Studies On The Ecology Of Pathogens And Its Antagonistic Bacteria In The Rhizosphere Of Cucumber And Mechanisms Of Induced Resistance By Plant Growth-promoting Rhizobactria

Cucumber is one of important vegetables which people like.The diseases caused by soil-borne plant pathogens not only influence cucumber yield and quality, but also cause large economic losses.Biocontrol method attached importance to people day after day because it gets over series of malpractice which caused by chemical control. Plant growth-promoting rhizobacteria(PGPR), as a type of benefial and potential bacteria in plant rhizosphere can directly and indirectly promote growth in varied plants. As a biocontrol agents, PGPR have been paid more and more attention by people. In this dissertation, the fluctuation of population on main pathogens and their antagonistic bacteria in the rhizosphere of cucumber varieties were studied. And studies were done on screening of PGPR in cucumber by two different methods, identification of PGPR and mechanisms of induced systemic resistance by PGPR. The results obtained as below:1. Using methods of selected culture medium and dual-cultured medium. The population of main pathogens in the rhizosphere of different cucumber varieties during the growing season was conducted. The results showed that a significant difference was among varieties and different growing stages at the same variety by statistic analysis, and which was accorded with normal disease regulation . At all growing stages the highest population of Pythium spp. was at mid-season stage of cucumber, and for Fusarium oxysporum f. sp. cucumerinum at principal-fruiting stage. The population of pathogens(Pythium spp. & Fusarium oxysporum f. sp. cucumerinum) was positively related with their antagonistic bacteria respectively and their correlation coefficients were 0.95 and 0.81, respectively.2. Three promising PGPR strains, termed as CH1,CH2 and CH3 were obtained by different screening methods. The results on petri dish, promoting seed germination, pot testing and plot testing showed that the disesses caused by Pythium aphanidermatum & Fusarium oxysporum f.sp. cucumerinum were suppressed significantly, while the growth of cucumber seedling was promoted.3. The selected PGPR were identified by studying on its characteristics of morphology, cultivation, physiology and biochemistry and 16S rDNA determination and analysis. The results showed CHI, CH2 and CH3 belong to Brevibacillus brevis, Bacillus Subtilis and Pseudomonas aeruginosa respectively.4. The resistance to cucumber seedlings damping-off was increased after treating with CH1 andthe surviving seedling of cucumber was promted as well. O^-₂ producing rate, H₂O₂ and MDA content were increased in roots and leaves after inoculation with Pythium aphanidermatum, however, were reduced by CHI treatment in the inoculated plants.Although the inoculation of Pythium aphanidermatum enhanced the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT), CHI treatment increased the activities of those enzymes above them higher in roots and leaves. It indicated that the increased activities was most systemic.5. The effects of the two PGPR strains on growth and physiological metabolism in cucumber were studied. The results showed that CHI and CH2 obviously promoted germinating of cucumber seeds, root and plant growth. Superoxide anion-producing rate, contents of H₂O₂ and MDA in roots and leaves of cucumber were considerably decreased after treatment with CH1 and CH2 and contents of chlorophyll, protein, soluble sugar and Vc which related with cucumber quality were magnificantly increased.6. When the plants inoculated with Pythium aphanidermatum, the activity of phenylalanine ammonia-lyase (PAL), polyphenoloxidase (PPO) and the contents of ferulic acid, chlorogenic acid,lignin, phenolic compound and flavonoid in roots and leaves of cucumber were significantly increased, but the activities of those above in cucumber were lower than that of treatment with CH1.7. PGPR CH1 could induce systemic resistance to Colletotrichum lagenarium which caused cucumber anthracnose. The results showed that numbers of necrosis spots and their size on the leaves wer...