スイカ

The present paper deals with the pathological studies on watermelon wilt caused by Fusarium niveum, EFS., with special reference to the mcce by which the fungus gains the entrance to the healthy root, to bring on the wilting of watermelon. Fusarium niveum invades at first the root cap (fig. 1), and then into the primodial meristem, or directly attacks the primary meristem of the host plant (fig. 2, 3, 4). Then the fungus makes its way into the stele, penetrating through the meristematic tissue intercellulary and intracellulary (fig. 5, 6, 7). After the fungs gains its position in the stele, it progresses vigourously along the xylem elements, destructing the invaded tissues, for which any response of the host plant can not be found. In this way, finally, the wilting is resulted. When Fusarium niveum reaches the host tissue, e. g. root cap or protoderm or piliferous layer, the fungus penetrates the host tissue intercellulary; and no intracellular penetration through the living tissue is observed. The fungus which penetrated through young piliferous layer intercellulary into primary cortex (fig. 8, 9, 10), may be destructive to this parenchymatous tissue and may cause a blight of this region, but it cannot enter the stele to cause wilt of the host, in consequence of the premature suberization of the endodermis. As soon as the physiological function of the root hair is ended, the piliferous layer cells may produce the suberized exodermis cells. And by the protective action of the suberized exodermis the causal fungus is entirely refused. to enter inner tissue (fig. 11, 12). Although the fungus ma y invade the cortex of a root through a rapture produced by the emergence of a seondary root, it may not be allowed into the stele, because of the presence of the endodermis which is just suberized by the wound stimulus of the root tissue, owing to the emergence of the new root. Within sixteen hours, at 23℃, the fungus may penetrate the root cap and enter the meristematic tissue. The mode of infection of the host tissue by this fungus may well be called as "Meristematic infection," because, its infection is successful only when the meristematic tissue is invaded.