PREVENTION AND CONTROL 267 



home which it cannot withstand. The European plums, 

 when introduced into America, met with a very severe 

 disease known as black knot (Plowrightia morbosa), 

 which is of comparatively little importance on American 

 plums. It is said that coffee when growing wild in its 

 original habitat was not subject to the rust (Hemileia 

 vastatrix), but that the fungus is a native of some part 

 of the East Indies, where it is parasitic on some wild 

 rubiaceous plant, from which it passed to the coffee on 

 its introduction into the locality of the parasite. 



Furthermore, the introduced plant frequently meets, 

 new climatic and soil conditions which decrease its 

 vitality, and make it more susceptible to organisms of 

 disease than it was in its original habitat. 



The practical agriculturist has also selected plants 

 with reference to the marketable value of the products, 

 and without regard to the susceptibility of the plants 

 to disease. This has frequently resulted in the growing 

 of varieties which could not resist the diseases. 



The increase in population and in agriculture has 

 also given opportunities for diseases to spread with 

 ease from farm to farm, and from one locality to adjoin- 

 ing communities. 



Prevention of Disease. In the adoption of methods 

 for the prevention of disease it must be remembered 

 that the plant is the central factor, and that the 

 interests of the practical agriculturist centres round a 

 healthy plant rather than the diseases of the plant. 

 The diseases and the organisms causing them receive 

 very little or no attention unless circumstances make 

 a study of them imperative. Therefore, from the stand- 

 point of the agriculturist, it is much more important to 

 grow strong, vigorous plants that are resistant to 

 diseases than to care for sick plants. This idea has led 

 to experimental studies for the purpose of selecting or 

 developing immune varieties. It is well known that 

 plants are subject to great variations, and that by 

 selecting those varieties which were the most suitable 

 for our purposes, agricultural and horticultural interests 



