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Relationship Between Take-all of Wheat and Rhizosphere pH in Soils Fertilized with Ammonium vs. Nitrate-Nitrogen. R. W. Smiley, Research Assistant, Department of Plant Pathology, Washington State University, Pullman, Washington 99163, Senior author presently NATO Postdoctoral Fellow, C.S.I.R.O. Division of Soils, Glen Osmond, S.A., Australia; R. J. Cook, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Pullman, Washington 99163. Phytopathology 63:882-890. Accepted for publication 22 January 1973 . DOI: 10.1094/Phyto-63-882.

Take-all of wheat caused by Ophiobolus graminis was reduced by ammonium-nitrogen (NH4-N) supplemented with 2-chloro-6-(trichloromethyl) pyridine (N-Serve 24) to slow nitrification, but was severe with no added N, or with Ca(NO3)2 at N rates equivalent to that supplied by NH4- N. The addition of lime (CaO) negated control with NH4-N. The correlation between disease severity and bulk soil pH (pHb) was relatively poor. A higher correlation existed between disease severity and rhizosphere pH (pHr). The pHr dropped with uptake of NH4 -N by roots, increased with uptake of NO3 -N, and remained generally unchanged with no added N. Disease severity in nonsterile soil was progressively less as the pHr decreased below 7.0 and was greatly reduced at pHr values below 6.6. In comparable soil treated with methyl bromide, disease was controlled only when pHr dropped below 5.0. Pathogen growth was nil in sterile and nonsterile soil at pH less than 5.0. Reduced disease apparently resulted from direct inhibition of the pathogen at pHr less than 5.0 and indirect inhibition (possibly a biological control) above 5.0. Best control in field plots occurred when (NH4)2 SO4 was mixed into the tilled layer rather than broadcast.

Additional keywords: biological control, soil fumigation.