LATE POTATO BLIGHT IX IOWA 



5S9 



the summers of 1903 and 1915. The same is true for l v <> 

 with the exception of the third decade of July. During this 

 decade the humidity, however, was above normal and gave one 

 of the highest readings on record. 



^-^1903 



Jur>« Jj"lv^ August 



Fig. 51 — Mean temperature by decades of each month. Des Moines station. 



The deduction is clear that the normal mean temperatures 

 for the summer months are too high for the development of 

 late blight in Iowa and are limiting factors. 



The exact optimum between the upper and lower tempera- 

 tures at which this disease thrives under field conditions is dif- 

 ficult to determine. Selby 3 in laboratory tests found that tem- 

 peratures ranging from 65° Fahrenheit to 75 Fahrenheit pro- 

 dueed favorable conditions for the disease and Galloway states 

 that "A normal temperature of from 72° Fahrenheit to 74" 

 Fahrenheit accompanied for any considerable time by moist 

 weather furnishes the best conditions for the spread of the 

 diseas 



since the normal mean in Iowa for July is 74.1 c Fahrenheit 

 and for August 71.8° Fahrenheit and as the disease has oc- 

 curred here only in the seasons of subnormal temperature dur- 



_ These months, it would seem that the last named figures are 

 perhaps high. 



'Selby, A. D., Ohio Naturalist, Feb., 



19 : 



