CHAPTER XIX 

 DISEASES OF PLANTS 



The Economic Importance of Plant Disease can 



hardly be overstated, for it is evident that the lessening 

 or prevention of crops is a very serious matter. The 

 United States Department of Agriculture estimates the 

 loss by plant disease in the year 1919 as follows: 190,- 

 000,000 bushels of wheat, 200,000,000 bushels of corn, 

 86.000,000 bushels of potatoes, 18,000,000 bushels of 

 apples, 1,742,000 bales of cotton. 



Rust alone, in some years, has entailed the loss of 200,- 

 000,000 bushels of wheat, while scab " caused in 1919 the 

 loss of nearly 60,000,000 bushels of wheat. " 



The Causes of Disease may be either lifeless or liv- 

 ing; they may be external to the plant or within it; they 

 may consist in unfavorable features of the environment 

 externally or the interference of other organisms. Only 

 the second type falls properly under the head of " associ- 

 ations of organisms "; but because there are other causes 

 of disease than parasites, we shall also consider examples 

 of disease due either (1) to unsuitable environment or 

 (2) to internal causes. 



UNSUITABLE ENVIRONMENT 



The Soil may be of such composition that it is said to 

 be alkaline or acid. In certain areas in the far west the 

 presence of unusual quantities of sodium or calcium salts 

 makes the soil alkaline; whereas, on the other hand, 

 especially in wet places, the soil may be acid because of 

 the presenre in it, or in its solutions, of acids formed in the 

 course of the decay of organic waste. Excessively hard 



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