Phenazine- and Phenolic-Rich Metabolites from Endophytic Pseudomonas aeruginosa CEU6 Suppress Ganoderma boninense Growth

Indonesia is the world’s largest palm oil producer, however, the expansion of oil palm plantations has not been matched by proportional yield increases. One of the major constraints is basal stem rot disease caused by Ganoderma boninense. Synthetic fungicides are still being used to suppress this pathogen, but their long-term application may affect beneficial soil microorganisms, including mycorrhizal fungi. This study evaluated the antifungal activity of metabolites produced by the endophytic bacterium Pseudomonas aeruginosa CEU6 against G. boninense and identified its phenazine and phenolic compounds. P. aeruginosa CEU6, previously isolated from Dayak onion (Eleutherine bulbosa), was tested for hemolytic activity on blood agar, followed by dual-culture and volatile organic compound (VOC) assays. Crude metabolites were obtained by liquid fermentation and ethyl acetate (EtOAc) extraction of the cell-free supernatant. The extract was tested against G. boninense using an agar well diffusion assay. Total phenolic content was determined using the Folin–Ciocalteu method, and metabolite profiling was performed by LC-MS. The isolate showed γ-hemolysis, indicating no hemolytic activity. Both dual-culture and VOC assays showed more than 74% inhibition of G. boninense. The EtOAc extract showed the highest inhibition at 42% when applied at full concentration, with a total phenolic content of 234.40 mg GAE/g. LC-MS analysis putatively annotated some phenolic compounds, including (E)-3,3′-dihydroxy-4,4′-dimethoxystilbene, 3,4-dihydroxyphenethylamine, o-aminophenol, p-hydroxyacetanilide, L-noradrenaline, and nobilin C, with the phenazine derivative, 1-hydroxyphenazine. These findings show that P. aeruginosa CEU6 produced antifungal metabolites that may be further developed as biological control agents against G. boninense.