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Ecology and Epidemiology

Effects of Soil Matric Potential and Cell Motility on Wheat Root Colonization by Fluorescent Pseudomonads Suppressive to Take-All. W. J. Howie, Former research assistant, Department of Plant Pathology, Washington State University, Pullman, WA 99164, Present address: Postdoctoral fellow, Advanced Genetic Sciences, Oakland, CA 94608; R. J. Cook, and D. M. Weller. Research plant pathologists, Agricultural Research Service, U.S. Department of Agriculture, Pullman, WA 99164. Phytopathology 77:286-292. Accepted for publication 4 August 1986. 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, 1987.. DOI: 10.1094/Phyto-77-286.

Strains of Pseudomonas fluorescens suppressive to take-all caused by Gaeumannomyces graminis var. tritici, when introduced on wheat seed, became distributed on the elongating root at progressively lower cell densities with increasing distance from the seed. Typically, a cell density of 10⁷ colony-forming units (cfu) per seed resulted in about 10⁵ cfu/cm of root 1–3 cm below the seed and 10³ cfu/cm of root 7–9 cm below the seed by 6–7 days after seed germination. Bacteria could not have washed down the roots, because water movement in the experimental system was only toward the root. Motility mediated by bacterial flagella was not responsible for the downward movement of the introduced bacteria, because three nonmotile mutants each colonized roots and suppressed take-all as well as their respective motile parents. We propose that root colonization by bacteria introduced on the wheat seeds occurred in two phases. In phase 1, the bacteria became distributed by passive carriage downward with root extension through soil, thereby accounting for the progressively lower populations on the roots at increasing distances away from the seed. Phase 2 was the multiplication and survival phase and occurred during and after phase 1, whereby the population increased to the limits of the ecological niche. There were no significant differences among the populations of total aerobic bacteria at rhizosphere matric potentials between –0.05 and –4.0 bars, but populations of the introduced bacteria were greatest at –0.3 bar in a Shano silt loam and at –0.7 bar in a Thatuna silt loam and a Quincy loamy fine sand. The relatively low proportion of the introduced strain relative to total bacteria at –0.05 and –4.0 bars may have been due to inadequate oxygen for multiplication at –0.05 bar and to inability of the bacteria to maintain adequate turgor at –4.0 bars and drier. The populations detected at –4.0 bars may have reflected phase 1 but not phase 2 of colonization.