(1) The activity in vitro of a new antifungal antibiotic named Blasticidin S and phenyl mercuric aceate against rice blast (Piricularia oryzae) was studied. The germination of spores was completely prevented at a concentration of 1μg/ml of both substances, when tested by the drop culture method. The inhibition effect of Blasticidin S against the formation of spores was also almost the same as that of phenyl mercuric acetate. But Blasticidin S was more effective to the inhibition of mycelial growth than was phenyl mercuric acetate. The concentration at which the growth of mycelium was almost completely inhibited, when grown in shaken liquid media, was less than 1μg/ml for Blasticidin S, and more than 10μg/ml for phenyl mercuric acetate.
(2) The stability of Blasticidin S on leaves of rice plants growing in the greenhouse was studied by the cup method. Blasticidin S could be detected at a concentration of 0.1μg/ml by the cup method assay using a spore suspension of Cladosporium fulvum as the test organism. The activity of Blasticidin S on leaves decreased gradually, and about one fifth of the original activity was found after four days.
(3) When the first internode of the rice seedling was coated with paste containing Blasticidin S, the antibiotic was found to be translocated to the upper stems and leaves. The paste was composed of 2 per cent antibiotic, 38 per cent petrolatum liquidum, 50 per cent petrolatum album, and 10 per cent glycerin. The leaves and stems above the treated stem were found to contain about 30μg/g of the antibiotic one day after the treatment.
(4) In the greenhouse Blasticidin S was found to be effective as a therapeutic fungicide against rice blast. When plants were sprayed with a solution containing 5μg/ml of Blasticidin S one or two days after inoculated with the spore suspension of P. oryzae, the number of leaf-spots developed on the plants was less than on those treated with 20μg/ml of phenyl mercuric acetate. However, when sprayed four days before inoculation, the protective effect of Blasticidin S was inferior to that of phenyl mercuric acetate.