592 



IOWA ACADEMY OF SCIENCE 



1915 



1903 



lune July August 



Fig. 53 — Per cent of sunshine by decades of each month. Des Moines station. 



Particular attention is called to the period covering the third 

 decade of July and the first decade of August of this table in 

 its relation to the period of secondary infection. 



Unfortunately the sunshine records for 1885 are not avail- 

 able as they were not taken in this form by the Weather Bureau 

 at that time. 



As expressed in terms of clear, partly cloudy and cloudy 

 days, the record for 1885 was as follows: 



The germicidal properties of sunshine are well known. The 

 delicate thin walled conidia are quickly destroyed by exposure 

 to bright sunshine. The predominance of cloudy days was 

 therefore an important aid in the propagation of the disease. 



SUMMARY. 



Climatic conditions in Iowa are generally unfavorable to the 

 development of Phytophthora infestans. 



The seasons in which it did occur were characterized by sub- 

 normal temperatures, high humidity, heavy dews, excessive 

 rainfall and a predominance of cloudy weather. 

 Iowa Agricultural Experiment Station. 



