CHAPTER V 



PLANT DISEASES 



BY E. M. FREEMAN, PH.D. 



Professor of Botany and Vegetable Pathology, College of Agriculture, 

 University of Minnesota 



What is a Plant Disease ? -- It is not always an easy 

 matter to define disease either in plants or in animals. As 

 with human beings, so with plants, one cannot always 

 distinguish sharply a condition which may be called 

 healthy from a condition of slight indisposition. All 

 sorts of gradations between perfect health and a badly 

 diseased condition are possible, there being no sharp 

 line where one can say, "These are diseased and those 

 are healthy." For instance, thickly planted wheat 

 may result in a growth of plants that are not quite so 

 vigorous as wheat sown at the normal rate, yet the 

 plants may not be very noticeably diseased. They 

 are nevertheless not so healthy as those planted under 

 normal conditions. On the other hand, a rusted wheat 

 plant is clearly seen to be diseased. We give a defini- 

 tion, as accurate as is possible when we say that when- 

 ever the ordinary functions of a plant are seriously 

 interfered with, the plant is diseased. 



Cause of Disease. --There are many ways in which 

 plants may be injured. Mechanical agents of one sort 

 or another account for a good deal of damage. For 

 instance, hail, wind, and frost all contribute to a con- 



263 



