Antagonistic Activities of Fungal Endophytes from Rhizophora mucronata Lamk. against Fusarium oxysporum Under Altered Nutrient Levels

Abstract

Mangrove ecosystems harbor diverse fungal endophytes that are increasingly recognized as sources of bioactive metabolites with potential applications in sustainable agriculture. However, their antagonistic roles against phytopathogens under variable environmental conditions remain poorly understood. This study investigated the antagonistic activities of 16 fungal endophytes isolated from the mangrove Rhizophora mucronata against the fungal phytopathogen Fusarium oxysporum. These mangrove fungal endophytes, representing the genera Pestalotiopsis, Neopestalotiopsis, and Pseudopestalotiopsis, were isolated from both disturbed and undisturbed mangrove ecosystems and evaluated for antifungal activities using a dual culture assay. Our results revealed diverse antagonistic interactions, with Types C (inhibition upon contact) and E (mutual inhibition with subsequent overgrowth) being the most frequently observed, occurring in 9 of the 16 mangrove fungal endophytic isolates. Interaction Types B (mutual inhibition upon contact or at a distance) and D (mutual inhibition between fungi at a distance with clear zones larger than 2 mm) were also observed among the tested mangrove fungal endophytic isolates. The percent inhibition of F. oxysporum by the mangrove fungal endophytes ranged from 68% to 86%, with Pseudopestalotiopsis sp. isolated from undisturbed sites displaying the highest antagonism. We further examined these interactions under altered phosphate and zinc concentrations in the growth medium. Low phosphate levels enhanced fungal inhibition, promoting Type C interactions, whereas elevated phosphate concentrations reduced antagonism, shifting interactions toward Type E. Modifications to zinc concentration produced more consistent inhibitory effects, with interactions primarily classified as Types C and E. These findings suggest that environmental factors, particularly mineral availability, play a critical role in shaping the antagonistic potential of mangrove fungal endophytes against phytopathogens, underscoring their promise as sustainable biocontrol agents.