Interactions of Antagonist and Pathogen in Biological Control of Onion White Rot

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Abstract
A study was undertaken to identify stable antagonists to isolates of Sclerotium cepivorum for biological control of onion white rot. Antagonism was measured by inhibition zone size in vitro and percent white rot infection in plants of ''Autumn Spice'' in pot trials. Regression analysis and deviation from regression were used to determine the stability of the antagonists for biological control of onion white rot. Significant differences in antagonism were detected among the antagonists. The order of antagonists, ranked according to antagonism and ability to control onion white rot, depended on the isolate of S. cepivorum being tested. However, the statistical model quantitatively identified useful characteristics in the B2 isolate of Bacillus subtilis. This antagonist showed a high degree of stability for inhibition zone in vitro and percent infection to white rot in pot trial for the 13 S. cepivorum isolates that were tested. The bacterial antagonist B2 has a potential for biological control of onion white rot over a range of isolates of S. cepivorum from 5 countries [Australia, UK, the Netherlands, New Zealand, Canada]. Relationships of values obtained by the statistical analysis will probably be useful in selecting stable biological agents for control of plant diseases.

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