CHAPTER XXVHI 

 SYMPTOMS OF DISEASE (SYMPTOMATOLOGY) 



The preceding pages have dealt with the causes of plant diseases, 

 that is their etiology. It remains to discuss the symptoms of disease 

 as that is a very important matter in deciding as to the nature of the 

 disease, and the harm that the various diseases may do to our agri- 

 cultural crops. It is easy to determine that there is something wrong 

 with the plant, because such well-known symptoms as withering, 

 as yellowing, as abnormal growth are evidences of it, but it is quite 

 another thing to decide as to the specific nature of the disease, its cause 

 and probable amelioration. Even to the trained plant pathologist, it 

 is not an easy problem to decide what the trouble is. It requires some- 

 times two or three years of research work with all the refined methods 

 of modern science to reach a satisfactory conclusion, and at times even 

 then the solution is baffling. To call a pathologist, or a botanist, an 

 ignoramus, because he cannot by a study of the symptoms name the dis- 

 ease, is unworthy of people who claim to be cultured, and yet it fre- 

 quently happens that the farmer's opinion of the book scientist is based 

 upon just such a flimsy pretext. General conclusions are reached in 

 this field of inquiry, just as in other fields, by the process of exclusion. 

 The pathologist puts questions to himself about the plant and gradually 

 he eliminates the impossible conditions, gradually narrowing himself 

 down to a few possibilities. For example, he might ask himself 

 whether the cause of the disease is external or internal. If external, 

 then whether it is due to climate, to animals, or plant parasites. If 

 plant parasites are concerned, then are they flowering plants or fungi. 

 We will suppose that he finds that the disease is of fungal origin. Then 

 with the cultural means at his disposal, the fungus must be obtained 

 in pure culture, and its pathogenicity tried out upon healthy individuals 

 corresponding racially, or specifically, with the diseased ones. If the 

 inoculation of the healthy host is successful, then the recovery of the 

 fungus from the tissues for comparative cultural study will follow. 



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