LATE POTATO BLIGHT IN IOWA 



591 



These readings were taken in the experiment station potato 

 field at a depth of six inches, whieh probably represents the 

 main zone of tuber development for the potato. It will be 

 noted that soil temperatures for 1915 are strikingly low as com- 

 pared with 1914. It is to be regretted that this comparison 

 cannot be made with a normal established over a long period 

 of years. Orton 7 reports that in the outbreak of 1893 in Penn- 

 sylvania, the soil temperatures for the summer were notably 

 low, and suggests that the soil temperatures are probably the 

 prinary factor in developing an epidemic. It is readily con- 

 ceivable that the soil temperature is a limiting factor in the 

 initial growth of the mycelium. However, once it reaches the 

 foliage and sporulation begins the controlling factors would 

 seem to be atmospheric rather than those of the soil. In fact, 

 the study of our field conditions leads to the suggestion that 

 through the planting of new seed the disease frequently makes 

 a start but fails to sporulate due to unfavorable atmospheric 

 conditions. Being unable to propagate it quickly perishes. 



Through the courtesy of Professor J. G. Hosier of the Illi- 

 nois Experiment Station we have the soil temperature rec- 

 ords for the year 1915 in comparison with a normal, covering 

 a ten year period at that station. 



Soil Temperatures at a Depth of Three Ixches. 

 (Champaign, Illinois.) 



In the following Sunshine Chart is shown the percentage 

 of possible amount of sunshine for the years 1903 and 1915 as 

 compared with the normal. 



TQrton, C. R., Contributions from Dept. Bot. Pa. St. Coll., 191G. 



