Degrees of sensitivity to metalaxyl within the Pythium spp. pathogenic to wheat in the Pacific Northwest.

sensitive species, including Phytophthora Cook, R. J., and Zhang, B.-X. 1985. Degrees of sensitivity to metalaxyl within the Pythium spp. infestans (10), Pseudoperonospora pathogenic to wheat in the Pacific Northwest. Plant Disease 69:686-688. cubensis (11), and Pythium aphanidermatum (13). In addition, resistance to A minimum of 20-25%, and in some cases more than 80%, of Pythium colonies selected at random metalaxyl has been induced to Phytophfrom soil-dilution plates of a Pythiumi-selective medium grew when transferred to the same medium me gasben inducedito iis(9 amended with metalaxyl at 1 ppm. The other colonies made no visible growth within 72 hr. Isolates thora megasperma f. sp. medicagini of P. ultimum var. ultimum, P. ultimum var. sporangiiferum, P. aristosporum, P. heterothallicum and Pythium ultimum (2). However, the (both male and female isolates), an unidentified heterothallic species (both male and female wheat fields where metalaxyl was isolates), and an unidentified homothallic Pythium sp., all from wheat roots, failed to grow within ineffective had no prior history of 72 hr on the selective medium amended with metalaxyl at 1 ppm. In contrast, isolates of P. metalaxyl use, ruling out any possibility torulosum, P. irregulare, and a second unidentified homothallic Pythium sp., also from wheat that a resistant strain had been favored. roots, grew on the metalaxyl-amended Pythium-selective medium at 15-50% of their respective In preliminary work (8), a portion of the normal growth rates on the same medium without metalaxyl. On cornmeal agar with metalaxyl at 1 Pythium population in Pacific Northwest ppm, the less sensitive isolates grew at 30-80% of normal growth on the same medium without wheat fields was relatively insensiti metalaxyl. With 0. 1 ppm of metalaxyl added to cornmeal agar, most isolates of the more sensitive metalaxyl. Of the four Pythium spp. species grew at 70-80% and those of the less sensitive species grew at 95-100% of normal growth. The failure of metalaxyl to inhibit growth of some species of Pythium involved in root rot of Pacific tested, P. torulosum showed the least Northwest wheat may account for observed failures of this fungicide to control this disease in the sensitivity. At least 10 species or subfield. species are involved in the Pythium root rot complex of Pacific Northwest wheat