160 Mortier F. Barrus 



prevalent during the cooler part of the growing season, is absent from the 

 fields there during the hot months of June, July, and August, even with 

 ample rainfall and when diseased seed was used for planting. Further- 

 more, Edgerton experienced great difficulty in keeping cultures of the 

 organism alive during the summer. The writer has never had any difficulty 

 in obtaining infections from inoculations made during the summer, although 

 for the success of the experiments he has made them at times when the 

 conditions promised to be most favorable. Some of the inoculations 

 were made in the greenhouse when the temperature during the following 

 days ran high, and yet fair to good infection was obtained in several 

 instances. Cultures of the fungus in the laboratory grow very slowly or 

 not at all during the excessively warm weather of the summer, and in 

 some cases they die when not transferred frequently. When placed in 

 a cellar where the temperature remained at about 16° to 20° C, a good 

 growth of mycelium and abundant production of spores resulted. During 

 the summer season in the North, the periods of excessively high tempera- 

 tures are short. The high temperature of the day is not sufficient to kill 

 the fungus in the host, and growth and spore production may take place 

 during the lower temperatures of the night. The effect of temperature 

 on growth of the fungus in culture media has already been discussed 

 (page 121). Lauritzen (1919:20), by employing inoculation chambers 

 where temperature and humidity were controlled, determined the range 

 of temperatures at which infection of bean seedlings takes place after 

 inoculation with Collefotrichum lindemuthianum, when kept at a favorable 

 constant degree of humidity, to be from 57° to 80° F. At the extremes 

 the number of infections occurring are small, but they are more abundant 

 at the more nearly optimum temperatures, as one would expect. Lauritzen 

 says that the data point to a lower temperature limit for infection when 

 forty-eight hours are used for the infection period than when twenty- 

 four hours are used. 



Moisture must be present or the humidity of the air high if the fungus 

 is to gain entrance to the host. Lauritzen (1919:29) found that infection 

 took place at humidities of 95.8 and above when the temperature was 

 kept at from 65° to 68° F. It was not necessary that a film of moisture 

 be kept on the leaf during the germination of the spore, for infection 

 occurred when the plants were dried off after inoculation and before being 

 placed in the inoculation chamber. Lauritzen did not determine how 



