Twenty-seven of 31 irrigation sources serving the Okanagan and Similkameen valleys of British Columbia were contaminated withPhytophthora. P. cactorum, the cause of collar rot of fruit trees in this region, was recovered from 15 sources;P. cambivoraandP. citricolafrom 1 each;P..megaspermafrom 4 sources. Many isolates of uncertain identity also were recovered.P. cactorum, P. cambivora, andP. citricolawere pathogenic to rootlets of young apple seedlings in laboratory tests, but onlyP. cactorumis considered an important pathogen in this region.