Study On Pathogenicity Differentiations Of Alternaria Alternata And Antagonistic Mechanism Of Biocontrol Agent Bacterium

Tobacco brown spot caused by Alternaria alternate is one of the main diseases in maturation and harvest stage of tobacco leaves, lowering seriously the yield and quality of tobacco leaves, Now, the pathogen pathogenicity differentiation of Tobacco brown spot disease have been reported many times, different strains have differences Toxic, but most of the Pathogenicity are in the middle level on the whole, showed strong virulence and serious harm, Bijie as the second major tobacco areas of Guizhou Province, brown spot disease has no related reports, so this study collected Bijie main tobacco areas of Pathogens and test the pathogenicity differentiation, biological characteristics of the isolates with different Pathogenicity, the four extracellular enzyme activities of A.alternate and their relationship to Pathogenicity were in vestigated. Meanwhile, The antagonism and mechanism of L2 which was separated in Bijie tobacco areas against the A.alternata were explored and provides a theoretical basis for the development of biocontrol agent against tobacco brown spot disease in this Paper, the main results are as follows:1、By using tissue culture method and single spore isolation of Pathogen, 35 strains were obtained by morphological observation of A.alternata, then 4 representative strains of this were identified, The results showed Gzuf A-10 and Gzuf A-39 was identified as Alternaria alternata, Gzuf A-26 and Gzuf A-40 was identified as Alternaria tenuissima.2、The detached leaves of tobacco were inoculated with agar disk of these 35 isolates. The results showed different strains had obvious differences on leaves of tobacc o cultivar K326. Four strains showed strong virulent, but four strains were stronger than those. While fourteen strains showed moderate virulent and eight those were weak virulent, five avirulent strains the majority strains showed moderate virulent. According to the observation also found that the penetrating and extending ability of the hypha on detaehed leaves was positively correlated to the pathogenicity.3、Based on the pathogenic ity test, pathogenicity of different isolates were selected to probe the relativities among the characters. The result showed that the mycelium growth rates, conidium production were gradually increase with pathogenicity weak, however toxin production were on the contrary.4、The activities of four extracellular enzymes of A.alternata and the relationship between extracellular enzyme activity and pathogenic ity were studied. The result showed that the activities of amylase, cellulose of the strong pathogenic strains, were all greater than those intermediate-pathogenic and weak pathogenic strains signif icantly. However, there was no signif ieant difference in the activities of laccase and guaiacol oxidase among the three pathogenic types of A.alternate. This shows that amylase and cellulase activity of Bijie tobacco brown spot pathogen is an important cause of pathogenicity differentiation.5、The antagonistic strain L2 was isolated from 164 soil samples of plant rhizosphere against Alternaria alternata, which was identif ied as B acil lus magaterium on the basis of its morphological, biochemical and physiological characteristics, and16 s r DNA gene sequence identification and analys is.6、Experiments were carried out on the stability of Metabolites of strain L2 and The best way to inhibited A.alternata and effects on pathogenic bacteria growth and development, results showed that: the sterile filtrate of strain L2 on the thermal stability, resistance to ultraviolet, but unstable in acidic conditions; In vitro leaves, strain L2 against A.alternata and optimum control way is passivation; L2 crude metabolites also have strong inhibited on mycelium growth, spore germination and sporulation. The changes of defense enzymes induced by L2 in tobacco leaves were studied. Results showed that the activities of defense enzymes(PAL, PPO, POD) were increased to some extent after inoculation with L2. It could come to a conclusion that induced resistance was one of the mechanisms of biocontrol.