The relevance of the study of the myxomycetes is caused by to the prevalence and ecological significance of these organisms: they make up to 50% of the biomass of soil amoebae and are the key regulators of the number of bacteria, algae and heterotrophic protists in terrestrial ecosystems. The importance of protection of myxomycetes, the necessity for timely assessment of the state of their local biota, is internationally recognized. Among the ecological groups of myxomycetes, one of the least studied is corticolous myxomycetes, which are characterized by the passage of the entire life cycle on bark of living trees. Corticolous myxomycetes in Ukraine were first described in the late 1980s, but no specialized studies on the diversity and ecology of corticolous myxomycetes have ever been conducted in the country, and in many biogeographical areas of Ukraine these organisms are not studied at all. This also applies to the southwestern part of the East European Upland, a physical-geographical section that covers the eastern edge of the forest and forest-steppe vegetation belts of Ukraine. Peculiarities of the southwestern part of the East European Upland as a unique for Ukraine landscape-coenotic complex, which is rich in refugiums of relict biota, determine the relevance of the study of species diversity and substrate ecology of corticolous myxomycetes in this area. The scientific novelty of the work is that for the first time the species composition of corticolous myxomycetes of the southwestern part of the East European Upland was established - 38 species of corticophilic myxomycetes, which belong to 18 genera, 10 families, 7 orders and 2 subclasses, 12 species are new for Kharkiv forest-steppe, 6 - new for the Middle Russian forests, 3 - new for the plain part of Ukraine, and 5 (Hemitrichia pardina, Licea floriformis, L. pygmea, Macbrideola argentea and Paradiacheopsis acanthodes) - new for Ukraine. The taxonomic, substrate-ecological and chorological structure of the biota of corticolous myxomycetes of the southwestern part of the East European Upland was studied for the first time. A detailed analysis of qualitative and quantitative characteristics of myxomycetes groups on different species of substrate-forming plants, extrapolation of the number of myxomycetes species associated with leading species of woody plants, a detailed comparison of groups, their division into five clusters. It is established that in the conditions of the north-east of Ukraine the species composition of corticolous myxomycetes is determined primarily by the composition of substrate-forming plants. For the first time in Ukraine, myxomycetes with European-North American and Panarctic habitats were found. The practical significance of the results. The obtained results on the species composition, taxonomic, substrate-ecological and chorological structure of corticolous myxomycetes biota enrich the knowledge of formation of regional biota of these organisms and contribute to the identification of factors influencing the diversity, abundance and distribution of these organisms. The obtained data can be used to justify the need to protect myxomycetes, in particular by creating nature reserves. The research results were used in the educational process at the H.S. Skovoroda Kharkiv National Pedagogical University in the preparation of materials for the course "Mycology", educational field practice of students of the Faculty of Natural Sciences and recommendations for term papers of students. The collected collection of myxomycetes is included in the scientific herbarium of the H.S. Skovoroda Kharkiv National Pedagogical University.
