VIEW ARTICLE

Ecology and Epidemiology

Influence of In Situ and In Vitro pH on Suppression of Gaeumannomyces graminis var. tritici by Pseudomonas fluorescens 2-79. Bonnie H. Ownley, U. S. Department of Agriculture, Agricultural Research Service, Washington State University, Pullman 99164-6430; David M. Weller, and Linda S. Thomashow. U. S. Department of Agriculture, Agricultural Research Service, Washington State University, Pullman 99164-6430. Phytopathology 82:178-184. Accepted for publication 1 August 1991. 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, 1992. DOI: 10.1094/Phyto-82-178.

Pseudomonas fluorescens 2-79 (NRRL B-15132) and its rifampicin-resistant derivative 2-79RN₁₀ are suppressive to take-all of wheat caused by Gaeumannomyces graminis var. tritici. Production of the antibiotic phenazine-1-carboxylic acid (PCA) is the primary mechanism of suppression by this strain, while a fluorescent siderophore and a second iron-regulated factor have minor roles. Hyphal growth of G. g. tritici was inhibited by strain 2-79RN₁₀ on modified Kanner agar (KMPE) (used for PCA production) adjusted to several pH values within the range of 4.9 to 8.0. Inhibition in vitro by strain 2-79RN₁₀ was greatest at pH 6.0–6.6, intermediate at pH 6.8–8.0, and least at pH 4.9–5.8. Inhibition of hyphal growth of the fungus at low pH (KMPE adjusted to 4.9 to 5.8) was due largely to acidity in the medium. Purified PCA in KMPE adjusted to pH 6.0, 6.3, 6.6, 6.8, 7.0, 7.2, 7.4, or 7.8 also inhibited colony growth of G. g. tritici at all pH values; inhibition was greatest at pH 6.0 and decreased as the pH of the medium increased. To determine the effect of soil pH on take-all suppression, wheat seeds were treated with strain 2-79RN₁₀ or mutants deficient in production of PCA (Phz^–, fluorescent siderophore (Flu^–), anthranilic acid (Aff^–), or a combination of these traits. Seeds were sown in steamed Ritzville silt loam (pH 7.6, 33.6% sand, 60.0% silt, and 6.4% clay) and the same soil adjusted to pH 4.9, 5.6, 6.1, 6.8, 7.3, or 8.0. Strain 2-79RN₁₀ and a Phz⁺ Flu^– Aff⁺ mutant derivative (strain 2-79-59.34) both significantly suppressed take-all at all soil pH values tested. All Phz^– mutants of 2-79RN₁₀ were generally less suppressive than Phz⁺ strains.