26 Harold Thurston Cook 



These tests supported the conclusions reached from field observations 

 and greenhouse practices. In the field it was always noted that the 

 fungus fruited only when water was present on the leaves in the form 

 of rain or dew, and in the greenhouse the diseased plants were always 

 sprayed with water and covered with a bell glass to obtain conidia. 

 On a number of occasions a few conidia were formed on plants in the 

 greenhouse which had not been sprayed with water and covered with 

 bell glasses. This occurred only at times when the humidity was very 

 high, and water had probably accumulated around the stomata during 

 transpiration as occurred in two of the plants included in table 3. 



SPORE GERMINATION 



Preliminary studies on spore germination made at the field laboratory 

 indicated that in order to obtain good germination it was necessary to 

 control the environmental factors as well as to have fresh spores. The 

 introduction of the fungus into the greenhouse at Cornell University 

 in the fall of 1929 made possible further studies on germination. 



TECHNIC 



Fresh spores for use in these tests were obtained by spraying an 

 infected plant in the greenhouse with water and placing it under a bell 

 glass for the night. On the following morning the fresh conidia that 

 had been formed during the night were brought into suspension by 

 dipping the mildewed leaves into water. In this manner only fresh 

 mature spores were obtained, and they were protected from desiccation 

 since they were not exposed for any length of time to the air. Drops 

 of the suspension were placed on clean microscope slides in moist cham- 

 bers and these were placed in incubators at different temperatures. 

 The necessary apparatus for the tests was placed in the incubators 

 the night before, so that it would be at the temperature at which the 

 spores were to be tested. 



Distilled water was used at first as a suspension medium for the 

 spores, but, since many of the spores were ruptured owing to high 

 osmotic pressure, it was necessary to find a more favorable medium. 

 A test in which the germination of the spores in distilled water, in lake 

 water, and in physiological salt solution, was compared, showed lake 

 water to be a suitable medium. The results of the test are given in 

 table 4. 



This test showed that lake water and physiological salt solution were 

 more favorable media than distilled water for spore-germination tests 

 because they were more nearly isotonic with the spore contents. Since 

 the best germination was obtained in lake water, this was used in all 

 the subsequent preparations of spore suspensions. In all cases the 

 water was collected in Erlenmeyer flasks, and was sterilized before 

 being used in order to prevent the development of bacteria and algae. 



