Genetic Resources, oregano (Origanum vulgare L.) is included in the List of 'Priority Species' of medicinal and aromatic plants (Barata et al., 2010). Wide use of oregano biomass in phytomedicine, production of essential oil, perfumery, culinary, food and beverage production, beekeeping, etc. is one of the reasons why the wild populations should be conserved. In addition, cultivation of oregano is providing material for further processing. Cultivation of oregano accessions resistant to biotic pathogens is required to ensure high yields and high product quality in agricultural systems.Oregano is susceptible to infectious diseases caused by pathogenic fungi. Several oregano fungal diseases have been reported.In the United States, fungi of the genera Alternaria, Botrytis, Helminthosporium and Stemphylium on leaves, as well as fungi from the genus Pythium, which cause symptoms of root rot have been reported (Farr et al., 1995). In Poland, fungi such as Fusarium spp., Boeremia exigua var. exigua and Rhizoctonia solani had been obtained from roots and stems, but Alternaria alternata, Colletotrichum fuscum and Stemphylium botryosum had been isolated from leaves (Wielgusz and Seidler-Łożykowska, 2017;Zimowska, 2015). Research of fungal diversity on oregano leaves in Latvia identified similar fungi as in the mentioned studies.Members of the Truncatella genus have a wide geographic distribution. They are commonly associated with plants, either as endophytes or as pathogens in a wide array of plant species. Truncatella angustata was first described by Persoon in 1801 under the name Stilbospora angustata, which was subsequently revised to T. angustata (Hughes, 1958). T. angustata has been recognized as an endophyte and a saprotroph but also as a pathogen on various vascular plants in both tropical and temperate regions. It has been reported