Studies on the downy mildew of onion.

The present paper deals with some ecological experiments of the oospores of Peronospora schleideni which may play an important role in the primary infection of downy mildew of onion. The causal fungus seems to be a physiological species of Peronospora schleideni Unger specialized in the size of oospores, the habit of their germination and the pathogenicity.The oospores are produced in the tissue of affected leaves under the air temperature above 17°C, and, it is likely that the formation is related to the age of the leaves. The germination of the oospore occurs in the water, in the soil and on the leaves of seedlings. The oospore germinates at 20°C-30°C, fails to germinate in the temperature range below 12°C and above 40°C.The germination is also accelerated by the pretreament with KMnO4 or low temperature. In conclusion, the oospores existing in the soil become a source of the primary infection against the seedlings in seedbed.