Abstract
Hymexazol (3-hydroxy-5-methylisoxazole), which has been developed as a soil-fungicide, was found to be effective against rice sheath blight caused by Pellicularia sasakii. In these experiments, its effectiveness with respect to the time at which application was made and its antifungal activities in vitro, in the greenhouse and in the field were investigated. The results were as follows: 1) Hymexazol was more effective as an eradicant than as a protectant: it showed no apparent inhibitory effect on the mycelial growth on leaves which had been treated previously by the chemical. In field trials, a significant controlling effect of hymexazol was noticed when the chemical was used shortly after inoculation. 2) The inhibitory effect of hymexazol when checked by the agar dilution method against P. sasakii was not so remarkable; however, the mycelium of P. sasakii which was grown of PSA medium containing hymexazol was thin and its sclerotium formation was inhibited at the low concentration of 10-100 μg/ml for over 10 days after treatment. The pathogenicity of this fungus was weakened by the treatment with hymexazol. On the other hand, the growth of P. sasakii was completely inhibited at a very low concentration of hymexazol (2.5 μg/ml) in a liquid culture. 3) The enlargement of lesions of rice sheath blight was retarded by direct contact of hymexazol with the pathogen. When hymexazol was applied to a mycelial colony of P. sasakii, the intensity of the pathogenicity of the peripheral mycelium which was not in direct contact with hymexazol was apparently weakened.
