Antimicrobial Potential of Endophytic Fungal Extracts From the Mangrove Avicennia germinans Against Phytopathogens

Abstract

The prolonged use of synthetic pesticides in agriculture has resulted in the emergence of resistant organisms, high residual levels, and negative impacts on beneficial organisms and human health. These issues highlight the need to explore alternative control strategies based on natural products from ecosystems such as mangroves. In this study, the antimicrobial activity of endophytic fungal extracts from the mangrove Avicennia germinans was evaluated against phytopathogens. Antibacterial activity was assessed by determining the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), while antifungal activity was evaluated based on the percentage of inhibition of mycelial growth (PIMG), spore germination rate (SGR), and reductions in disease incidence (DI%) and severity (DS%) in tomato fruits. The toxicity of an extract (LC50) was determined using Artemia salina. Thirty-one endophytic fungi were isolated from the mangrove, eight of which showed bioactive properties. Extracts of Fusarium pernambucanum and F. arcuatisporum showed activity against Gram-positive and Gram-negative bacteria, while F. keratoplasticum exhibited significant antifungal effects. F. pernambucanum showed very strong inhibitory (7.8 μg/mL) and bactericidal (31.2 μg/mL) effects against Clavibacter michiganensis, as well as moderate inhibitory (250 μg/mL) and bactericidal activity (1000 μg/mL) against Pseudomonas syringae. F. arcuatisporum extract exhibited good inhibitory (125 μg/mL) and bactericidal (1000 μg/mL) activity against Rhizobium radiobacter. At 1 mg/mL, F. keratoplasticum extract showed very high inhibition (85.37%) against Colletotrichum asianum and high inhibition (62.77%) against Acremonium strictum. It also reduced the SGR, DI%, and DS% of C. asianum by 70%, 50%, and 68.75%, respectively, at 1 mg/mL and completely suppressed its DI% and DS% (100%) at 2.5 mg/mL. None of the bioactive extracts exhibited toxic effects on A. salina. These findings highlight the potential of endophytic fungal extracts from A. germinans as promising sources of bioactive compounds for phytopathogen control.