Pythium spp. were isolated from roots of carrots with symptoms of root dieback, other muck crops, and weeds collected at the Holland–Bradford Marsh in Ontario. The isolates were identified and their pathogenicity to cultivars of carrot and other crop plants was examined. P. irregulare, P. sylvaticum, P. sulcatum, P. torulosum, P. intermedium, P. afertile and an undescribed species designated Pythium sp. NNK1 were recovered from carrot roots. Of these, only isolates of P. sulcatum, Pythium sp. NNK1, and P. irregulare produced severe root dieback when inoculated to carrots. P. sulcatum and Pythiúm sp. NNK1 produced severe symptoms not only in cultivars with known susceptibility to root dieback, but also in those with field tolerance. Pythium spp. found in carrots, except P. sulcatum, P. torulosum, and P. afertile, occurred in several other crop and weed hosts. Control measures for root dieback and the erratic distribution of the disease are discussed.