Sporothrix flocculosa, a biocontrol agent reported to act by antibiosis, had a differential effect against four target fungi, based on video microscopy records of the interactions. Two of the fungi, Botrytis cinerea and Cladosporium cucumerinum, were very susceptible and reacted ahead of the advancing colony of S. flocculosa. Typical reactions included retraction of the plasmalemma, aggregation of the cytoplasm, and cell death. When S. flocculosa was inoculated directly onto a colony of B. cinerea, conidia of the biocontrol agent developed toward the pathogen, and the hyphae of B. cinerea ceased cytoplasmic activity and became highly vacuolated within 8 h. Individual conidia of S. flocculosa produced several germ tubes, but did not attack live, necrotic, or dead hyphae of the pathogen. The other two tested fungi, Fusarium oxysporum f. sp. radicis-lycopersici and Idriella bolleyi, were unaffected by the presence of S. flocculosa. These results correlated with previous observations that C. cucumerinum and F. oxysporum f. sp. radicis-lycopersici were, respectively, sensitive and tolerant against purified toxins of S. flocculosa. This indicates that the host range of S. flocculosa is dictated by host sensitivity to its toxins.
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Richard R. Bélanger (1996) studied this question.