VIEW ARTICLE

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 (pH_b) was relatively poor. A higher correlation existed between disease severity and rhizosphere pH (pH_r). The pH_r 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 pH_r decreased below 7.0 and was greatly reduced at pH_r values below 6.6. In comparable soil treated with methyl bromide, disease was controlled only when pH_r 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 pH_r 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.