VIEW ARTICLE

Disease Control and Pest Management

Relation of Ammonia and Nitrous Acid to Suppression of Phytophthora in Soils Amended with Nitrogenous Organic Substances. Peter H. Tsao, Professor, Department of Plant Pathology, University of California, Riverside 92521; J. J. Oster, staff research associate, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 71:53-59. Accepted for publication 10 March 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-53.

Amendment of natural soils with chicken manure and urea at 2 and 0.1%, respectively, caused soil pH to rise from near neutral to above 8.6 and then fall to the acid range. Soil nitrogen analyses confirmed correspondence of this pH pattern to the processes of ammonification and nitrification. Results of in vitro tests with NH₄OH and KNO₂ solutions demonstrated that Phytophthora propagule germination was inhibited by low concentrations of NH₃ and HNO₂. A minimum of 4 ppm NH₃ or 0.3 ppm HNO₂ reduced Phytophthora cinnamomi sporangium germination to ≤ 1%; 6 ppm NH₃ or 0.5 ppm HNO₂ reduced P. parasitica sporangium germination to ≤1%; 17 ppm NH₃ or 0.9 ppm HNO₂ reduced P. parasitica chlamydospore germination to less than 10 and 5%, respectively. Additional in vitro tests with ammonium and nitrite salts showed that toxicity resided in the NH₄⁺ or NO₂^– portion of a compound. Furthermore, the pH dependency of germination inhibition in the ammonium/ammonia and nitrite/nitrous acid solutions demonstrated that NH₃ and HNO₂, respectively, are primarily responsible for the inhibition. NH₄OH at a given concentration was more toxic at pH 8 than at pH 6, but KNO₂ was more toxic at pH 6 than at pH 8. Sufficiently high concentrations of NH₄⁺ and NO₂^– were detected in amended soils at soil pH values favoring their presence in the nonionized forms, NH₃ and HNO₂, respectively, to account for significant germination inhibition. Therefore, these toxicants are responsible, at least in part, for Phytophthora suppression in amended soils.