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Several Pythium species causing leak on potato are managed by the systemic fungicide metalaxyl-M. Metalaxyl-M-resistant (MR) isolates of Pythium spp. have been identified in potato production areas of the U.S.A., but information is lacking on the distribution of MR isolates in the Pacific Northwest. Soil samples from numerous fields (312) cropped to potatoes in Idaho (140), Oregon (59), and Washington (113) were assayed using metalaxyl-Mamended agar for the presence of MR isolates of Pythium in 2004 to 2006. Altogether, 1.4%, 42.4% and 32.7% of the fields from these states, respectively, were positive for MR Pythium. Isolates of Pythium ultimum that were highly resistant to metalaxyl were recovered from 53 fields representing ID, OR, and WA. Greater than 50% of the Pythium soil population consisted of MR isolates in ten of 64 fields from Oregon and Washington. Nine species of Pythium were recovered from soil samples, of which MR P. ultimum and P. spinosum were identified. Isolates of MR P. ultimum recovered from soil were pathogenic on potato tubers and may pose a serious threat to the management of Pythium leak and seed rot of diverse crops rotated with potato.Resumen Varias especies de Pythium que causan la pudrición acuosa en papa son controladas por el fungicida sistémico metalaxilo-M. Cepas de metalaxilo-M-resistente (MR) de Pythium spp. han sido identificadas en áreas de producción de papa de los Estados Unidos, pero no hay información de la distribución de cepas MR en el Pacifico Noroeste. Muestras de suelo de numerosos campos (312) cultivados con papa, en Idaho (140), Oregon (59), y Washington (113) fueron ensayadas utilizando agar metalaxilo-M-enmendado para la presencia de Pythium MR del 2004 al 2006. En total, 1.4%, 42.4% y 32.7% de los campos de estos estados, respectivamente, dieron positivo a Pythium MR. Cepas de Pythium ultimum altamente resistentes al metalaxilo fueron recuperadas de 53 campos representando a Idaho, Oregon y Washington. Más del 50% de la población de Pythium del suelo, consistió de cepas MR en 10 de los 64 campos de Oregon Am.

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In vitro dose responses of several calcium and potassium salts were determined on some commercially significant plant pathogens, including: Helminthosporium solani, Fusarium oxysporum f. sp. pisi race 2, Colletotricum coccodes, Phytophthora cactorum, Phytophthora cinnamomi, Phytophthora erythroseptica, Phytophthora infestans, Phytophthora megasperma, Pythium ultimum, and Venturia inaequalis. Mycelial growth inhibition was both salt-specific and dose-related. Pythium ultimum was completely inhibited by 75 mg·L−1 or greater calcium propionate, but needed 300 mg·L−1 or greater of calcium acetate and 40 mL·L−1 or greater of potassium silicate for complete inhibition. Phytophthora infestans was completely inhibited by 150 mg·L−1 or greater calcium acetate, 150 mg·L−1 or greater calcium propionate, or 5 mL·L−1 or greater potassium silicate. Phytophthora cactorum was completely inhibited by 300 mg·L−1 or greater calcium propionate, but required 600 mg·L−1 or greater calcium acetate and 10 mL·L−1 or greater potassium silicate for complete inhibition. Phytophthora cinnamomi was completely inhibited by calcium propionate at 600 mg·L−1 or greater or by 10 mL·L−1 or greater potassium silicate. Only potassium silicate inhibited Phytophthora megasperma, Phytophthora erthroseptica, V. inequalis, and H. solani at concentrations of 5 mL·L−1 or greater, 20 mL·L−1 or greater, 40 mL·L−1 or greater, or 80 mL·L−1 or greater, respectively. Potassium acetate did not completely inhibit any of the pathogens in this study when tested at concentrations 1200 mg·L−1 or less.