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Species of Pythium from Greenhouses in Pennsylvania Exhibit Resistance to Propamocarb and Mefenoxam

Pythium isolates from commercial greenhouses in Pennsylvania were tested in vitro and in vivo for sensitivity to propamocarb. Isolates of Pythium aphanidermatum, P. irregulare, and P. ultimum consistently infected geranium seedlings that had been treated with propamocarb. P. ultimum and P. irregulare isolates exhibited dual fungicide resistance by overcoming propamocarb, mefenoxam, and a mixture of propamocarb and mefenoxam to infect seedlings. The sensitivity of isolates to propamocarb in vitro was not a good predictor of in vivo sensitivity. This is the first report of Pythium spp. resistance to propamocarb and dual resistance to propamocarb and mefenoxam.