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Carrot roots with cavity spot lesions from eight different counties in Norway were sampled and Pythium species were isolated on selective medium. Pythium spp. were characterised morphologically and by species-specific PCR. Laboratory experiments with inoculations of carrot roots were performed. A total of 130 isolates out of 230 Pythium-like isolates tested with PCR were identified as pathogenic species of Pythium. These were P. intermedium (29%), P. sulcatum (23%), P. sylvaticum (16%), P. violae (15%) and a possible new Pythium species designated P. ‘ vipa’ (18%). There were some differences between geographical regions and ages of cavities regarding the frequency of the different species isolated. When rating sunken lesions in the laboratory inoculation experiments, P.‘ vipa’ was the most aggressive and P. violae the least aggressive species. P. intermedium and P.‘ vipa’ caused more discolouration of the infected carrot tissue than the other species. The importance of the different Pythium spp. as agents of cavity spot in Norway is discussed. [ABSTRACT FROM AUTHOR]