Abstract: Tobacco target spot is an important leaf disease that endangers tobacco production, and it is important to screen biocontrol strains against R. solani and optimize the fermentation conditions of the strains. To control tobacco target spot, biocontrol strains were isolated from the rhizosphere soil of healthy tobacco by dilution coating plate method and plate confrontation test. Strain MLY-3 with 80.92% inhibition rate against R. solani was screened out. Microscopic examination revealed that strain MLY-3 induced distortion and deformity of the mycelium during confrontation culture. Strain MLY-3 was identified as P. aeruginosa by morphological observation, physiological and biochemical detection and molecular analysis. Single factor test and orthogonal test results showed that the optimal fermentation medium components of strain MLY-3 were as follows: mannitol 10 g/L, beef extract 5 g/L and MgCl₂ 7.5 g/L. The optimal fermentation conditions were as follows: initial pH 5.5, inoculum 3%, temperature 30℃ and time 36 h. The inhibition rate of its aseptic fermentation broth against R. solani was increased by 53.8% under optimal fermentation conditions. The field control effect of strain MLY-3 fermentation broth against tobacco target spot was 70.03%, indicating great potential for biocontrol application. This study enriched the biocontrol resource of tobacco target spot and laid the foundation for its application.

Key words: tobacco target spot, Pseudomonas aeruginosa, optimization of fermentation conditions, control effect