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Naturally Occurring Fluorescent Pseudomonads Involved in Suppression of Black Root Rot of Tobacco. E. W. Stutz, Institut fur Phytomedizin, Eidgenossische Technische Hochschule, 8092 Zurich, Switzerland; G. Defago(2), and H. Kern(3). (2)(3)Institut fur Phytomedizin, Eidgenossische Technische Hochschule, 8092 Zurich, Switzerland. Phytopathology 76:181-185. Accepted for publication 5 September 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-181.

Fluorescent pseudomonads were isolated from tobacco roots grown in soils naturally suppressive to black root rot caused by Thielaviopsis basicola. In the suppressive soil, fluorescent pseudomonads could be detected to a depth of 1 m: below 1 m, however, no fluorescent pseudomonads were found and suppressiveness was lost. Heat treatment of the suppressive soil at 60 C for 30 min nullified suppressiveness and fluorescent bacteria could no longer be isolated. Suppressiveness was readily transferred by addition of 5% or more suppressive soil to a conducive soil. Fluorescent pseudomonads could then be isolated following but not prior to this addition. Several strains of these fluorescent pseudomonads were isolated, cloned, and tested for suppressiveness by introducing them into a conducive soil. A highly suppressive strain. CHAo, was chosen for further tests and was identified as Pseudomonas fluorescens. Black root rot of tobacco was suppressed in 36 of 39 conducive soil samples by adding strain CHAo at 107 cfu/cm3 of soil. Strain CHAo could not be reisolated from the soil samples that remained conducive.