Mycogenic management of Pythium irregulare root rot and wilt in pepper via endophytic Aspergillus terreus MF3 and Mentha spicata extracts

Background Pepper ( Capsicum annuum L.) is a critically important crop for the global economy; however, its production is severely affected by harmful soil-borne pathogens such as Pythium irregulare , which causes root rot and wilting. This study aimed to evaluate a sustainable biological control strategy combining the halophytic endophytic fungus ( Aspergillus terreus MF3 ( A. terreus MF3)) isolated from Avicennia marina in South Sinai, Egypt and the methanolic leaf extract of Mentha spicata ( M. spicata ), with the goal of suppressing the disease, mitigating oxidative stress, and inducing defense responses in the host plant. Results GC–MS analysis of the M. spicata extract tentatively identified 11 phytochemical constituents, predominantly consisting of oxygenated monoterpenes, fatty acid esters, and phenolic derivatives, with key bioactive compounds including 1,2-Benzenedicarboxylic acid (46.32%), 9-Octadecenoic acid (Z) (12.53%), and Piperitenone oxide (6.01%). In vivo greenhouse experiments showed that P. irregulare infection resulted in a high disease index (85.1%) in untreated infected plants, whereas treatment with A. terreus MF3 fungal filtrate and M. spicata extract reduced the disease index to 40.1% and 45.0%, respectively. Both biological treatments partially alleviated pathogen-associated oxidative stress compared with the infected control, reducing MDA accumulation from 9.10 to 6.20 mg g −1 DW with A. terreus MF3 and to 7.50 mg g −1 DW with M. spicata , while H 2 O 2 levels decreased from 7.80 to 5.60 and 6.40 mg g −1 DW. Furthermore, the treatments improved photosynthetic pigment levels and enhanced biochemical defense responses, as reflected by increased free proline and total phenolic contents and enhanced activities of peroxidase (POD) and polyphenol oxidase (PPO). Conclusions This study introduces a novel, eco-friendly biocontrol strategy combining the halophytic endophytic fungus Aspergillus terreus MF3 and Mentha spicata leaf extract to combat Pythium irregulare in pepper. By effectively suppressing the disease, mitigating pathogen-induced oxidative stress, and stimulating host defense responses, this sustainable approach offers a valuable alternative to chemical pesticides for integrated disease management.