Screening Of Antagonistic Bacterium Against Pathogens Of Tobacco Brown Spot Disease And Analysis Of Their 16S RDNA PCR-RFLP

Tobacco brown spot disease has been an important leaf disease in tobacco. In tobacco maturation period, this disease causes tobacco brown spot disease seriously, and makes large economic losses. Its main pathogen is Alternaria alterrrata (Fries) Keisslar. In order to avoid the pollution of environment and tobacco leaf, it is necessary to use biological control. In this dissertation, antagonistic bacterium that can control tobacco brown spot disease are screened by dual cultures on PDA.2,846 bacterial strains were isolated from the soil and leaf samples. These strains were screened for antagonists against A. alterrrata by dual cultures on PDA. The result showed that 22 strains were strongly antagonistic to the pathogen, and one hundred and 114 strains were antagonistic to the pathogen too.136 antagonistic strains were analysed by 16S rDNA PCR-RFLP. The result showed that all tested strains were divided into 3 groups. 11 antagonistic strains wered analysed by the sequence analysis of 16S rDNA. The result showed that these tested strains were also divided into 3 groups.Strain B75 was strongly antagonistic to tobacco brown spot disease in pot test and field test. According to its morphological characteristics, strain B75 was considered to be the member of genus Bacillus. According to its physiological and biochemistrical characteristics, the strain was confirmed to be one strain of B. subtilis. Strain B75 shared 100% homology with published sequence of B. subtilis(accession number: AY03031) by this means of sequence analysis of 16S rDNA. In the phylogenetic tree, both strain B75 and B. subtilis were on the same branch, which showed that strain B75 was closely related with B. subtilis. So we can make a conclusion that the isolate B75 was one strain of B. subtilis.