42 James G. Horsfall 



diseases has failed to reveal a single record of the fungus on any species of 

 these two genera. 



Mains (1923:312) inoculated strains of red clover from many sources 

 in the greenhouse and found that the North American varieties were 

 generally susceptible, while the European showed in general rather high 

 resistance. Another thing noticeable was the presence of resistant indi- 

 viduals in all except three of the varieties tested. 



Monteith (1924:63) says, "The Italian clovers which are most sus- 

 ceptible to anthracnose are most resistant to mildew." That European 

 red clovers are resistant to this disease is borne out by the observations of 

 Delwiche (1926) in Minnesota and Noll and Irvin (1926) in Pennsylvania. 



The disease 



The name powdery mildew is so characteristic of the disease that when 

 it is coupled with the name of the suscept, it is sufficiently descriptive. 

 In France the disease is called le blanc (Noffray, 1920). 



History and range 



Sheldon first reported clover powdery mildew in this country in 1908 

 from West Virginia (Plant Disease Bulletin 6:53. 1922), and it was 

 recorded occasionally up to 1921. Late in 1921, however, an epiphytotic 

 of powdery mildew on red clover appeared in the East. Professor F. C. 

 Stewart in conversation with the writer states that he saw the disease 

 early in September, 1921, in New York. According to Stakman 

 (1922:243), "one of the noteworthy pathological features of the year 

 was the unusual presence of clover powdery mildew." Reports such as 

 the following poured into Washington in 1922: " clover mildew new dis- 

 ease in all sections of State; powdery mildew of clover very prevalent; 

 abundant." Haskell and Wood in summing up clover diseases for 1922 

 state (1923:250), "during 1922 it [powdery mildew] was much more 

 widely distributed [than in 1921] and occurred all over eastern United 



States from South to North and in eastern Canada 



Apparently, it did not extend beyond the line of the 100th meridian." 



The disease in 1923 appeared to spread westward (Haskell, 1924:302) 

 but was less severe than in 1922. In 1924 there was little change in the 

 situation as a whole (Melchers, 1925:181) except that it was more prev- 

 alent in some of the Western States, particularly Oregon, Washington, 

 Idaho, Montana, and Utah. This indicates that the eastern form was 

 spreading. 



Haskell (1926) summarizing the situation for 1925 says in effect that 

 the appearance of the disease in the Pacific Northwest in 1924 and its 

 increase in 1925 tend to confirm the theory that a new physiologic from 

 of mildew had been introduced into the United States where it was spread- 

 ing from east to west. By 1925 (Haskell, 1926:369) the disease had 



