Studies on the Downy Mildew of Onions 



25 



TABLE 3. Relation of Humidity to the Formation of Conidiophokes and 



Conidia 



Plant no. 



Date 



Treatment 



Relative 

 humid- 

 ity 

 (per cent) 



Leaves 



wet or 



dry 



Conidia 

 present 



or 

 absent 



First series of tests 



1 

 8 

 25 

 55 

 55 

 66 

 67 

 67 

 69 

 69 

 :,l 

 51 

 34 

 34 

 4(1 

 HI 



81 

 81 

 73 

 73 

 50 

 50 



Dec. 1 

 Dec. 1 

 Dec. 1 

 Dec. 5 

 Dec. 6 

 Dec. 4 

 Dec. 6 

 Dec. 7 

 Dec. 4 

 Dec. 5 

 Dec. 6 

 Dec. 7 

 Dec. 6 

 Dec. 7 

 Dec. 6 

 Dec. 7 

 Dec. 4 

 Dec. 7 

 Dec. 4 

 Dec. 7 

 Dec. 6 

 Dec. 7 



Humidity chamber 



Humidity chamber 



Humidity chamber 



Humidity chamber 



Inoculation chamber 



Humidity chamber (B. G.)* 



Humidity chamber 



Inoculation chamber 



Humidity chamber 



Bell glass on bench 



Humidity chamber 



Inoculation chamber 



Humidity chamber (B. G.) . 



Inoculation chamber 



Humidity chamber 



Inoculation chamber 



Humidity chamber 



Inoculation chamber 



Humidity chamber 



Inoculation chamber 



Humidity chamber 



Inoculation chamber 



Second series of tests 



10 

 10 

 12 

 12 

 23 

 23 

 29 

 29 

 29 

 30 

 30 



75 

 75 

 81 

 SI 

 81 

 83 

 89 

 89 

 92 

 92 



Feb. 19 

 Feb. 21 

 Feb. 19 

 Feb. 21 

 Feb. 19 

 Feb. 21 

 Feb. 19 

 Feb. 20 

 Feb. 21 

 Feb. 19 

 Feb. 20 

 Feb. 19 

 Feb. 21 

 Feb. 19 

 Feb. 20 

 Feb. 21 

 Feb. 19 

 Feb. 19 

 Feb. 21 

 Feb. 20 

 Feb. 21 



Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 

 Humidity 



chamber 



chamber (B. G.) 



chamber 



chamber 



chamber 



chamber 



chamber 



chamber (B. G.) 



chamber 



chamber 



chamber (B. G.) 

 chamber (B. G.) 

 chamber (B. G.) 

 chamber (B. G.) 

 chamber (B. G.) 

 chamber (B. G.) 



chamber 



chamber (B. G.) 



chamber 



chamber (B. G.) 

 chamber (B. G.) 



* " B. G.", plants covered with a bell glass. 



exposed to high humidities without water on their leaves. This was 

 probably because sufficient water bad accumulated at the stomata 

 during transpiration in the humid atmosphere to allow the formation 

 of the fruiting structures. It is possible that conidia would always be 

 formed in an atmosphere of 100 per cent humidity, provided that suf- 

 ficient time were allowed for water to accumulate around the stomata. 



