| Summary: | Mycoparasites are a collection of fungicolous eukaryotic organisms that occur on and are antagonistic to a wide range of plant pathogenic fungi. To date, this fungal group has largely been neglected by biodiversity studies. However, this fungal group is of interest, as it may contain potential biocontrol agents of pathogenic fungi that cause beech Tarcrust disease (BTC), which has contributed to the devastation of European beech (<i>Fagus sylvatica</i>) forests. <i>Biscogniauxia nummularia</i> has been demonstrated to cause BTC. However, a trophic association between mycoparasites and pathogenic <i>Biscogniauxia</i> spp., has not been established. This study aimed to taxonomically identify and characterize <i>Biscogniauxia</i>, a fungus causing destructive BTC disease in European beech at Lovćen national park, Montenegro and to uncover the diversity of mycopathogens that are natural regulators of xylariaceous <i>Biscogniauxia</i> stroma formation, associated with beech decline. This finding is supported by distinctive phylogenetic and evolutionary characteristics, as well as unique morphological-microscopic fungal features indicating that <i>Biscogniauxia</i> from Montenegro, which is a major cause of BTC occurring in ancient beech forests at the edge of southern <i>Fagus sylvatica</i> distribution, may be described as a novel fungus specific to <i>Fagus.</i> Its evolutionary nuSSU–complete ITS–partial nuLSU rDNA phylogeny indicates its likely emergence by asexual fusion or introgressive hybridization between diverged <i>B. nummularia</i> and <i>B. anceps</i> species. The name <i>Biscogniauxia destructiva</i> is proposed for the novel fungus, as it is aggressive and highly destructive BTC disease.
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