Antifungal activities of metal-tolerant endophytes against Ganoderma boninense under the influence of metal stress

Five endophytic fungal isolates (Bipolaris sp. LF7, Diaporthe miriciae LF9, Trichoderma asperellum LF11, Phomopsis asparagi LF15, Saccharicola bicolour LF22) were tested for their tolerance to various concentrations of Al³⁺, Cu²⁺, Pb²⁺, Zn²⁺ and Cd²⁺. All isolates exhibited Tolerance Index (TI) exceeding 1, suggesting growth stimulation of the endophytes under metal stress. The antifungal activities of the endophytes against Ganoderma boninense (GB) (via the production of metabolites) were observed in metal and non-metal treatments. Production of alkaloids, terpenoids and sterols by the endophytes were enhanced under metal stress. In addition, endophytes demonstrated inhibition towards GB via the production of inhibitory compounds (Bipolaris sp. LF7, P. asparagi LF15), competition (D. miriciae LF9, S. bicolour LF22) as well as mycoparasitism (T. asperellum LF11), in the absence and presence of metals. The endophytes also induced changes in GB hyphae with highly branched hyphal tips observed in non-metal treatment. The inhibitory activities of the endophytes were also observed under metal stress, with prominent swollen hyphae and scarcer GB mycelium. Metals such as Cd²⁺ and Cu²⁺ affect enzymes required for hyphal formation thus resulting in swelling of hypha. Of all the isolates, T. asperellum LF11 has the most potential to inhibit GB, in both conditions (presence and absence of metal stress). Results from this study provide early indications on the potential use of metal-tolerant endophytes as biocontrol agents towards G. boninense in soils with metals.