A 3-decalinoyltetramic acid, for which the trivial name hymenosetin is proposed, was isolated from crude extracts of a virulent strain of the ash dieback pathogen, Hymenoscyphus pseudoalbidus (= "Chalara fraxinea"). This compound was produced only under certain culture conditions in submerged cultures of the fungus. Its planar structure was determined by NMR spectroscopy and by mass spectrometry. The absolute stereochemistry was assigned by CD spectroscopy and HETLOC data. Hymenosetin exhibited broad spectrum antibacterial and antifungal activities (including strong inhibition of MRSA, as well as moderate cytotoxic effects.So far, the metabolite proved inactive in assays for evaluation of phytotoxicity, whereas viridiol, another secondary metabolite known from H. pseudoalbidus, was regarded as phytotoxic principle of the pathogen against its host, Fraxinus excelsior. Further studies will show whether hymenosetin constitutes a defence metabolite that is produced by the pathogenic fungus to combat other microbes and fungi in the natural environment.

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Fungal endophytes can be part of the defensive system of plants against multiple pathogens by competing for resources, hyperparasitism or producing bioactive compounds with antimicrobial properties. There is an ever‐increasing interest for obtaining new and environmentally friendly products to use in the fight against pathogens. With this purpose, Byssochlamys spectabilis (anamorph Paecilomyces variotii), which is a fungal species that commonly occurs, was evaluated as an antagonistic organism towards three phytopathogens (Biscogniauxia mediterranea, Fusarium moniliforme and Phytophthora cinnamomi). First, an in vitro experiment was designed to test the effect that the endophyte filtrate had on the three pathogens. The endophyte filtrate decreased the radius growth rate of F. moniliforme by nearly 10%. Consequently, the antagonism between B. spectabilis and F. moniliforme was evaluated in Lolium rigidum plants under greenhouse conditions by means of co‐inoculations. The endophyte produced a decrease of 50–75% in the disease severity caused by the pathogen in the earliest infection stages. Crude extracts of B. spectabilis were obtained to determine the secondary metabolites responsible for such an effect. The bioactivity‐guided chromatography and HPLC‐MS (high performance liquid chromatography coupled with mass spectrometry) of the active fraction suggested that the antibiotic activity was caused by viriditoxin. In conclusion, the fungal endophyte B. spectabilis and/or its bioactive compounds showed antagonism towards several phytopathogens and deserves further study to investigate its actual potential for use as a biocontrol agent.

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