CHAPTER XLV 

 PLANT DISEASES 



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Among the earliest records of cultivated plants occur references to plant 

 diseases, but for centuries the causes were thought to be mysterious and 

 supernatural. The ancient Hebrew, Greek, and Roman writers frequently 

 mentioned smuts and rusts, and diseases of the olive, the vine, and the 

 fig. From the fall of the Roman empire ( 476 a.d. ) until the 19th century, 

 however, little was added to the knowledge of plant diseases. Even in 

 modem times explanations of the causes of diseases and plants were 

 impossible as long as people subscribed to the doctrine of spontaneous 

 generation of living organisms. As late as the first quarter of the 19th 

 century many fungi were regarded even by scientific men as merely 

 transformations of the cellular structure of the plant upon which they 

 grew. Plant diseases were thought to be due to internal disturbances 

 resulting in the degeneration of the tissues themselves. 



About the middle of the 19th century the recognition of fungi as causes 

 of plant diseases came about through the researches of certain mycolo- 

 gists, the most famous of whom was Anton de Bary, who discovered the 

 true nature of rusts in 1853 and published an account of tlie life history 

 of stem rust of wheat in 1865.^ The idea that some diseases are caused 

 by parasites had often been expressed, but de Bary proved it beyond any 

 doubt. 



About this time the researches of Louis Pasteur and others were form- 

 ing the foundations of the science of bacteriology. These investigations 

 soon led to the rejection of the belief that fungi and bacteria, associated 

 with diseased parts of plants, were produced in the lesions by the tissues 

 of the diseased plants. That bacteria may cause diseases of plants was 

 first proved definitely in 1880 when Burrill of Illinois discovered that 

 bacteria cause fire blight of pears and apples (Fig. 256). Proof of in- 



1 Farmers had long believed the barberry to be associated in some mysterious way with 

 rust epidemics. As early as 1660 planting the barberry was prohibited in certain sections 

 of France. Massachusetts in 1760 decreed that all barberry plants in the colony must be 

 removed. 



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