SOIL MICROBIOLOGICAL STUDIES ON AGROBACTERIUM RADIOBACTER AND BIOLOGICAL CONTROL OF CROWN GALL

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Abstract
This paper is an account of studies at the University of Adelaide on Agrobacterium radiobacter, the bacterium responsible for crown gall induction. The original intention was an ecological study of this organism but first it was necessary to be able to identify A. radiobacter and count it in its natural environment. It turned out that neither of these requirements was satisfied at the start of the investigation. A taxonomic study of the genus Agrobacterium had to be undertaken and this led to the definition of two distinct biotypes 1 and 2 of A. radiobacter, both containing pathogenic and nonpathogenic forms. Pathogenic biotype 2 agrobacteria are predominantly responsible for crown gall induction in Australia. A medium highly selective for biotype 2 agrobacteria was developed and this enabled their distribution and abundance to be measured.The ecological study provided strong circumstantial evidence that stone-fruit trees became galled only when they were growing in soil with a high ratio of pathogenic to nonpathogenic biotype 2 agrobacteria. This ratio could be changed by inoculating seed at time of sowing, or roots at time of transplanting, with nonpathogenic agrobacteria. Efficient biological control of crown gall was achieved by this inoculation which is now widely practiced by commercial growers in Australia. The mechanism of biological control operates through the production of a bacteriocin by the controlling organism. This paper is an account of studies at the University of Adelaide on Agrobacterium radiobacter, the bacterium responsible for crown gall induction. The original intention was an ecological study of this organism but first it was necessary to be able to identify A. radiobacter and count it in its natural environment. It turned out that neither of these requirements was satisfied at the start of the investigation. A taxonomic study of the genus Agrobacterium had to be undertaken and this led to the definition of two distinct biotypes 1 and 2 of A. radiobacter, both containing pathogenic and nonpathogenic forms. Pathogenic biotype 2 agrobacteria are predominantly responsible for crown gall induction in Australia. A medium highly selective for biotype 2 agrobacteria was developed and this enabled their distribution and abundance to be measured. The ecological study provided strong circumstantial evidence that stone-fruit trees became galled only when they were growing in soil with a high ratio of pathogenic to nonpathogenic biotype 2 agrobacteria. This ratio could be changed by inoculating seed at time of sowing, or roots at time of transplanting, with nonpathogenic agrobacteria. Efficient biological control of crown gall was achieved by this inoculation which is now widely practiced by commercial growers in Australia. The mechanism of biological control operates through the production of a bacteriocin by the controlling organism. © Williams & Wilkins 1974. All Rights Reserved.

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