Antagonistic Mechanism of Brevibacillus brevs A57 against Cotton Mainly Pathogenic Fungus

The bacterial strain A57(Brevibacillus brevs) isolated from cotton soil showed stronger antagonism against Rhizoctonia solani,Fusarium oxysporum f.sp.vasinfectum and Verticillium dahliae,its average inhibitory rate is 33.5%,39.5% 29.5% respectively on PDA plate.The extracted filtrate from cotton seedling which is treated by the antagonistic bacteria's extract filtrate,tested activity against Fusarium oxysporum f.sp.vasinfectum.The result showed inhibition rate was 5.7%.Cotton seed was treated by the filtrate liquid of A57 strain with different dilution times,and was planted in the nutrition soil which was inoculated by three pathogens respectively,the control effect of the filtrates produced from antagonistic bacteria A57 with 10 folds dilution against damping off caused by Rhizoctonia solani,Fusarium wilt and Verticillium wilt,was 49.8%,56.8% and 48.9% respectively.The antagonism of exudates filtrate of A57antagonistic strain with different dilution times was produced too on modified PDA plates,we put the asepsis seed embryo on it,we found that the extract filtrate enhanced the germination of cotton seed.Hypocotyl length by extract filtrate treatment is 20.2 mm,and hypocotyl length by control treatment is 16.44 mm,but the dilution 10 folds of the filtrate liquid caused radical intumescence and abnormality.The examination of antagonistic mechanism under light microscope showed that the antagonistic substance of A57 appeared to inhibit pathogens by leading to mycelium rupture,distortion,abnormality,and inhibition bourgeon of conidiophore,and leading to abnormality of conidiophore.The crude protein of A57 obtained by(NH4)2SO4 precipitation still has the stronger antagonistic activity against the pathogen,the inhibitory rate of the crude protein is 23.7%,A57 crude protein is centrifugalized and the supernatant also has the antagonistic activity against the pathogen,that suggest the antagonistic substance of A57 contain not only protien but also small molecule substance.The antagonistic bacteria's fermentation condition was produced,the results indicated that the best fermentation condition of A57 with the strongest antagonistic activity against Fusarium oxysporum f.sp.vasinfectum was as follows : LB culture medium,pH8,requiring oxygen.