(1) In the present paper, the results of the writer's experiments on the effect of copper sulphate mixed in the culture solution or in the soil in WAGNER'S pots upon the susceptibility of the rice plant to the Helminthosporium disease were described. (2) In the first series of experiments by solution-culture, the development of the rice plant was accelerated more or less in the 1/1, 000, 000 Mol. or 1/500, 000 Mol. addition of copper sulphate than on the control-solution. (3) The susceptibility of the rice plant grown by solution-culture to the Helminthosporium disease showed a tendency to decrease inversely with the increase of the amount of copper sulphate added. (4) In the second series of experiments using the soil in pots, the 1 ngth of rice-seedlings grown on soils containing a little amount of copper sulphate and the number of their leaves exceeded those as controls, although no difference was visible for the naked eye. (5) In all the experiments of the second series, the diseased spots per unit area of leaves were always less in number in the seedlings grown on the soil containing copper sulphate than in those of controls. (6) The growth of the causal fungus, Ophiobolus Miyabcanus, was more vigorous in the decoction of the seedlings grown as controls than in the decoction of those on soils with copper sulphate.
1. The present paper deals with the results of comparative studies on the morphologies of the downy mildews from cucumber (Cucumis sativus L.) and squash (Cucurbita moschata DUCHESNE), about the difference of whose pathogenicities the writer had reported in a previous paper. 2. Conidia and conidiophores were removed from the diseased leaves, gently wiping the spots with a moistened brush, and they were produced anew for the present experiment, leaving the leaves in a moist chamber for a night. 3. The length and the width of the conidia were measur d (300 in total), and also the total length, the length and the width of the main axis, and the number of branching were measured of the conidiophores (100 in total). The materials of both fungi were from two distinct localities, Tokyo and Tu. 4. Considering the fact that the sizes of the conidia and conidiophores of the Peronosporaceous fungi vary according to the temperature at the time of their production, as many authors had shown, the temperatures at which they were produced were regulated. 5. Comparison were made with the conidia and conidiophores between both fungi produced at various definite temperatures and also with the relations of their variabilities to the temperature. 6. Conidia as well as conidiophores show considerable differences of sizes according to the temperature at the time of their production. 7. In both fungi the length and the width of the conidia are smaller under lower temperatures and vice versa, while as to the conidiophores, the total length, the length and the width of the main axis, and the number of branching are observed to be small under low temperature, becoming larger as it rises, but become smaller after they have reached the maximum states with rising temperature. 8. The differences in the average sizes of the conidia of the fungi from cucumber and squash are not so very significant, though those from squash are somewhat larger than those from cucumber at each temperature tested. 9. In both fungi collected at Tu, the width of the main axis and the number of branching of the conidiophores show no remarkable differences at each temperature, while as to the total length of the conidiophores and the length of the main axis, significant differences are observed in the relations of their variabilities to the temperature, i.e. those of the fungus from cucumber attain their maximum lengths at 22°C.-26°C., while those from squash attain 14°C.-18°C., their values being nearly equall, but they are almost equally small at 11°C. and 30°C. And also little differnce is observed between both fungi as to the mean value derived from the measurements made at six different temperatures ranging from 11°C.-30°C. 10. The remarkable fact that the conidiophores of the fungus from cucumber collected at Tokyo are far larger than those from the same collected at Tu was discussed. 11. From the general considerations on the results of the present and the previous experiments it is appropriate to conclude that the downy mildews from cucumber and squash are not different morphological species, but different biological species of Pseudoperonospora cubensis (B. et C.) ROSTOW. judging from the difference of their pathogenicities.
In Western-Chosen, four species of Fusaria are found to be the cause of the take-all of cotton seedlings. No less than ninety per cent of cotton seedlings are affected by these fungi every year, which can only be detected when the roots of the seedlings were examined. The great damage of the disease happens once for three or five years. The circumstances which favour the infection of the disease are as follows: (1) the minimum temperature is lower than 12°C, (2) the heavy rainfall which makes the soil water content to its maximum capacity or nearly so. Affected seedlings will easily be recovered from this disease under such condition: (1) when the minimum soil temperature rises above 13°C, (2) when the water content of the soil becomes about 50 per cent of its water capacity. The process of the recovery of the diseased seedlings was studied on the basis of oecology and pathological anatomy. The outbreak of the take-all of cotton seedlings in Western-Chosen in 1938 was explained oecologically on the basis of this study. It is proposed that the causes of the outbreak of the disease in this year are as follows: (1) during the later half of May, the minimum daily temperature fell below 5°C successively for several days, (2) at the same time there was a heavy rain fall which was enough to bring the soil water content to its full water capacity. (3) there occurred, in succession, a continuous rainfall until the begining of June, (4) on the contrary, at the former half of June, the soil became arid rapidly because of the continuous (for more than six days) dry weather and high temperature.