| Summary: | 碩士 === 國立中興大學 === 植物病理學系所 === 107 === The seedlings and the fresh fruits of passion fruits are of high value in local and global trade. Fusarium solani (Teleomorph: Neocosmospora solani (Martius) is one of the main disease causal agents affecting passion fruits. The objectives of this study are to develop Bacillus-based biocontrol agents for the management of fusarium disease on passion fruits; to assess the potential fungicides to control fusarium disease on passion fruits; to evaluate of the compatibility of potential Bacillus species strains with chemical fungicides; and to develop integrated strategy for controlling Fusarium disease on passion fruits; to investigate the putative plant protection mechanisms by Bacillus species. Our previous studies indicated that Bacillus subtilis YBC and 151B1 native in Taiwan showed antagonistic activity to several fungal pathogens on passion fruits including Colletotrichum karstii and F. solani PF7. They also inhibited mycelial growth and conidial germination of F. solani PF7. Application of B. subtilis 151B1 and YBC culture broth caused a reduction in the disease severity of fusarium wilt on leaves of passion fruits and enhanced the survival rates of passion fruit seedlings with challenge of F. solani. Application of B. subtilis 151B1 and YBC culture broth promoted the growth of plant height, increased the size and numbers of leaves, and fresh and dry weights of passion fruit seedlings. Among thirteen fungicides commosnly used for the control of Ascomycetes tested, the results showed that tebuconazole showed the best effecacy on the inhibition of hyphal growth of F. solani PF7 and the best survival of the passion fruit cuttings in the infested soil. We also found that the growth of B. subtilis 151B1 and YBC was not affected by the above tested fungicides and the efficacy in enhancing survival rates of passion fruit cuttings in the Fusarium-infested soil remained, suggesting the compatibility between B. subtilis and tested fungicides. To investigate the putative mechanisms of disease control, the results indicated that the treatments of culture filtrates from B. subtilis 151B1 and YBC both caused aberrant conidial morphology and loss of cell membrane integrity. Additionally, the treatments caused reduction in mitochondrial membrane potential and interfered with energy metabolism of Fusarium solani PF7. The treatments also enhanced the reactive oxygen species accumulation, and resulted in externalization of phosphatidylserine, chromatin condensation, and DNA fragmentation, suggesting the functions of culture filtrates in triggering apoptotic-like cell death. To investigate the mechanisms of plant growth promotion by B. subtilis 151B1 and YBC, our results indicated that both Bacillus strains exhibited activities of producing indole-3-acetic acid (IAA) and solubilizing phosphate. In conclusions, B. subtilis 151B1 and YBC are potential biocontrol agents for passion fruit disease caused by F. solani. The control efficacy may be contributed to their ability in triggering apoptotic-like cell death, reducing the mitochondrial membrane potential and interfering with energy metabolism of the pathogen. B. subtilis 151B1 and YBC also exhibited growth promotion traits by producing IAA and solubilizing phosphates. Chemical fungicide, tebuconazole, B. subtilis 151B1 and YBC, or combination of tebuconazole and B. subtilis could be used to prevent the passion fruits from the infection by F. solani PF7. The integrated control strategies by combination use of Bacillus species and chemical fungicides could be further implemented to control the collar rot disease of passion fruits caused by F. solani.
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