LATE POTATO BLIGHT IN IOWA 5S3 



LATE POTATO BLIGHT EPIDEMICS IN IOWA AS COR- 

 RELATED WITH CLIMATIC CONDITIONS 



A. T. ERWIN. 



At least three pronounced outbreaks of the late blight of the 

 potato, Phytophthara infestans, have occurred in Iowa within 

 the past forty-five years. These were in 1885 1 , 1903, and 1915 2 . 

 The northern limits of the region generally designated as the 

 corn belt seems to represent in a general way the southern lim- 

 its of this disease under normal climatic conditions. This is in- 

 dicated by the fact of its frequent occurrence in the region just 

 north of us. 



From these regions of the north we receive our annual seed 

 supply and this disease is therefore probably introduced into 

 some portions of the state every year. The fact that under nor- 

 mal climatic conditions in Iowa it does not survive even when 

 so introduced and yet in occasional years breaks forth in a vir- 

 ulent form, presents an interesting problem. 



In many parts of the New England states late blight, usually 

 is an ever present disease and fails to develop only in dry years. 

 In Iowa the conditions are reversed. It is normally absent and 

 its presence in every known instance has been accompanied by 

 abnormal weather conditions. This fact indicates an interre- 

 lationship between these outbreaks and the atmospheric condi- 

 tions. In the following pages is presented a study of the cor- 

 relations between climatic conditions and late blight epidemics 

 in this state. 



The relationship existing between many diseases and certain 

 climatic factors is well known. It is usually difficult, however. 

 to separate out the operative and nonoperative factors and to 

 determine which are finally causative. The fact that in Iowa 

 the outbreaks of this potato blight have always been accompan- 

 ied by abnormal conditions permits of a careful study of its re- 

 lation to those conditions and hence presents an approach to the 



J The outbreak for 1885 was reported by I lalsted, B. D. (Bot. Dept. Bull. 

 la. Agrl. College, 95, Feb., 1SSS), and tbose for 1903 and 1915 came under the 

 observations of the writer. 



Specimens for the years of 1903 and 1915 are filed in the Bot. Dept. her- 

 barium of this institution and are identified by L. H. Pammel. 



