Pathological Studies on Rice Blast, caused by Piricularia oryzae

Abstract

Some histological observations on the diseased tissues of adult rice plants affected by Piricularia oryzae have been done.The diseased portion of an affected leaf may be divided into three zones. They are venenate, necrotic, and disintegrated ones.The venenate zone occupies the border of a diseased portion, often showing a long, light yellowish stripe, and blends gradually into healthy tissue. The necrotic zone forms a brown coloured, narrow streak, developing along the inner side of the venenate zone, or along the vascular bundle within affected part. The disintegrated zone is grayish brown and is in somewhat dried state occupying the most part of a diseased portion.A toxic substance, excreted by the fungus which has entered the host plant, seems to infiltrate into the surrounding tissues of the invaded portion, resulting in the formation of the venenate zone. The pathological changes of tissues of this zone are observed as follows: the discoloration and the decrease of size of chloroplast, the degeneration of cell-membrane, and the vacuolar or granular disintegration of protoplasm.When the fungous hyphae spread vigorously within the tissues of the venenate zone, the cell-inclusions as well as the cell-walls will be severely collapsed. The disintegrated zone is formed in this manner.If the poisoning action of the fungus is not so strong or the venenation of the tissue is not so severe, owing to the position of the tissue, e. g., the portion arround the vessel, some part of the cell-inclusions together with the cell-walls will remain without being wholly disintegrated, and will increase their staining properties. Thus the necrotic zone will make its appearance.The results of some observations on the culm blasts have been given. The majorities of them are the blasts of the nodes of racheae, of the bases of ears, and of the nodes of flag leaves.Many of the pathological changes of these parts are similar to that of leaf blast, with the exception of the presence of reserve starch grains, though of comparatively small amount, distributed irregulary within the tissues severely affected.The decrease of starch in the affected tissues may be attributed to the transformation of reserve starch into sugar, carried out within the tissues, affected but still living. Because, the transformation of starch will begin when sugar in the cell-sap decreases in its concentration, being digested by the pathogene. It is also advocated that the irregular distribution of starch remains in the severely affected tissues depends upon the feeble diastatic action of the pathogene, Piricularia oryzae.