02 isolate was chosen for the formulation development. The formulation contained 40 ml of cell suspension, 89 g of rice flour, 1 ml of vegetable oil and 10 g of sucrose. The biocontrol of leaf spot diseases was tested by using a formulation applied in greenhouse experiments. The result showed that spraying 1 hour before or after the pathogen inoculation on lettuce plants was more effective in suppressing leaf spot disease than simply pathogen inoculation alone. Moreover, the antagonistic bacteria in formulation have the ability to survive for more than 6 months under storage at room temperature and survive for up to 15 days on lettuce leaves.
Onion bulb rot was observed in low-temperature warehouses in Mae Wang District, Chiang Mai, Thailand in 2022. The pathogens were isolated and identifi ed as Pantoea agglomerans and Burkholderia gladioli based on morphological and biochemical characterization, pathogenicity tests, and 16S rRNA gene sequences. Biological control is regarded as a safer and more sustainable strategy than chemical control and has elicited great attention for disease control. Bacillus sp. isolate BB22 was isolated from rhizosphere soil and was evaluated for its preliminary potential to inhibit the growth of onion bulb rot bacterial pathogens by the dual culture technique. The results showed inhibition zones of P. agglomerans of 30.1 mm and B. gladioli of 13.0 mm. Molecular identifi cation of Bacillus sp. isolate BB22 was performed by using 16S rRNA and gyrA gene sequences and confi rmed as Bacillus velezensis. PCR amplifi cation of the bmyB, fenD, ituA, and srfAA genes coding for bacillomycin, fengycin D, iturin A, and surfactin, respectively, showed the potential for the production of these antibiotics. This study revealed the potential application of B. velezensis as a biocontrol agent against bulb rot-causing bacterial pathogens of onions.
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