The effect of environmental pH on the toxic secretions of the fungus

The fungus Meira argovae was recently isolated from the cadavers of the citrus rust mite. M. argovae is an antagonist towards phytophagous mites (91% mortality vs. 31% at control), against phytopathogenic fungi (Penicilliumsp., Botrytis sp., Sclerotinia sp. and Fusarium spp.) and bacteria (Erwinia sp., Xanthomonas sp. and Agrobacterium sp.). HPLC analysis led to the resolution of a single bioactive compound that is secreted by the fungus. Mortality rates differed between citrus rust mites on ripe (pH 6.8) and raw (pH 5.3) red grapefruits (81% and 60%, respectively). Experiments in which the fungus was grown on buffered liquid of different pH values (between 5-7) showed that most toxic secretion occurred at pH 7, and that the more the basic the medium, the more toxin produced. It is suggested that the antagonism of the fungus is mainly due to a single toxic compound. We postulate that the fungus secretes substances into the medium in order to alkalinize the environment, which allows high levels of toxin secretion. In summary, the fungus M. argovae is a potential biocontrol agent with capabilities to control many plant pests. Its main mechanism of antagonism probably involves a pH-dependent secretion of a toxic metabolite. Thirteen non-antagonistic M. argovae mutants were isolated by insertional mutagenesis; their further characterization could contribute to the understanding of the antagonistic mechanism of this fungus.