590 



IOWA ACADEMY OF SCIENCE 



The average of the means for July and August for the three 

 years in question is 70° Fahrenheit. This temperature would 

 seem to represent the danger line. So far as the temperature 

 conditions are a factor, a mean below 70° Fahrenheit for the 

 latter part of July and early August provides favorable condi- 

 tions for an outbreak of late blight. In this connection it is 

 interesting to note the statement made by Smith that "The 

 critical districts (for late blight) would be along the line of 

 70° Fahrenheit." 



Conversely, regions lying within a mean above 70°, which in- 

 cludes Iowa, would be but little affected. The study of its his- 

 tory in this state supports this conclusion. 



Since the disease is always more or less present through the 

 introduction of infected seed, there is always the probability 

 of an outbreak at a mean temperature below 70° Fahrenheit 

 provided the humidity factor is also favorable. 



SOIL TEMPERATURES. 



The initial growth of the mycelium in an infected tuber is 

 probably largely a matter of temperature conditions as moisture 

 is supplied directly by the tuber. 



1914 



1915 



Fig. 52 — Mean soil temperatures, Ames, Iowa. Readings taken at a depth 



of six inches. 



The soil temperatures for the season of 1915 in comparison 

 with those for 1914 are presented in the following table. 



6 Smith, J. W. ( Monthly Weather Review, 43-5-234, May, 1915. 



