Waterways are ideal pathways forPhytophthoradispersal and potential introduction to terrestrial ecosystems. While manyPhytophthoraspecies from phylogenetic clades 6, 9 and 10 are predominant oomycetes in watercourses due to their adaptation to a lifestyle as saprotrophs and opportunistic pathogens of riparian plants, species from clades 2, 7 and 8 are predominantly soil- or airborne using aquatic habitats as temporal niches for spreading and invading terrestrial sites along the watercourses. In contrast to forest ecosystems, knowledge ofPhytophthoradiversity in watercourses in Central Europe is limited. Between 2014 and 2019 extensive surveys of streams and rivers were undertaken across Austria, in South Moravia, Czech Republic and Žilina province, Slovakia to unveil the diversity and distribution ofPhytophthoraand related oomycetes. In addition, in Austria riparian forests of black alder (Alnus glutinosa) and grey alder (A. incana) in lowlands and in the Alps were examined. A variety ofPhytophthoraspecies from clades 2, 6, 7, 8, 9 and 10 were isolated, with clade 6 species showing the widest distribution and abundance. Furthermore, interspecific clade 6 hybrids and other oomycetes such asHalophytophthora fluviatilisand undescribedNothophytophthoraspp. were also obtained. In riparian alders, symptoms ofPhytophthorainfections were associated with species from theP.×alnicomplex andP. plurivora.Phytophthora plurivorawas most common in alder stands whereasP. uniformiswas the oomycete species occurring at the highest altitude in alpine riparian areas.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11557-023-01898-1.