Biocontrol activities of metal-tolerant endophytes against Ganoderma boninense in oil palm seedlings cultivated under metal stress

Two metal-tolerant endophytes (Diaporthe miriciae LF9 and Trichoderma asperellum LF11) were studied in vitro for antifungal activities using dual culture assay. Both isolates inhibited Ganoderma boninense (GB) under metal and non-metal stress, with higher inhibition by T. asperellum LF11 than D. miriciae LF9. T. asperellum LF11 was unaffected by the metals, consistently inhibiting GB (100%) in both conditions. In contrast, inhibitory activities by D. miriciae LF9 was encouraged under metal (32.81–70.64%) compared to non-metal (14.07%) stress; higher in Cu ²⁺ (70.64%) and Zn ²⁺ (63.10%) than Pb ²⁺ (51.79%) and Cd ²⁺ (32.81%). These in vitro activities differed from in vivo findings. After 12 weeks, infected seedlings of oil palms inoculated with D. miriciae LF9 showed lower disease incidence (DI) in non-metal (33%) and multi-metal (67%) treated soils, thus suggesting enhanced tolerance of seedlings towards infection. Treatment with D. miriciae LF9 also delayed the onset of symptoms in the seedlings. Disease severity (DS) was lower for seedlings in non-metal (11%) and multi-metal (22%) treated soils, suggesting D. miriciae LF9 slowed the disease development. On the contrary, T. asperellum LF11 was less effective. Seedlings did not show improved tolerance under non-metal (67%) and multi-metal (100%) stress conditions. The isolate was also less successful in slowing disease development with DS of 33% and 56% observed in seedlings in non-metal and multi-metal treated soils, respectively. The lower endophytic inhibition towards GB in multi-metal treated soils was possibility attributed to higher metal toxicities by the several metals present, thus portraying complex interactions among metals, endophytes and seedlings. Nevertheless, the endophytes suppressed GB in both conditions, hence can be further studied for enhanced performance in metal-laden soils.