On the Relation of Airhumidity to Conidial Formation in the Rice Blast Fungus, Piricularia Oryzac, and the Characteristics in the Germination of Conidia produced by the Strains showing Different Pathogenicity

1. In the present paper, the results of the experimental studies on the relation of air-humidity to conidial formation of the rice blast fungus on the affected leaves were first described. Then, the characteristics of the strains of the same fungus differing in their pathogenicities were compared in germinating conidia.2. Rice seedlings of about 20 cm height were sprayed with the suspension of conidia. As soon as the diseased spots appeared on the inoculated leaves, they were put into large desiccators, in which the air was controlled at various constant humidities by the use of different salts or sulphuric acid of varying concentrations. The results show that the causal fungus is capable of producing the conidia on the spots in the air maintaining 93% or higher in relative humidity. Although the size of the spots increased gradually, the conidia were not produced in the air maintaining 88% or lower in the relative humidity. At 90 or 89% they were scarcely produced.3. The percentage of the germinated conidia and also the length of the germ-tubes seem to have no correlation to the difference of pathogenicity shown by strains of the fungus. However, in the present experiment the tendency of producing two germtubes from a single conidium was higher in a strain of the strongest pathogenicity than in a strain of the weakest pathogenicity.