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Influence of Soil Treatments on Growth and Yield of Wheat and Implications for Control of Pythium Root Rot. R. J. Cook, Research plant pathologist, U. S. Department of Agriculture, Agricultural Research Service, Pullman, WA 99164; J. W. Sitton(2), and W. A. Haglund(3). (2)Agricultural research technician, U. S. Department of Agriculture, Agricultural Research Service, Pullman, WA 99164; (3)Plant pathologist, Department of Plant Pathology, Northwestern Washington Research and Extension Unit, Washington State University, Mt. Vernon 98272. Phytopathology 77:1192-1198. Accepted for publication 6 February 1987. 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, 1987. DOI: 10.1094/Phyto-77-1192.

The population of Pythium spp. (based on plate counts) was more than 300 propagules per gram of soil in 33 of 39 wheat fields sampled in eastern Washington and northern Idaho during 19831986, and none had less than 100 propagules per gram of soil in the top 15 cm. Treatment of plots with either methyl bromide, chloropicrin, or 1-3, dichloropropene and 17% chloropicrin eliminated 9599% of the inoculum and resulted in adult wheat plants that were 310 cm taller than those grown in nontreated soil. Solarization and burning a layer of straw (1012 t/ha) on the soil surface before sowing eliminated 8090 and 4050% of the Pythium inoculum, respectively, and likewise resulted in taller adult wheat plants. The correlation (r2) between population of Pythium before sowing and adult-plant heights was 0.54, significant at P = 0.001. Wheat yields were 1336% greater in response to fumigation in fields where wheat was grown every other year, 312% greater where wheat was grown every third year, and 19 and 14% greater, respectively, in response to solarization and burning straw. The greater yield of wheat in response to soil treatments resulted mainly from more heads per plant rather than more plants. In the greenhouse, heat treatment of soil at 45 C/20 min eliminated Pythium and resulted in seedlings up to 25% taller than those in untreated soil; the reestablishment of Pythium ultimum var. sporangiiferum at 500 propagules per gram in heat-treated soil again resulted in stunted seedlings. Treatment of seed with metalaxyl resulted in taller seedlings in natural soils and treated soils reinfested with Pythium, but not in soil free of Pythium. Roots were commonly stripped of root hairs as well as fine rootlets in the top 15 cm of soil in nontreated plots, and Pythium-like hyphae were observed in the root hairs from the field. The results indicate that Pythium is an important pathogen of Pacific Northwest wheat in areas receiving 4045 cm of precipitation annually.

Additional keywords: root disease, soilborne pathogen.