Naturally occurring fluorescent pseudomonads involved in suppression of black root rot of tobacco.
Fluorescent pseudomonads were isolated from tobacco roots grown in soils naturally suppressive to black root rot (Thielaviopsis basicola). In the suppressive soil, fluorescent pseudomonads could be detected only to a depth of 1 m; below this 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 be isolated following but not prior to this addition. Several strs. of these fluorescent pseudomonads were isolated, cloned and tested for suppressiveness by introducing them into a conducive soil. A highly suppressive str., CHAo, chosen for further tests, was identified as Pseudomonas fluorescens. Black root rot of tobacco was suppressed in 36 of 39 conducive soil samples by adding CHAo at 107 cfu/cm3 of soil. CHAo could not be reisolated from the soil samples that remained conducive.