Studies on selective toxicity of fungicides

Abstract

Correlation between laboratory and field effectiveness of various fungicides against Piricularia oryzae, which causes rice blast, was studied. The fungicides tested were seven chemicals, copper sulfate, phenyl mercuric acetate, Zineb, Urbazid, Dichlone, Nirit and s-Triazine, and two antibiotics, Antimycin A and Blasticidin S. Among them phenyl mercuric acetate, Antimycin A and Blasticidin S had been known to be more effective as protectant fungicides against rice blast in the field than others. In the laboratory we observed the inhibition effects of these fungicides on spore germination, mycelial growth by shaking culture, and spore formation of P. oryzae. And also the minimum inhibitory concentration by the agar dilution method and the size of inhibition zone on agar plate by the cup method were measured.The spore germination was perfectly inhibited by all the fungicides tested, and the minimum inhibitory cencentrations of all the substances tested except copper sulfate were almost the same. The result measured by spore germination or agar dilution method seemed not to agreed with the field results.The mycelial growth and the spore formation were markedly inhibited by phenyl mercuric acetate, Blasticidin S and Antimycin A, but littele or less by others. Satisfactory correlation between the laboratory results measured by mycelial growth or spore formation method and the field results was obtained. Therefore the inhibition effect on mycelial growth and spore formation seemed to play an important role in determining a compound as a practical fungicide against rice blast in the field than that on spore germination.The sizes of inhibition zone were larger in the case of the three fungicides having field effects than in the case of others.