CHAPTER 7 



NATURE OF PLANT DISEASES 



The successful greenhouse operator will realize 

 the necessity of recognizing readily any plant dis- 

 ease. Very often this is overlooked and attention is 

 attracted only when the trouble takes the form of an 

 epidemic, and a large number of plants are thus 

 carried off by it. 



Plant diseases are usually of four kinds : 



1. Those of a mechanical nature. 



2. Those brought about by physiological dis- 

 turbances or unfavorable environment. 



3. Those brought about by parasitic flowering 

 plants, fungi or bacteria. 



4. Diseases the cause of which is unknown. 



A familiarity with the symptoms of diseases will 

 enable us to determine the contagious nature of the 

 trouble and often the methods of control to pursue. 

 The following outline* briefly summarizes the prin- 

 cipal symptoms of disease in plants : 

 1. Discoloration or change of color, 

 a. Pallor, yellowish or white instead of the nor- 

 mal green. 



♦Adapted with slight modifications from Heald, F. D., Texas 

 University Bui. 135, Sc. ser. No. 14: 7-8, 1909. 



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