ABSTRACT
This study evaluated the potential of Bacillus species as an antifungal agent against Fusarium oxysporum f.sp elaeidis, a fungal pathogen that causes vascular wilt in oil palm trees. The research assessed the physicochemical properties of the bacterial and fungal population in both healthy and diseased oil palm rhizosphere. The bacterial and fungal populations were analyzed using standard analytical methods, and the optimum medium for spore count for pathogenicity was ascertained using ten different media to grow the fungal culture. The pathogenicity test was carried out on four-month-old nursery seedlings sprouted from three different progenies collected from field 25 located at the main station of Nigerian Institute for Oil Palm Research, Edo state, Nigeria. The cell-free metabolites of rhizospheric Bacillus safensis applied in a dose-dependent concentration of 100%, 75%, 25%, and 12.5% respectively, exhibited antifungal properties against the pathogen. The study also assessed the influence of pH, temperature, metal ions, nitrogen sources, and carbon sources on the crude enzyme produced by Bacillus safensis.
The physicochemical studies revealed that nitrate concentration was significantly higher (p < 0.05) in the healthy palm rhizosphere than in the diseased palms rhizosphere. The dominant bacterial species isolated from the oil palm rhizosphere were Salmonella sp., Pseudomonas sp., Bacillus sp., Micrococcus sp., Staphylococcus sp., and Escherichia coli, while the fungal isolates were predominantly Penicillium sp, Aspergillus niger, Fusarium oxysporum, and Mucor sp. The medium that gave optimum sporulation of the fungus was achieved with potato dextrose broth after ten days of incubation. Pathogenicity tests on the nursery progenies confirmed that the isolated fungus, F. oxysporum f. sp elaeidis, had the same virulence impact as the adult palms from which it was isolated. There was no significant difference (p > 0.05) in the progenies' height measurements (cm), however, there was a considerable difference (p < 0.05) between the test seedlings and the control seedlings treated with sterile distilled water.
The bacterium used in this study was identified as Bacillus safensis with accession number MT151602 using molecular techniques. The growth curve of the Bacillus safensis revealed that the optimal secretion of its chitinase and protein concentration (3 mg /ml) was observed after twenty-four hours of incubation. The optimal relative activity (14.50 µmol /min) of chitinase secreted by Bacillus safensis was achieved when the pH value of the medium was 6.0 and a temperature of 50°C. Ammonium sulfate as a nitrogenous source gave the optimum activity of 17.30 µmol /min. The single peak on the elution chart on ion-exchange chromatography of Sephacryl-S200 showed that the chitinase enzyme produced by Bacillus safensisis monomeric in nature. This study has shown that oil palm rhizospheric Bacillus safensis secreted the enzyme chitinase with a molecular weight of 46 kDa and exhibited antifungal potential against the pathogen, F. oxysporum f. sp elaeidis.
