Characterization of major secondary metabolites produced in soilless mix by a formulated strain of the biocontrol fungus Gliocladium virens

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Abstract
A strain of Gliocladium virens (GL-21 = G-20) formulated in alginate prill was grown for 3 days in a commercial soilless potting medium. Chloroform extractions were separated by thin-layer chromatography and individual bands from G-20 cultures in soilless medium were identified by standards, mass spectrometry, and nuclear magnetic resonance as (i) a mixture of fatty acids, (ii) viridin, (iii) gliotoxin, (iv) dimethylgliotoxin, (v) viridiol, and (vi) a mixture of phenolics including ferulic acid. None of these compounds was detected in the medium not amended with prills of isolate G-20 of G. virens. Other isolates of G. virens, including G-3 and G-9, produced gliovirin and did not produce gliotoxin. Of all the metabolites produced by G-20 in soilless medium, only gliotoxin strongly inhibited germination of sporangia and growth of Pythium ultimum (growth prevention at 1.0 μg/mL), growth of Rhizoctonia solani mycelium (0.5 μg/mL), or germination of sclerotia and growth of Sclerotium rolfsii (50 μg/mL). Viridin was more inhibitory to growth of R. solani (1.0 μg/mL, minimum inhibitory concentration) than to P. ultimum (25 μg/mL) or S. rolfsii (50 μg/mL). Growth of G. virens isolate G-20 was inhibited by 25 μg/mL of viridin but not by 100 μg/mL gliotoxin. In contrast, growth of isolate G-3 was inhibited by 50 μg/mL gliotoxin but not by 100 μg/mL of viridin. Dimethylgliotoxin, fatty acids, and phenolics were not inhibitory to any of the fungi tested. These results suggest that gliotoxin is the major antibiotic metabolite inhibitory to Pythium and Rhizoctonia of this formulated strain of G. virens. This information will be useful for monitoring metabolite production by G. virens, determining optimum gliotoxin production in situ, and improving strain performance for biocontrol of plant pathogens. Key words: antibiotic, biological control, antagonist.

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