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Relationship Between Root Colonization and Suppression of Gaeumannomyces graminis var. tritici by Pseudomonas fluorescens Strain 2-79. Carolee T. Bull, Graduate research assistant, Department of Plant Pathology, Washington State University, Pullman, WA 99164-6430, Research assistant, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; David M. Weller, and Linda S. Thomashow. Research plant pathologist, and research geneticist, respectively, Agricultural Research Service, U.S. Department of Agriculture, Pullman, WA 99164-6430. Phytopathology 81:954-959. Accepted for publication 1 April 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, 1991. DOI: 10.1094/Phyto-81-954.

Pseudomonas fluorescens 2-79RN10 produces the antibiotic phenazine-1-carboxylic acid (Phz+) and is a biocontrol agent of take-all of wheat. This study demonstrated a positive relationship between root colonization by 2-79RN10 and biological control of take-all. In natural soil, either infested or not infested with Gaeumannomyces graminis var. tritici, the population sizes of 2-79RN10 (Phz+), 2-79-B46 (phenazine deficient, Phz), and 2-79-B46R (genetically restored, Phz+) detected on wheat roots were directly related to the dose of bacteria applied to the seed. Furthermore, the population size of 2-79RN10 or 2-79-B46R on wheat seeds and roots and the number of lesions formed by G. g. tritici on seminal roots were inversely related. This study also demonstrated for the first time that phenazine-1-carboxylic acid is a major factor in suppression of G. g. tritici during primary infection of roots because strain 2-79-B46 failed to suppress lesion formation. No inverse correlation occurred between the population size of strain 2-79-B46 on seeds or roots and the number of root lesions.