Screening And Application Of Antifungal Bacteria Against Fusarium Oxysporum F.sp. Cubense

This paper introduced the screening and possible applications of two antagonistic bacteria strains against Fusarium oxysporum f.sp. cubense race 4. The disease prevention, colonization action of the two isolates and growth promoting, mechanism of disease prevention of the isolate BTSF were investigated. This would help develop strategies for biocontrol of banana wilt.A total of 26 strains of bacteria were isolated from healthy banana plants. According to in vitro assays, all of them had inhibitory effects against the banana wilt. After primary and secondary screening using different ways, two strains that showed the strongest inhibitory effects were selected. The two strains were named BTWC and BTSF respectively.Pot planted banana seedlings were irrigated with the cell cultures of the two antagonistic bacteria respectively. The results indicated that pouring 5×106 cfu/mL fermentation liquids of the two antagonistic bacteria could yield significant disease prevention effects against banana wilt. Pouring the fermentation liquids into banana roots 3 days before pathogen innoculation resulted in better disease prevention effects, with disease control efficiencies of the two strains being 72.49 % and 83.10 % respectively; the disease control efficiencies of BTWC and BTSF decreased to 46.43 % and 38.77 % respectively if they were used 3 days after pathogen inoculation. The changing patterns of the control effieciencies of the two strains were similar. The strain BTSF had a better control efficiency than BTWC and control (50 % carbendazol wettable powder). Two mechenisms might be responsible for the disease prevention of the antagonistic bacteria. The first one was that the abundant antagonistic bacteria occupied the entry points of pathogens. The second one was that some metabolites of the antagonistic bacteria might have inhibitory effects on pathogenes and could induce defense responses of host plants.The fermentation liquor of the two antagonistic bacteria strains were infiltrated into banana plants by using root-irragating and leaf-daubing methods respectively. Sampling at different times indicated that marked Rif-resistant mutants could be recovered from banana plants inoculated by either of the two methods. This suggested that the two bacteria strains could penetrate into as well as move within and reproduce in the penetrated plants. This observations provided the basis for investigating the effects of the antagonistic bacteria on the resistance mechanism of host plants.The enzyme activities of five protective enzymes including POD, SOD, PAL, PPO and CAT in banana plants treated with the cell cultures of banana wilt or antagonistic bacteria were determined respectively, with an aim to elucidate the mechanisms underlying the disease prevention effects of the two antagonistic bacteria. The results indicated that except SOD, whose activity decreased after treatment, activities of all other four enzymes were significantly higher than control after treatment with the cell cultures of antagonistic bacteria. This indicated that treatment with the cell cultures of antagonistic bacteria could induce some protective responses of banana plants.The growth-promoting effects of the metabolites of the antagonistic bacteria BTSF (which was identified as Bacillus subtilis) were studied by using fermentation liquids in which the bacteria have been filtered out. The results indicated that fermentation liquids at the volume 10%(V/V),20%(V/V),30%(V/V)had obvious effects on the callus induction, especially to the banana and growth-promoting of the callus, buds and roots of the plant.