Studies on the Downy Mildew of Onions 23 



relatively cool and heavy dews occurred frequently. Prolonged rainy 

 and muggy periods also were very favorable for the development and 

 spread of the disease. The fungus was found capable of producing 

 conidiophores and conidia at rather low temperatures. The fresh fruit- 

 ing structures were found on numerous occasions at Elba during the 

 latter part of September, when the night temperature was as low as 5° C. 

 Conidia were found also on Egyptian onions as late as the middle of 

 November in 1929 at Ithaca. 



experimental work 

 Relation of temperature to the formation of conidiophores and conidia 



Experiments on the relation of temperature to the formation of 

 conidia were not conducted in carefully controlled chambers. However, 

 a few tests were made on this phase of the problem in the greenhouse 

 and .the laboratory. The results of these tests are presented here, since 

 they show the approximate temperature range over which conidia may 

 be formed. 



Infected plants were placed at four different temperatures in the 

 greenhouse. One test was sufficient to show that the fungus would 

 fruit at all temperatures tested (10° to 28° C). Following this test, 

 two infected plants were brought into the laboratory, sprayed with 

 water, and treated as follows: one was placed in a 3°-C. incubator 

 under a bell glass to prevent evaporation of the water, while the other 

 was placed under a bell glass in a room where the temperature rose as 

 high as 34° C. during the night. The next morning fruiting structures 

 were found on both plants. There is no question about the conidia 

 having been formed at 3° C, because the temperature remained con- 

 stant in the incubator. It is possible, however, that 34° C. is too high 

 a temperature for the formation of conidia, and that in this case they 

 were formed before the temperature rose to that degree. Murphy and 

 M'Kay (1926) found that conidiophores were first produced when the 

 maximum temperature was 10° C., but that they were much more 

 abundant several days later, when it rose to 23°. Katterfeld (1926) 

 found that they were formed at 9.5° to 11° C. These workers did not 

 report the determination of the full range of temperature over which 

 conidiophores are formed. Hiura (1930 b) found that the optimum 

 temperature for the production of conidia was near 15° C, the mini- 

 mum was 6°, and the maximum was 25°. 



Relation of humidity to the formation of conidiophores and conidia 



As stated above, preliminary observations and experiments in the 

 field and the greenhouse indicated that an abundance of moisture is 

 necessary for the formation of conidiophores. In order to determine 



