On the Influence of Sunlight upon the Lesion Enlargement of the Helminthosporium Disease of Rice Seedlings

Abstract

1. The writer has reported the results of the investigations on the influence of sunlight upon the lesion enlargement of the rice blast disease in the previous paper. The present paper deals with the results of the similar experiments with the Helminthosporium disease of rice seedlings.2. In the experiments, the writer used the glass boxes and the cages made of wire-netting kept under the approximately same condition except light intensity. The potted rice seedlings were kept in the boxes or cages uncovered, coverd with a single or two sheets of white cotton cloth and also with two sheets of black cloth as soon as they had been taken out from the inoculation chamber. After previously determined intervals the sizes of the diseased lesions were measured under microscope.3. The lesion enlargement in the early stage of the interception of sunlight seemed to be maximum on the seedlings kept under the darkest condition and minimum on those in the uncovered boxes and cages. This point is not identical with the case of the rice blast disease, in which in the same stage the lesion enlargement was maximum under the medium degree of shading.4. In the late stage, the lesion enlargement of the Helminthosporium disease changed to be maximum on the seedlings kept under the medium degree of shading. In the case of the rice blast disease, it was, however, maximum in the uncovered boxes.5. The sunlight has a tendency to retard the growth of the causal fungus on culture media. This is quite similar to the case of the blast disease.6. The relation of sunlight to the lesion enlargement of the Helminthosporium disease in the late stage was shown to be homologous to that of the blast disease in the early stage. In all probability the disagreement of the results by these diseases is due to the difference of the sensibility of the causal fungi to the host vitality.