6 Harold Thurston Cook 



each State, in the Plant Disease Reporter, are shown in figure 1. 

 Apparently it has been observed most frequently in New York, Cali- 

 fornia, Oregon, Louisiana, and Ohio. Its wide distribution, however, 

 suggests that it may occur commonly and in other States than those 



Figure 1. geographical distribution of onion mildew in the united states 



The numerals show the number of times that the disease has been reported in the Plant Disease 

 Reporter, for each State 



shown by the reports. Probably the reason why it has not been re- 

 ported from more localities is that the crop or the disease is not of 

 sufficient economic importance to attract attention. 



HISTORY 



The first report of onion mildew was made by Berkeley in England 

 in 1841. Although the disease was frequently reported during the next 

 forty-three years in Europe (Schleiden 1842, Unger 1847, de Bary 1863, 

 Fuckel 1869, Prank 1880, and Smith 1884), it was not until the paper 

 by Shipley (1887) appeared that a real contribution to our knowledge 

 of the disease was made. Shipley was appointed by the British Gov- 



