CHAPTER VI 



CAUSES OF DISEASES IN CROPS 



IT is, indeed, very difficult to define the term dis- 

 ease. Health and disease are only relative terms, and 

 it is not easy to draw a line where health leaves off 

 and disease begins. Disease, however, may be ap- 

 plied to all deviation from the normal which threatens 

 the life of the plant. Perhaps the nearest conception 

 of health and disease is that of Marshal Ward, who 

 says: "If we agree that a living plant in a state of 

 health is not a fixed and unaltering thing, but is ever 

 varying and undergoing changes as its life works out 

 its labyrinthine course through the vicissitudes of the 

 ever- vary ing environment, then we cannot escape the 

 conviction that a diseased plant, so long as it lives, 

 is also varying in response to the environment. The 

 principal difference between the cases is, that whereas 

 the normal healthy plant varies more or less regu- 

 larly and rhythmically about a mean, the diseased 

 one is tending to vary too suddenly or too far 

 in some particular direction from the mean. The 

 healthy plant may, for our present purposes, be 

 roughly likened to a properly balanced top spinning 

 regularly and well, whereas the diseased one is lurch- 



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