CHAPTER XIII 

 PLANT DISEASES 



138. Definition. In the cultivation of plants there must 

 be some knowledge of the diseases to which they are subject. 

 Sometimes whole crops are destroyed by some disease, or 

 at least much reduced in quantity and quality. The great 

 losses from this cause have led the national and state gov- 

 ernments to provide for the study of plant diseases in the 

 hope of preventing them. Very much has been accom- 

 plished, but very much more remains to be done. A multi- 

 tude of facts in reference to diseases and treatments have 

 been accumulated, but these cannot be detailed here. Only 

 the general facts in reference to plant diseases and the general 

 principles of treatment can be indicated ; for special details 

 the student must consult the larger works in which the 

 known facts are assembled. 



It is difficult to define exactly what is meant by a plant 

 " disease." In a large sense it is anything that interferes 

 with the normal activities of a plant, so that it is not " doing 

 well." It is evident that this would include a great variety 

 of causes, such as soil conditions, climatic conditions, me- 

 chanical injuries, attacks of animals (especially insects), and 

 attacks of parasitic fungi ; in fact, anj^thing that affects 

 unfavorably the vigor of a plant. It is clear that we can 

 include no such indefinite range of causes, and must restrict 

 ourselves to the diseases induced by parasitic fungi, for 

 these are the most common and most studied of the diseases. 



The distinction between a disease and its cause must be 

 kept clear. A parasite (like wheat rust, for example) is a 

 cause, but the disease is the condition of the attacked plant 



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