Phytophthora quercina is a papillate homothallic soil-borne plant pathogen causing root rot of oak tree species in Europe. It is associated with necrotic fine roots. In culture, this species shows a uniform, dome-shaped and cottonwool-like colony growth pattern. It also frequently presents sympodially branched primary hyphae, a high proportion of elongated, ellipsoid or ovoid oogonia, the absence of amphigynous antheridia. Its sporangia vary greatly in size and shape.