Biocontrol efficacy of Bacillus siamensis LZ88 against brown spot disease of tobacco caused by Alternaria alternata

Tobacco brown spot disease, caused by Alternaria alternata, is responsible for causing significant losses in tobacco production in China. The use of biocontrol microorganisms to prevent and control this disease represent an important, environment-friendly, management strategy. In the present study, 17 bacterial strains with antimicrobial activity were isolated from 40 rhizosphere soil samples and evaluated for antifungal activity to identify an effective biocontrol agent for the control of tobacco brown spot disease in tobacco. Among the isolated strains, Bacillus siamensis, strain LZ88, exhibited the strongest antifungal activity with an inhibition rate of 81.96%. We confirmed that strain LZ88 controlled both mycelial growth and spore production in A. alternata by secreting protein-like antifungal metabolites and through the release of volatile organic compounds. Strain LZ88 also had the ability to induce plant basal immunity as evidenced by increased expression of the defense-related enzymes, peroxidase (POD) and polyphenol oxidase (PPO), in tobacco leaves. Results of greenhouse experiments indicated that LZ88 could effectively reduce the occurrence and degree of severity of tobacco brown spot disease. To the best of our knowledge, this is the first report regarding the use of B. siamensis to control brown spot disease in tobacco.