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Thanks to environmental changes, globalization, long distance trade and plant transport, invasive organisms have become a major threat for world biodiversity and ecosystem services. Over the last 20 years, common European ash trees (Fraxinus excelsior L., Fraxinus angustifolia Vahl. etc.) have been subjected to heavy dieback and mortality because of the introduction and spread of the ascomycetous fungal pathogen Hymenoscyphus pseudoalbidus Queloz (syn. Chalara fraxinea Kowalski). Once the disease is established, its management is hardly possible. Therefore, one of the main objectives of European researchers is to find effective and respectful control methods, such as biological control. The discovery of viruses which reduce the virulence of the chestnut blight fungus Cryphonecria parasitica (Murr.) Barr., has intensively stimulated the research of fungal viruses as potential biological control agents (BCA). The occurrence of putative dsRNA particles in the decaying fungus H. pseudoalbidus was investigated as an important indicator of the mycoviruses' presence. In total, 106 samples of this pathogen were obtained from eight different European countries. According to the results, dsRNA segments were confirmed in 32.1 % of examined samples (two similarly sized at 2--2.5 kb and a third one of approximately 5 kb). Statistical results have revealed no significant relation between the presence of dsRNA and growth rate, colour or any other characteristic of the mycelium.

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This bachelor thesis deals with necrosis of ash caused by pathogenic fungus Chalara fraxinea. The work contains summarized informations about phenology, bionomy and charakteristic symptoms which were used during the practical research. Next part of the thesis -- the practical research itself -- is focused on charting of intensity and spreading direction of the disease in one specific forest section. In this part are also mentioned possible factors, which could modify (accelerate or restrict) the growth of the fungus in specific cases. Than, in the last part, are walorized financial damages caused by this pathogenic fungus and suggested some interventions, which can contribute to survival of ashes in the forests.