Studies on the Downy Mildew of Onions 



33 



testing for infection by placing moist bell glasses over the plants. In 

 order to avoid injury by Botrytis, the plants were divided into two 

 groups and these groups were tested on alternate days by covering with 

 bell glasses. In this manner neither group was exposed continuously 

 to a humid atmosphere. The temperature of the greenhouse in which 

 these tests were conducted was from 10° to 24° C. The results obtained 

 in two of these tests are shown in table 5. 



TABLE 5. Incubation Period of Peronospora destructor in Allium cepa 



From these results it is seen that the incubation period is from eleven 

 to fifteen days under the conditions of this experiment. Of twenty- 

 three other plants that were placed under bell glasses between thirteen 

 and twenty days after inoculation, twenty-two produced conidia, indi- 

 cating that the incubation period had already been completed. These 

 results agree with those of Katterfeld (1926), who found the 

 incubation period to be ten to fifteen days in the greenhouse and 

 thirteen to eighteen days in the garden. They differ from those of 

 Murphy and M'Kay (1926), who found the period to be twenty-three 

 days. 



effect of temperature on infection 



Eight plants were inoculated under favorable conditions for infection 

 on January 25, and on the following day they were transferred to the 

 greenhouse, where half of them were placed in a warm room (19° to 

 33° C.) and the other half in a cool room (10° to 24° C). Beginning 

 six days after inoculation, half of the plants in each house were tested 

 for conidial production every other day by placing them under bell 

 glasses after spraying them with water. The results of these tests are 

 shown in table 6. 



This experiment indicated that high temperatures are unfavorable 

 for the development of the fungus in the host tissues. Even though 

 all of the plants were inoculated and treated alike during the first 

 twenty -four hours, none of those kept at the warm temperature de- 

 veloped conidia although all four of those kept in the cool house 

 produced them in eleven to sixteen days after inoculation. The tests 



