Abstract
1. This paper deals with the results of the experiments on the pathogenicity of culture strains of the rice-blast fungus as well as on the varietal susceptibility of the rice plant to the disease. In the experiments, the leaves of the rice plant grown to the booting stage and the pedicels of spikes were inoculated with the spore suspensions of nine culture strains of the fungus.
2. The pathogenicity of each culture strain was obviously lower when inoculated on the leaves in the booting stage than when inoculated in the seedling stage. The writer was able to recognize, however, a distinct differentiation in the pathogenicity of culture strains of the fungus tested. The strains No.XIII and No.IX showed the strongest pathogenicity. The pathogenicity of the strain No.V was weakest, that of No.XXII being medium. The strains No.XVIII and No.XX seemed to have a rather weak pathogenicity. In all experiments the strains No.XVII and No.VII indicated no pathogenicity on the leaves of the grown plants.
3. The culture strain, which showed the strongest pathogenicity on the pedicels of spikes, was No.IX. The strains No.XIII, No.XII and No.XXII followed to No.IX in order. The strains No.XX and No.XVIII showed the medium and No.V the weak pathogenicity on the pedicels of spikes, while No.VII and No.XVII showed the weakest. The results of the above experiment agree nearly with those of the inoculation experiment on the leaves of the grown plants. Namely, there seemed to be a distinct differentiation in the pathogenicity of culture strains of the present fungus.
4. Pedicels of spikes of 16 varieties of the rice plant, which seeds had been sent from four different localities of Japan (Yamagata, Nagano, Okayama and Hukuoka prefectures), were inoculated with three culture strains of the causal fungus. Without exception, the culture strain No.IX showed the strongest pathogenicity on all varieties of the rice plant tested. Although the pathogenicities of the strains No.V and No.XVIII were almost indistinguishable, the latter seemed to be slightly stronger than the former.
5. The seeds of a few varieties of the rice plant from the above localities were selected. Comparing their susceptibility for the strongest strain No.IX, the writer recognized clearly that the varieties, “Igô” (Yamagata), “Bôzutamagawa, Mubôaikoku” (Nagano), “Kamezi No.3” (Okayama) and “Aikoku” (Hukuoka) are most resistant. The varieties “Waseôno” (Yamagata), “Kinaiwase No.68, Kinaiwase No.69” (Nagano) and “Kômyônisiki” (Okayama) were most susceptible for the same strain. The susceptibility of “Toyokuni, Tôgô” (Yamagata) and “Kinaiwase No.22” (Nagano) were intermediate.
