During the monitoring of oak decline phenomenon in Slovakia, symptoms indicative of Phytophthora diseases were observed in sessile oak stands in western Slovakia. The study aimed to test the presence and diversity of Phytophthora species associated with declining oak stands. From rhizosphere soil samples, Phytophthora plurivora, P. quercina and Pythium intermedium were detected. Soil inoculation tests with P. plurivora and P. quercina on 1‐year‐old oak plants inflicted damage to the root system, and significant differences in measured root parameters between the inoculated and control treatments after 5 months. Although more wilting symptoms were observed in P. plurivora—infected seedlings (7 out of 12 inoculated seedlings), P. quercina caused the highest amount of damage to root systems, and for example, fine root length was 1.6 times shorter compared to the control. To our knowledge, this is the first report of P. plurivora and P. quercina in sessile oak stands in Slovakia.