There are many records in the literature demonstrating that species belonging to the genus Phytophthora and Pythium cause damage to fine root and trunk collars of many forest tree species. Natural water courses can be an important but often neglected source of these phytopathogens. The detection of pathogenic Oomycetes organisms in the Ukrainian and Polish rivers was performed with the methods of baiting and filtering of water. The former was applied in Ukraine, and the latter in Poland. Branches with young leaves of beech, oak and black alder were used as baits and floated on the river surfaces for 3 days. Three Phytophthora species (Ph. gonapodyides, Ph. lacustris and Ph. cactorum) were detected in Ukrainian rivers presumably moving naturally towards the EU border. Species of the Pythium genus, and among them the most common ones like P. lycopersicum, P. sylvaticum, P. citrinum and P. terrestris, were detected in Polish rivers.

Keywords: water courses, Oomycetes, phytopathogens, seedlings, watering, nurseries.