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Factors Influencing Antagonism of Chaetomium globosum to Venturia inaequalis: A Case Study in Failed Biocontrol. Mark A. Boudreau, Department of Plant Pathology, University of Wisconsin, Madison 53706; John H. Andrews, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 77:1470-1475. Accepted for publication 24 March 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1470.

The apple scab antagonist Chaetomium globosum, applied as ascospores to apple leaves in the growth chamber or field, did not colonize the leaf surface even where scab control was observed. In further growth chamber trials, heat-killed ascospores controlled scab as effectively as viable spores. Antagonism of C. globosum ascospores or culture extract to Venturia inaequalis was not observed when either was applied several days in advance of the pathogen to seedlings or agarose-coated slides. Inhibitory activity of culture extract in vitro declined somewhat over time or when incubated at pH 5.5, 7.0, or 8.8; activity was substantially reduced at pH 11.1 or when extract was dried or aerated for 6 days. The results suggest that antibiotics from C. globosum ascospores can diffuse passively onto the leaf surface and inhibit infection by V. inaequalis. On the phylloplane they can be degraded abiotically, thus accounting for loss of biocontrol. This does not preclude additional mechanisms for the loss of control.