In several studies of oak decline in Europe, one semi-papillate (Phytophthora psychrophila sp. nov.) and two non-papillate homothallic Phytophthora species (P. europaea and P. uliginosa spp. nov.) were isolated, together with other Phytophthora species, from rhizosphere soil samples which could not be assigned to existing taxa. P. psychrophila differs from other semi-papillate species of Waterhouse's morphological Group IV, like P. ilicis and P. hibernalis, by its uniform, dome-shaped and cottonwool-like colony growth pattern on V8 juice agar and malt extract agar, the occurrence of sympodially branched primary hyphae, the high variation in size and shape of the sporangia, shorter pedicels, lower optimum temperature for growth, and ITS sequences. P. europaea is distinguished from related non-papillate Group V and VI species, namely P. fragariae, P. cambivora, and the ‘alder phytophthora’, by producing oogonia with tapered bases, irregular walls and exclusively paragynous antheridia, its cardinal temperatures for growth, and ITS sequences. P. uliginosa differs from related Group V and VI species by its large oogonia with exclusively paragynous antheridia, the predominant occurrence of ellipsoid sporangia with markedly wide exit pores, its slow growth, low cardinal temperatures, its colony growth patterns, and ITS sequences. P. uliginosa is separated from P. europaea by its larger oogonia without tapering bases, lower cardinal temperatures and growth rates, different colony growth patterns, and greater aggressiveness on Q. robur.