THE PRINCIPLES OF DISEASE CONTROL 87 



Crop rotation. There is no small number of fungous diseases 

 which reappear year after year on account of the fact that the soil 

 has become contaminated with the spores or mycelium of the fun- 

 gus, these stages being often able to remain alive throughout a 

 considerable period of time. It is only possible to prevent many 

 of these diseases by the practice of a suitable rotation. Land 

 infested with the organism causing club root of cabbage and tur- 

 nips should be kept free from cruciferous plants for two years. 

 The fungus producing scab of potatoes is far more persistent in 

 the soil than the last mentioned ; and Urocystis Cepiclce, the onion- 

 smut organism, is^ supposedly able to retain the capacity for ger- 

 mination in the soil for a number of years. In addition, there are 

 many other fungous, as well as bacterial, diseases for which it is 

 essential to practice the strictest rotation principles. 



The application of fungicides. The application of fungicides to 

 the growing crop has been for about twenty years a principal means 

 of disease control or prevention. In this connection it is under- 

 stood that the application of a fungicide to the host plant is gener- 

 ally for the purpose of protecting it from an attack of a fungus. 

 In only a few cases is it possible to actually kill an organism which 

 is already causing injury. In the case of some of the powdery mil- 

 dews the use of any fungicidal sprays or dusts may be beneficial, 

 in part, from the direct killing action of the fungicide upon the 

 superficial growth of the fungus. In the great majority of instances 

 the fungicide is applied with the view of covering a healthy plant, 

 which is thus to be kept in healthy condition. The germination of 

 the fungous spore, which may follow upon the host subsequent to the 

 application of the fungicide, should thus be prevented. It has been 

 fairly well demonstrated that the germinating spore will, for in- 

 stance, absorb from the nearly insoluble copper compounds of 

 Bordeaux mixture sufficient toxic substances to cause its death. 



At the same time, it is, of course, necessary that the fungicides 

 shall be of a nature and strength which will be in general nonin- 

 jurious to the plant which is to be protected. It is not, however, 

 possible to determine this point precisely, since apparently under 

 different climatic conditions the injurious action of the fungicide 

 may vary greatly. Weak Bordeaux mixture will be noninjurious to 

 the foliage of peach and plum, or even to apple, one season and 



