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Interactions of Antagonist and Pathogen in Biological Control of Onion White Rot. R. S. Utkhede, Research scientist, Agriculture Canada Research Station, Summerland, British Columbia, Canada V0H 1Z0; J. E. Rahe, associate professor, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada V5A 1S6. Phytopathology 73:890-893. Accepted for publication 17 November 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-890.

This 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 cultivar 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. This indicates that the bacterial antagonist B2 has a potential for biological control of onion white rot over a range of isolates of S. cepivorum from five countries. It is also suggested that relationships of values obtained by the statistical analysis will be useful in selecting stable biological agents for control of plant diseases.