Pythium spp. Associated with Crown Rot of Cucumbers in British Columbia Greenhouses

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Abstract
Studies were undertaken to determine the causal agent responsible for a root disease ("crown rot") of cucumbers [Cucumis sativus] occurring in British Columbia greenhouses. Three Pythium spp..sbd.P. aphanidermatum, P. irregulare, and Pythium sp. Group G.sbd.were regularly isolated from infected root crown tissue. A fourth species, P. coloratum, was isolated from grower propagation mix. Isolates representative of species were tested for pathogenicity against cucumber plants in greenhouse and growth chamber studies. All four species were pathogenic, as shown by their ability to reduce plant growth or cause death. Symptoms observed in commercial greenhouses were reproduced. P. aphanidermatum caused the most deaths in all studies, followed by P. irregulare and then Pythium sp. Group G. A disease survey in a commercial greenhouse revealed that plant-to-plant transmission of crown rot within sawdust bags occurred at a relatively low frequency. Of several potential sources of Pythium inoculum examined, peat-based propagation mixes used in B.C. greenhouses commonly contained Pythium spp., including those isolated from cucumber tissues infected with crown rot.