Summary:

Occurrence of Phytophthora spp. in Polish rivers and pathogenicity of iso-lates toward some plants were studied. Rivers Jasieniec, Pisia and Rawka were surveyed during all the year with one month interval. First 2 rivers are situated in horticultural areas in Warsaw and Łódź districts whereas the third one is running through agricultural fields and forests. Rhododendron leaves were used as the baits for detection of Phytophthora from water. Number of necrotic spots on baiting leaves was a measure of population densities of Phytophthora spp. in rivers. The occurrence of Phytophthora in water was noticed during all year with the lower density in the 3rd quarter of the year. Five Phytophthora species and taxon Salixsoil were identified among isolates recovered from 3 rivers. During all year from 3 surveyed rivers (except from Pisia in the first quarter) P. citricola was recovered as the most often occurring species. Additionally P. cactorum, P. cambivora, P. cinnamomi and P. megasperma were detected from rivers. In laboratory trials water isolates of P. cactorum and P. citricola colonized roots, stem parts and leaves of birch with the significantly faster spread of necroses on plant parts inoculated by P. citricola. In greenhouse trial both species caused root and stem base rot symptoms of birch seedlings within 4 weeks of growth. Isolates of P. citricola recovered from river from March to October colonized rhododendron leaf blades with significantly slower spread of necroses on tissues inoculated with cultures detected from water in June, September and October. It was shown that P. citricola may be bring to thuja nursery with water used for plant sprinkling and is the causal agent of tip blight.