Isolation and Characterization of Plant Growth-Promoting Rhizobacteria from Mud Flats in South Korea: Potential for Lettuce Growth Promotion and Control of Sclerotinia Rot

Various strategies have been developed to control lettuce diseases on farms and in food-packing plants. Biological control is considered a promising alternative owing to its eco-friendly nature. In the present study, bacteria isolated from coastal mudflats were evaluated for their efficacy in controlling Sclerotinia rot, and the plant growth-promoting activity in lettuce was also assessed. Among the screened microorganisms from the coastal mudflats, 12 bacterial strains exhibited antifungal activity against Sclerotinia sclerotiorum selected. These isolates have shown beneficial characteristics, such as nitrogen fixation, indole-3-acetic acid production, phosphate solubilization, and siderophore production. Additionally, the selected isolates showed antifungal effects on the pathogens of major plant disease, such as Alternaria porri, Colletotrichum acutatum, Fusarium oxysporum, Phytophthora capsici, Pythium ultimum, Rhizoctonia solani, and Stemphylium lycopersici. Among the selected bacterial strains, Bacillus subtilis GCM190 exhibited a high sclerotinia rot control rate, similar to that of the tebuconazole-treated group, and showed a significant effect in promoting the growth of lettuce leaves, stems, and roots (least significant difference, P = 0.05). The selection of rifampicin-resistant mutants and their tracing on lettuce roots and soil confirmed that they were well established in both the soil and lettuce roots. The selected microorganisms also exhibited antifungal effects in vitro against other crop diseases affecting cucumbers, tomatoes, red peppers, and green onions, suggesting high potential for practical applications.