On the Effect of Sunlight upon the Enlargement of Lesions of the Rice Blast Disease

Abstract

1. The effect of four different intensities of sunlight upon the enlargement of the diseased lesions on the leaves of rice seedlings caused by Piricularia Oryzac BR. et CAV. was investigated.In the experiment the writer used the glass boxes kept under the same conditions except light intensity. The potted rice seedlings were kept in the boxes uncovered, covered with a single or two sheets of white cotton cloth and also with black paper as soon as they had been taken out from the inoculation chamber. After previously determined intervals the length and width of the diseased lesions were measured with the aid of a divider.2. Judging from the results of the writer's experiment, the degree of the lesion enlargement soon after the incubation period seemed to be maximum on the seedlings kept under a weak shading and minimum on those in the darker boxes. The seedlings, kept in the lighter boxes than the former, showed the medium degree of enlargement.3. The above relation of shading to the lesion enlargement changed gradually with the lapse of time and in the later period the lesion enlargement on the seedlings kept in the lightest boxes became maximum. It is recognized that the darker the boxes are, in which the seedlings grow, the smaller the lesions become.4. The influence of sunlight upon the growth of the causal fungus on the potato decoction agar containing 1% sucrose was investigated under different light intensities controlled as same as the previous experiment. The results of the experiment show that the darker the light condition is, the better the causal fungus grows, although the growth difference is not remarkable.5. The relation of shading to the lesion enlargement soon after the incubation period is thought to be due to the direct effect of sunlight on the growth of the causal fungus, and also to the indirect effect caused by changes in the host vitality.6. The relation of shading to the lesion enlargement in the later period is thought to be clearly due to the diminution of the assimilation products of the host, which affect the growth of the causal fungus.