Section II — Prevention and Control 

 CHAPTER IX 



GENERAL CONSIDERATIONS 



Arable Crops and Parasitic Diseases. 



A field is to be planted with a certain crop, which is subject 

 to specific diseases. The grower requires to know what can be 

 done to prevent their appearance, and, should they appear, how 

 the resulting damage may be kept to the minimum. For every 

 crop and for every disease of it the answers to these questions 

 must vary, but the general principles of prevention and control 

 are nevertheless capable of useful discussion. 



Sources of Infection. 



For a parasitic disease to develop in a crop a primary infection 

 is necessary, and the sources from which infection may arise will 

 first be considered. 



I. The parasite may be in the soil, continuing its existence in 

 the soil itself, or on the dead or living remains of the previous 

 crop. To remedy this condition the process of starving out the 

 parasite by denying to it its appropriate host is commonly 

 adopted. This may take the form of resting the field for a period, 

 but usually the result is more efficiently and more economically 

 attained by planting a different crop. In the practice of crop 

 rotation from the point of view of eliminating diseases the 

 principles to be kept in mind are (a) that the successive crop- 

 plants should not be susceptible to attack by the same parasites, 

 and (b) that a crop needing deep or frequent cultivation will 

 produce the quickest results. Even where no alternative crop is 

 available much may be done in the desired direction by early 

 removal of the remains of the previous crop and thorough culti- 

 vation before replanting. Many attempts have been made 

 to find some direct means of soil sterilization by chemical or 

 .physical agencies. It may be stated with confidence that no 

 method involving the use of chemicals or heat is known which 

 *is at the same time cheap enough and effective enough to be 

 applied to areas larger than those of seed or nursery beds or the 

 sites of a few infested plants or trees. 



The practice of flooding fields where it is practicable may be 

 very useful. 



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