Degrees of Sensitivity to Metalaxyl Within the Pythium spp. Pathogenic to Wheat in the Pacific Northwest. R. J. Cook, Research Plant Pathologist, U.S. Department of Agriculture, Agricultural Research Service, Washington State University, Pullman 99164. Bing-Xing Zhang, Visiting Plant Pathologist to Washington State University from Zhejiang Agricultural University, Hangzhou, Zhejiang, Peoples Republic of China. Plant Dis. 69:686-688. Accepted for publication 8 February 1985 . This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/PD-69-686.

A minimum of 20–25%, and in some cases more than 80%, of Pythium colonies selected at random from soil-dilution plates of a Pythium-selective medium grew when transferred to the same medium amended with metalaxyl at 1 ppm. The other colonies made no visible growth within 72 hr. Isolates of P. ultimum var. ultimum, P. ultimum var. sporangiiferum, P. aristosporum. P. heterothallicum (both male and female isolates), an unidentified heterothallic species (both male and female isolates), and an unidentified homothallic Pythium sp., all from wheat roots, failed to grow within 72 hr on the selective medium amended with metalaxyl at 1 ppm. In contrast, isolates of P. torulosum, P. irregulare, and a second unidentified homothallic Pythium sp., also from wheat roots, grew on the metalaxyl-amended Pythium-selective medium at 15–50% of their respective normal growth rates on the same medium without metalaxyl. On cornmeal agar with metalaxyl at l ppm, the less sensitive isolates grew at 30–80% of normal growth on the same medium without metalaxyl. With 0.1 ppm of metalaxyl added to cornmeal agar, most isolates of the more sensitive 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 Northwest wheat may account for observed failures of this fungicide to control this disease in the field.

Keyword(s): Triticum aestivum.