Factors Influencing Antagonism ofChaetomium globosumtoVenturia inaequalis: A Case Study in Failed Biocontrol

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Abstract
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 aseffectivtly 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 oragarose-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 infeciton by V. inaqualis. On the phylloplane they can be degraded abiotically, thus accounting for loss of biocontrol. This does not prelude additional mechanism for the loss of control.