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Co-existence of four Phytophthora species in a small river in Bulgaria was established—P. plurivora, P. gonapodyides, P. chlamydospora and P. gallica. The species identification was performed by DNA sequence analysis of the ITS region and the presence of a rare Phytophthora species, P. gallica, which was isolated for the first time in Bulgaria, was recognized. Additional sequencing of the mitochondrial cox1 gene and two other nuclear genes (tef1 and Btub) confirmed the species identification of the four isolates. P. gallica belongs to clade 10 and was characterized using classical morphological and physiological methods. Pathogenicity testing of collected Phytophthora isolates on detached leaves and cuttings of poplar (Populus nigra), black locust (Robinia pseudoacacia), hawthorn (Crataegus monogyna) and blackberry (Rubus fruticosus) was conducted. All Phytophthora isolates were able to cause leaf necrosis on each of tested plant species with varying intensity, except for P. gallica that did not infect only poplar leaves, but induced disease symptoms on the other three plant species. Additional experiments with cuttings of the analyzed plants confirmed the ability of P. gonapodyides, P. chlamydospora and P. plurivora to infect poplar, black locust and hawthorn. All isolated Phytophthora species were determined as a potential threat for the riparian ecosystems in the region based on the result of pathogenicity analyses. [ABSTRACT FROM AUTHOR]