Study On Antagonistic Bacterium And Its Inhibiting Mechanisisms Against Pathogens Of Strawberry Replant Disease

Fourty bacteria strains were isolated from decomposed medicinal plant materials and soil samples treated by mixed medicinal plant materials. Only 1 strain had inhibition effect to three strawbeery replant pathogen strongly. The inhibition zone to Rhizoctonia solani, Fusarium oxysporum and Verucillium dahliae all surpassed 5 mm. And the inhibition zone width did not change along with time.The results of the identification demonstrated that the morphological characterization, culturmg pattern, physiology and biochemical characteristics were accordant with those of Bacillus subtilis descibed by Bergey's Manual of Determinative Bacteriology (Eighth Edition). Therefore, Bs-21 was identified as Bacillus subtilis.Through studying on the antagonistic mechanism, the results showed that both Bs-21 strain and its metabolizable production can inhibit the pathogens. Micropscope observation showed that the mycelium of the three pathogenic fungi appeared distortion, with protoplasm agglomerated and leaking of cell contents after treated by antagonistic bacteria. It seemed that Bs-21 could secrete polypeptide and hydrolytic enzyme.Conidial germination experiments were used to test the inhibition effects of different fermentation liquids. The result showed that the spore germination of E oxysporum, and Verticillium dahliae was inhibited by different treatments. And the spores were deformed. Autoclaved ferment of Bs-21 significantly inhibited the conidial germination of Fusarium oxysporum in marked level compared with ferment of Bs-21 sterilized by filtrate. It indicated that Bs-21 could secrete some kind of substance with stronger antagonistic activity than unheated.Autoclaved ferment and sterilized ferment of Bs-21 by filtrate were tested for their inhibitive effects to three pathogens of strawberry replant disease at different dilutions. Results showed that both ferment had inhibitory effects at different levels to different pathgens. The inhibition effect to Rhizoctonia solani is feeble, while is strong to Fusarium oxysporum and Verucillium dahliae. It is concluded that Bs-21 could secrete partially thermostable substance.Because of good antibiosis against three phytopathogenic fungi, bacterium Bs-21 had the potential to be used as biocontrol agent. For the further studies, solid state fermentation (SSF) was chosen as fermentation method. The medium was made up of three mixed medicinal plant materials with corn straw powder in a proportion of 1:1:1:20. The optimal culture conditions were finally determined as: inoculation level 10%, culture temperature 34°C, 80% initial moisture content and initial pH 7.5. The above conditions were the most suitable for strain Bs-21 to grow with the ultimate bioasis.