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Nullification of Antagonism of Phoma betae by Bacillus subtilis var. niger in Soil and in a Simulated Rhizosphere. F. Schönbeck, Lecturer at the Institute for Plant Diseases, University of Bonn, Germany; W. A. Kreutzer, Professor of Botany and Plant Pathology, Colorado State University, Fort Collins 80521. Phytopathology 61:1447-1450. Accepted for publication 12 July 1971. DOI: 10.1094/Phyto-61-1447.

Bacillus subtilis var. niger, a sugar beet rhizosphere colonist, was antagonistic to the seed-borne sugar beet pathogen Phoma betae in culture. The bacterium caused initial retardation of sugar beet seedling attack by the fungus, but gave no control either in steamed (autoclaved) or nonsteamed soils. Competitive parasitism of sugar beet seedlings by indigenous species of Pythium in raw soil was prevented by soil treatment with p-dimethylaminodiazobenzene sodium sulfonate. This chemical had no effect on P. betae or B. subtilis at the dosage used. In a simulated rhizosphere, it was found that the antagonism of B. subtilis niger to P. betae was reduced by Bacillus megaterium or a mixed population of soil bacteria, and almost completely nullified by Escherichia coli, an atypical soil organism. The degree of antagonism of E. coli to B. subtilis niger was postively correlated with the population of E. coli. This antagonism was not eliminated by the addition of nutrients. Bacillus subtilis niger held in contact with E. coli for 24 hr became nonviable, and failed to recover even when transferred to new media.

Additional keywords: Pyrenochaeta terrestris, Fusarium spp., Rhizoctonia solani, Pythium ultimum.