Methods of Control 373 



as the particular crop which the disease attacks is 

 grown for a number of years on the same field, the 

 soil becoming thoroughly permeated by the mycelium 

 and spores of the parasitic organism. If the infected 

 land is planted with crops not subject to the disease, 

 the parasitic organism will sooner or later die for 

 want of a suitable host to live upon. For this reason 

 crop rotation plays an important part in the control 

 of numerous truck crops. To meet with success 

 in rotation, the trucker must know what crops are 

 subject to the disease to be controlled, so as to avoid 

 them temporarily in the sick land. Weeds, too, are 

 often subject to the same diseases as the cultivated 

 crops. Crop rotation often fails if we overlook the 

 importance of clean culture. 



VARIETIES RESISTANT TO DISEASE 



It is a well-known fact that not all varieties of 

 plants are alike subject to the same disease. In 

 going over a diseased field, we find that while a large 

 percentage of the plants may be dying, some few 

 individuals will stand up and thrive in spite of the 

 disease. If these individual plants are perpetuated 

 in the same sick field, we may succeed in developing 

 a strain or variety of plant which will produce one 

 hundred per cent, healthy individuals in the same sick 

 soil. On this principle are based the selection and 

 development of resistant varieties. Much has al- 

 ready been accomplished in this direction and still 

 more is to be expected in the future. 



