32 Harold Thurston Cook 



ity was shown by a loss of tension and a withdrawal of the cytoplasm 

 from the walls. Katterfeld found that immature conidia were capable 

 of germination but produced shorter germ tubes than did mature co- 

 nidia. Conidia exposed to an atmosphere of 100 per cent humidity 

 lost their germinating power after fifteen to seventeen hours, while 

 those exposed to dry laboratory air were not able to germinate after 

 one and one-half to two hours. The writer is in agreement with Kat- 

 terfeld on these points. 



Hiura (1930 b) found that the time required for the germination 

 of conidia varies with the temperature and with the conidia. His 

 conclusions in regard to the optimum, minimum, and maximum tem- 

 peratures for conidial germination are approximately the same as those 

 of the writer. He found the optimum to be near 10° C, the maximum 

 near 20°, and the minimum below 5°. His results regarding the time 

 required for germination do not agree wholly with those obtained in 

 this study, however. According to Hiura 's results, the time required 

 for germination decreases up to the optimum temperature, but does 

 not increase appreciably above the optimum as was shown by the 

 writer's results. 



INFECTION 



Infection is dependent on an abundance of viable conidia and environ- 

 mental conditions favoring their rapid germination. In practice it was 

 found that by applying the information obtained in the experiments on 

 the relation of temperature and moisture to spore formation and germi- 

 nation, infection could always be obtained. Infection was determined 

 by the production of conidia when the plants were placed under 

 favorable conditions for their development. 



Spores were obtained for inoculation as described under Technic 

 (page 26), and were immediately taken to the laboratory. Here potted 

 plants were sprayed thoroughly with the spore suspension and were 

 then placed under moist bell glasses in the incubators at temperatures 

 below 18° C. for twelve to twenty-four hours. The plants were then 

 removed to the greenhouse, where they were kept in a cool room with 

 a maximum temperature of approximately 20° C. and a minimum 

 temperature of about 10°. Plants inoculated in this manner always 

 became infected ; but infection seldom took place when the inoculum 

 had been exposed to dry air or to high temperatures, or when the plants 

 were not kept cool for at least twelve hours after inoculation. 



INCUBATION PERIOD 



The incubation period was determined by inoculating a group of 

 potted plants under favorable conditions, as described above, and then 



