The European mistletoe Viscum album is an evergreen hemiparasitic plant that negatively affects the vitality, growth, and stability of its host trees by extracting water and mineral nutrients through specialized haustorial structures. Under current climatic conditions, particularly increasing drought frequency, mistletoe infestation has intensified in many forest ecosystems, including stands of European silver fir Abies alba. While interactions between mistletoe and associated microorganisms have attracted growing interest, knowledge of fungal communities colonizing fir mistletoe V. album subsp. abietis remains limited. Understanding the diversity and composition of these fungi is essential for assessing their ecological role and potential influence on mistletoe development. This study investigated the diversity of fungi associated with necrotic tissues of fir mistletoe collected from silver fir stands in southeastern Poland. Leaves and shoots showing symptoms of necrosis were sampled and subjected to fungal isolation under laboratory conditions. In total, 242 tissue fragments were plated on malt extract agar, yielding 88 fungal isolates. The isolates were grouped into morphotypes based on colony characteristics and subsequently identified using molecular analyses of the internal transcribed spacer (ITS) region of ribosomal DNA. The obtained isolates were assigned to 11 fungal species representing a diverse assemblage of taxa with different ecological roles. The most abundant species was Sphaeropsis visci, accounting for 67% of all isolates, indicating its strong association with necrotic mistletoe tissues. Other frequently isolated taxa included Phoma herbarum and Sordaria fimicola, while several additional species were recovered at lower frequencies, such as Plenodomus visci, Coniothyrium genistae, Cladosporium sp., Nemania diffusa, Fusarium tricinctum, Epicoccum nigrum, Hypoxylon howeanum, and Colletotrichum godetiae. Most fungal growth originated from leaf tissues, with only a single species isolated exclusively from shoots. Sequence similarity to reference data in the GenBank database ranged from 99.59% to 100.00%, confirming reliable species identification. The results demonstrate that necrotic tissues of fir mistletoe provide a habitat for a diverse fungal community comprising both potential pathogens and saprotrophic species. The dominance of Sphaeropsis visci suggests its possible role in influencing mistletoe vitality and development. These findings contribute to a broader understanding of mistletoe−fungus associations and provide a basis for future studies on the ecological significance of fungal colonization of mistletoe and its potential implications for forest health and management.

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