| Tobacco brown spot caused by Alternaria alternata (Fries) Keissler is an important disease on the crop worldwide. Presently pesticides application is the major measure of controlling the disease, but the use of pesticides is restricted because this may induce problems as fungicide resistance, product quality decrease and chemical residues in the environment. Biocontrol may be an alternative to avoid these problems in tobacco disease control. In the present study, antagonistic bacteria against A. alternata were screened from epiphytic and endophytic isolates of tobacco leaves, and the biocontrol efficiency and mechanisms of the antagonistic bacteria against the pathogen was analyzed.1. A total number of 302 endophytic bacterial isolates were obtained from healthy tobacco leaves through surface sterilization and molar-mortar grinding. From these, 11 isolates were shown antagonistic against Tbs3 strain of A. alternata by antagonism test on PDA plates. The endophytic isolate Itb162 was found most effective in brown spot control and a rate of disease control reached 52.0% on tobacco plants three days after bacteria spraying.2. Surface washing and printing methods were applied to isolate epiphytic bacteria and 236 isolates were obtained. Five of these epiphytic isolates exhibitedmarked suppression of A. alternata strain Tbs3 on PDA plates and the isolate Ata28 had the highest brown spot control effect, with a disease reduction of 50.4% three days after treatment.3. The filtrates of Itb162 and Ata28 bacteria were shown separately to suppress mycelium growth and spore germination of A. alternata. Spore germination rate was significantly reduced and malformation of the spore germlings and rupture or dissolving of mycelium were observed. The combination to the two bacterial filtrates did not modify the fungal suppression effect compared with the application of individual filtrates.4. The suspension mixed with Itbl62 and Ata28 bacteria (1:1, v/v) was sprayed and the pathogen Tbs3 was inoculated on to tobacco seedlings in greenhouse three days afterwards. The disease control effect was 58.4%, which was significantly higher than spraying suspension of individual bacteria isolates. Tests and analyses showed that the enzyme activity of POD, PPO and PAL markedly increased in tobacco seedlings three days after spraying Itb 162 and the mixed suspension, and the pathogenesis-related protein (PRP) content was also accumulated. |
