44 



James G. Horsfall 



crop probably would reach 20 per cent, but for all fields it would not be 

 more than from 1 to 2 per cent in most seasons. 



An experiment to be discussed under protection gives some clue as 

 to the damage caused by this disease. The experiment was made in 

 the greenhouse during the mildew period of the 1928 season, and the 

 plants were subjected to no more unnatural conditions than they would 

 have had in the field during the same period. At the end of approxi- 

 mately a month, during which time the plants were continually exposed 



Figure 8. effect of powdery mildew on red clover 

 The plants on the left were kept healthy by dusting; those on the right were diseased 



