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.
View full abstract