Abstract
Field experiments carried out in East Africa have shown that seed dressings with Thiram (tetramethylthiuram disulphide) preparations may reduce the incidence of crown rot in groundnut ( Arachis hypogea L.) seedlings by at least 50 %. Seed treatment with organo-mercurials, on the other hand, has generally produced a substantial increase in the incidence of crown rot. Experiments designed to elucidate this effect of organo-mercurial seed dressings have indicated that it cannot be ascribed to any phytotoxic action. The effect can probably be attributed to the selectively toxic action of organo-mercurials upon the fungus flora of the soil in the vicinity of the treated seed. This selective toxicity has been shown to operate to the advantage of Aspergillus niger , certain strains of which have proved to be much more tolerant of organo-mercurials than other fungi of Urambo soil. Tolerant species of Penicillium have occasionally been encountered. Mercury tolerant strains of Aspergillus niger have been isolated from Urambo soil; they are distinguished from mercury sensitive strains both by a greater and more rapid development of acidity in the culture medium, and by a greater virulence in the production of crown rot when inoculated to groundnut seedlings.
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