Bioactive metabolites produced by the fungal pathogens Diplodia fraxini and Hymenoscyphus fraxineus isolated from infected Fraxinus spp. trees

During the last two decades widespread ash population decline has happened in Europe. This epidemic disease, commonly known as ash dieback, is the result of biological invasions by the two ascomycetes fungi Diplodia fraxini and Hymenoscyphus fraxineus. On Fraxinus spp. both fungi are able to cause the same symptoms, corresponding to the typical ones due to the infections of Diplodia species on woody hosts characterized by V-shaped necrotic sector visible in cross section.1 D. fraxini is a fungus belonging to the Botryosphaeriaceae family. It was found to be a pathogen associated to symptomatic Fraxinus angustifolia trees in South Europe and isolated also from branch and twigs of Fraxinus excelsior in initial and advanced stages of dieback in Poland. A new phytotoxic isochromanone, named fraxitoxin (1, Fig. 1), was isolated together with (–)-mellein and tyrosol, already known toxic fungal metabolites, from the organic extract of the D. fraxini culture filtrates.1 The structure of fraxitoxin was characterized using spectroscopic methods (essentially NMR and HR-EI-MS) while its absolute configuration was assigned by electronic circular dichroism (ECD) measurements and calculations. H. fraxineus is a fungus belonging to the Helotiaceae family. It was identified as the pathogen causing the F. excelsior dieback in Europe. H. fraxineus was introduced in Europe from Asia and has gradually spread from East to West continental Europe including the British Isles.2 Viridiol (2, Fig. 1) was isolated as the most abundant and phytotoxic metabolite, from the organic extract of the H. fraxineus culture filtrates.3 The purification and chemical and biological characterization of these two metabolites as well as those of other related compounds will be illustrated in this communication.