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Identification and Comparative Pathogenicity of Pythium species from Wheat Roots and Wheat-Field Soils in the Pacific Northwest. C. Chamswarng, Assistant professor, Department of Plant Pathology, Kasetsart Univ., Bangkok 10903, Thailand; R. J. Cook, research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Pullman, WA 99164. Phytopathology 75:821-827. Accepted for publication 4 March 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/Phyto-75-821.

Ten species and varieties of Pythium, two unidentified, were recovered from wheat roots and wheat-field soils in eastern Washington and northern Idaho. Of these, P. ultimum var. ultimum, P. u. var. sporangiiferum, P. aristosporum, P. volutum, P. torulosum, P. irregulare, P. sylvaticum complex and Pythium sp. D (unidentified) are homothallic, and P. heterothallicum and Pythium sp. E (unidentified) are heterothallic. Of 302 isolates from a single site, 165 were homothallic, 137 were heterothallic, and seven species were identified. Pathogenicity was tested on Daws wheat at 15 C with either mycelial inoculum in a mixture of cornmeal, sand, and vermiculite, or with oospores added to fumigated soil (500 and 1,000 oospores per gram). All 10 species and subspecies were pathogenic. P. aristosporum and P. volutum were the most pathogenic and caused seed decay, severe root rot, and root browning. P. ultimum (both varieties), P. sylvaticum complex, and P. irregulare were next most pathogenic and caused seed decay, root rot, and root necrosis. P. torulosum, P. heterothallicum, Pythium sp. E and Pythium sp. D were least pathogenic and caused mild root necrosis and little or no seed decay. In general, the Pythium species resulted in less seedling emergence; shorter roots, shoots, and first leaves; and less dry weight of roots and shoots compared with the healthy plants from the controls.

Additional keywords: soilborne pathogens.