1 66 PRINCIPLES OF FARM PRACTICE 



weeds are hosts of fungi that may infect and cause diseases 

 of cultivated plants. 



A sixth method is that of sanitation. It is usually accom- 

 plished by rotation of crops so that one crop is not exposed to 

 infection from the residue of the preceding crop. Rotation 

 of crops is important in controlling corn smut, tomato wilt, 

 wheat scab, and the root-knot disease which is very severe 

 in the South. 



A seventh method of disease control is the use of resistant 

 or disease-escaping varieties of plants. The method is based 

 upon the fact that some plants are more hardy and less liable 

 to infection than are others. One object of plant breeding is 

 to produce plants with a high resistance to disease, and con- 

 siderable progress has been made in this direction. The use 

 of resistant varieties is especially important where it is im- 

 possible to control diseases by the other methods. For 

 example, asparagus rust seemed impossible of control until 

 resistant varieties were developed. Yellows-resistant cabbage 

 is another example. 



Importance of controlling plant diseases. The total loss 

 of cultivated plants in the United States, due to plant diseases, 

 has been estimated at $600,000,000 annually. When we re- 

 member that there is no kind of plant that may not be 

 infected by some disease, these figures are not surprising. 

 In the control of plant diseases, as in the control of human 

 diseases, preventive measures are the most important. 



The common diseases should be known so well as to be 

 recognized easily, so that measures for control may be taken 

 in time to prevent much damage. The diseases of the culti- 

 vated crops of each state have been studied by members of 

 the staff of the State Agricultural Experiment Station and 

 of the U. S. Department of Agriculture. The results of these 



