Filamentous fungi from Plantago lanceolata L. leaves: Contribution to the pattern and stability of bioactive metabolites

► Plant-associated fungi can decompose chief constituents in extracts of Plantago lanceolata. ► Fungi produce high amounts of phenolic antioxidants from extracts of P. lanceolata. ► Acteoside decomposition is slower in the presence of phenolic acids. ► Antioxidants can significantly inhibit breakdown of acteoside in herbal material. ► Different fungi can inhibit/accelerate breakdown of acteoside in herbal material.