Abstract
One hundred and seventy Pythium isolates obtained from carrot cavity spot lesions from a field in Western Australia were found to belong to either P. coloratum or P. sulcatum. In laboratory pathogenicity tests on mature carrots, all isolates of P. coloratum produced large brownish-black water soaked and depressed lesions. In comparison, only a few isolates of P. sulcatum produced lesions, all of which were small. In glasshouse pathogenicity trials, P. coloratum produced substantial and numerous lesions at an inoculum density of 0.5% (weight of millet seed-based inoculum/weight of soil), whilst P. sulcatum produced only a few small lesions at inoculum densities of 0.8 and 1% and none at 0.5%. This is the first record of P. coloratum as a casual agent of cavity spot disease of carrots.