| Tobacco is one of cash crops, which request rather highly for the quality and security. Whereas tobacco brown spot, tobacco black shank and tobacco bacterial blight are the most serious diseases, except tobacco virus disease in the loss extent of economy. Biocontrol is an unpromising way to control these tobacco diseases in many methods of disease-control. Bacillus spp. takes on potential disease-control remarkablely, which is an important part of preventing and curing plant diseases. Pseudomonas fluorescent consists in soil abroadly. Majority of Pseudomonas fluorescent strains are provided with the functions of promotion or resistion. The antagonistic bacteria of tobacco brown spot, tobacco black shank and tobacco bacterial blight were screened and preliminary identify from soil samples in the rhizosphere of tobacco in Chongqing in this study.1. Seven hundred and fifty two bacterial isolates in the rhizosphere of tobacco were obtained from eighty five soil samples in Shizhu County, Qianjiang County, Fengjie County, etc., in Chongqing. Four hundred and ninety nine of these isolates were Bacillus spp. with high temperature, and other two hundred and fifty three isolates were Pseudomonas fluorescent, on KMB plate.2. It was found that three hundred and forty eight isolates showed antagonistic effects on Alternaria alternata, Phytophthora nicotianae and Ralstonia solanacearum with the method of pairing culture on PDA or NA plate. These antagonistic bacteria could be classified four grades. By further pairing culture on plate for the second time, the results showed that the inhibition zone of twenty nine strains against A. alternata ranged from 8.0 to 12.0mm, twelve strains were Bacillus spp. and seventeen strains were P. fluorescent. The inhibition zone of twenty-six strains against P. nicotianae ranged from 9.0 to 14.0mm, two strains were Bacillus spp. and twenty-four were P. fluorescent. The inhibition diameter of thirty-one strains against R. solanacearum ranged from 30.0 to 42.0mm, nine strains were Bacillus spp., and twenty-two were P. fluorescent. 3. The results in the greenhouse experiment showed the controlling efficiency were from 31.31% to 87.51% against tobacco brown spot, and the P-72-10 reached 87.51%. The controlling efficiency of four strains was above 80.00%. The results in the greenhouse experiment showed the controlling efficiency was from 30.44% to 73.91%, and the P-71-2 reached 73.91%. The controlling efficiency of two strains was above 70.00%. The results in the greenhouse experiment showed the controlling efficiency was from 46.00% to 86.00%, and the P-76-2 reached 86.00%. The controlling efficiency of five strains was above 80.00%.4. Comparison of the antagonistic effect of different volume ratio with the same concentration of B-43-6 and P-72-10's mixed-zymolysis on A. alternate, the combination v/v = 4:1 of these two bacteria showed the best controlling efficiency. As a result, the inhibition zone was 7.6mm against A. alternate with the method of pairing culture on PDA, while the disease biocontrol efficiency attained 91.83% by spraying inoculation in the greenhouse experiment. The results of two strains B-57-4 and P-70-3's mixed-zymolysis (v/v = 3:1) in the greenhouse experiment showed the best controlling efficiency. The inhibition zone of these two strains' mixed-zymolysis (v/v = 3:1) was 5.1mm against R. solanacearum with the method of pairing culture on NA, while the effect of disease controlling reached 92.00% on tobacco bacterial blight by the inoculation with irrigating roots.5. The 10 antagonistic bacteria screened from tobacco rhizosphere by pairing culture and greenhouse experiment It was found these isolates showed wide antifungal spectrum through pairing culture on plate, which indicated these bacteria had better repressing impact on B. maydis, G. zeae and T.basicola.6. According to the observation of cell morphology and biochemical test data, P-3-1, P-69-6, P-70-3, P-71-2 and P-72-10 strains were identified to be Pseudomonas fluorescent, B-16-4 and B-61-10 strains were identified to be Bacillus subtilis, B-43-6, B-46-2 and B-57-4 strains were identified to be Bacillus megaterium.
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