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Factors Influencing the Incidence of Embryo Infection by Pythium spp. During Germination of Wheat Seeds in Soils. R. Fukui, Graduate research assistant, Department of Plant Pathology, Washington State University, Present address: Department of Plant Pathology, The University of Hawaii at Manoa, Honolulu 96822; G. S. Campbell(2), and R. J. Cook(3). (2)professor, Department of Crop and Soil Science, Washington State University; (3)research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Pullman, WA 99164-6430. Phytopathology 84:695-702. Accepted for publication 30 March 1994. 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, 1994. DOI: 10.1094/Phyto-84-695.

The incidence of embryo infection (EI) by Pythium spp. in soil held at a matric potential (ψm) of 10 J/kg was a function of inoculum density (ID) at lower IDs and reached a plateau (EImax) at higher IDs. In field soils, EImax (69.989.0%) was achieved at 150200 propagules per gram. In pasteurized soil reinfested with P. ultimum var. sporangiiferum, EImax (≅100%) was achieved with only 50 propagules per gram. The incidence of EI was greatest at 10 J/kg, progressively less as soil was drier, and negligible at drier than 200 J/kg. Both ID and ψm could be limiting for EI at lower IDs, whereas only ψm affected EI when soil contained enough ID for EImax. When the pH levels of two natural soils were adjusted to 4.37.6 with H2SO4 or Ca(OH)2, the incidence of EI was greatest at pH 5.16.7. The treatment of soil with chloramphenicol resulted in a higher incidence of EI in soil adjusted to alkaline pH, suggesting that antagonistic soil bacteria had some involvement in the low incidence of EI in alkaline soil. The incidence of EI was higher as seeds aged and lower when seeds were soaked in water before planting.

Additional keywords: Pythium carrying capacity, resident antagonists, seedling blight, soilborne pathogens.