Occurrence and pathogenicity of Oomycota species causing root rot were investigated in 10 forest tree nurseries in western Turkey. Soil samples (129 in total) taken from the rhizosphere of symptomatic seedlings were baited for Oomycota using young leaves of Quercus suber, Rhododendron simsii and R. ponticum. Oomycota (178 isolates) were obtained by culturing on selective media, and identified using morphological methods followed by PCR and sequencing of the ITS rDNA and cox1 regions. Phytophthora aff. cactorum, P. citricola sensu lato, P. crassamura, P. syringae, Pythium aphanidermatum, Py. intermedium, Py. irregulare, Py. ultimum and Phytopythium vexans were common amongst the isolates. The highest diversity of Oomycota was found in the forest nurseries at Adapazari-Hendek and I˙zmir-Torbali. Pathogenicity tests showed that the isolates caused lesions on a range of host plants. The importance of these nursery infections in transferring potentially damaging oomycete species to Turkish forests is discussed.