Identification and Comparative Pathogenicity ofPythiumspecies from Wheat Roots and Wheat-Field Soils in the Pacific Northwest

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Abstract
Ten species and varieties of Pythium, 2 unidentified, were recovered from wheat roots and wheat-field soils in eastern Washington and northern Iadho. 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 7 spp. were identified. Pathogenicity was tested on Daws wheat at 15.degree. C with either mycelial inoculum in a mixture of cornmeal, sand and vermiculite, or with oospores added to fumigated soil (500 and 1000 oospores per gram). All 10 spp. 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.