During 2012–2014 in southeastern Poland the species Colletotrichum fuscum was isolated from the leaves of oregano (Origanum vulgare L.) showing the symptoms of necrotic, concentrically zoned spots with a lighter center and a slightly raised edge. Morphology of nine randomly chosen isolates from the fungus population and reference isolate CBS obtained from CBS-KNAW Fungal Biodiversity Centre were studied. Each isolate was cultured on PDA medium, at the temperatures 24°C for 14 days. The character of the cultures, the color of the averse and the reverse, the formation of morphological structures of the fungus, i.e. acervuli, conidia, appressoria and chlamydospores were studied. Ultrastructural observations of morphological structures were undertaken using scanning electron microscopy. The presence of setose acervuli, conidia, chlamydospores and appressoria was visible. Moreover, studies on the biotic effect between C. fuscum and other species of phyllosphere fungi of oregano showed that all tested fungi inhibited the growth of C. fuscum, but the size of the limiting effect was different. Fungi from genera Trichoderma and Clonostachys were found out to be the most effective and positive antagonists. Alternaria alternata, Botrytis cinerea and Fusarium spp. – despite the high values of IBE were considered negative antagonists.

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