586 



IOWA ACADEMY OF SCIENCE 



very wet. The excess in two of them was quite pronounced. The 

 conditions in this third decade of July are without doubt sig- 

 nificant in relation to the outbreaks which occurred during this 

 period in at least two of the epidemics. The decade in which 

 the outbreak of 1885 occurred was not recorded, but judging 

 from analagous climatic conditions it also occurred probably 

 during the third decade of July and during early August. 



1903 



1865 



1813 

 June TulV A u qy*T 



Fig. 49 — Rainfall by decades of each month. Des Moines station. 



The first decade of August was wet in all three years. This 

 third of the month was also covered in at least two of these 

 outbreaks by a period of secondary infection. The second de- 

 cade of August was dry in all three instances. Since the vines 

 were dead by this time, the atmospheric conditions for the sec- 

 ond and third decade of August would not be significant in 

 relation to foliage destruction. 



HUMIDITY. 



Humidity and rainfall are usually closely associated though 

 such is not necessarily the case. From the standpoint of the 

 host plant, rainfall is the more important factor while humidi- 

 ty bears a direct relationship to the growth and development 

 of foliage diseases. A liberal supply of atmospheric vapor and 

 dew combined with the right degree of temperature provides 

 ideal conditions for spore production and germination. In the 

 following table are presented the humidity data for the three 

 years in question. 



