I 



CHAPTER FORTY 



PLANT DISEASES 



It is difficult to define a plant disease, although it is usually 

 not difficult to distinguish in a particular example between a 

 healthy and a diseased plant. The difficulty of definition arises 

 from the fact that diseases or abnormal conditions are produced 

 by so great a variety of causes, and the effects or symptoms are so 

 diverse, that it is impossible to include all examples of diseases 

 and at the same time exclude others that are merely effects of 

 unfavorable environmental conditions. 



Plant diseases are usually defined as derangements of the nor- 

 mal structures and physiological processes of plants or parts of 

 plants. The formation of galls and tumors, the development of 

 gray, brown, and black spots on leaves, the sudden wilting and 

 drying of shoots, and the rotting of seedlings are familiar external 

 signs of disease. 



Prevalence. Plant diseases have been recognized since the 

 earliest historic times. They have been extensively studied, 

 however, with a view to their control only during the last half 

 century. They could not be well understood until the life 

 histories of the bacteria and fungi were discovered. 



The increasing prevalence of destructive diseases in America is 

 the natural result of the more extensive cultivation of crop plants. 

 The present world-wide exchange of plants and plant products 

 has resulted in the accidental widespread transference of the 

 bacteria, fungi, insects, and other organisms that injure plants. 



Losses from plant diseases. The present losses from plant 

 diseases are enormous when we view them for a state or for the 

 United States as a whole. Sometimes on a single farm a disease 

 may reduce the yield so greatly that the crop was grown at a 

 loss instead of a profit. Some idea of the extent of the injuries 

 inflicted may be gained from the following table of estimates 

 by the United States Department of Agriculture for 191 7 : 



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