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TEXTBOOK OF BOTANY 



plants and plant products from one part of the world to another. When 

 a new plant is introduced nito a country great care must be exercised to 

 prevent the simultaneous introduction of its associated parasitic organ- 

 isms. When certain plants of a given area are badly infected with patho- 

 genic organisms, the government sometimes prohibits the transpor- 

 tation of such plants, or plant parts, into other sections of the country. 

 This is known as "plant quarantine." 



In spite of our ever-increasing knowledge about plant diseases, they 

 continue to cause enormous losses every year throughout the world 

 (Table 16). Farmers, orchardists, and nurserymen frequently cultivate a 

 crop at a loss instead of a profit because of diseases or unfavorable 

 weather conditions. 



Table 16. Average Annual Reduction in Yield Due to Diseases, in the 

 United States for the Years 1933 to 1937 Inclusive. Estimates by U. S. Dept. 

 of Agriculture. 



^Estimates reported For UVM only. 



Environmental conditions and plant diseases. We are aware that the 

 existence of an\' plant depends upon its environment. Some of us have 

 seen whole crops destroyed bv unusual environmental conditions, such 

 as flood, drought, tornadoes, and fires. As might be expected, plant dis- 

 eases are influenced bv the environmental factors of the soil and the 

 atmosphere. Whate^'er affects the host, such as temperature, light, oxygen 

 supplv, and the water content of the soil, will also directly or indirectly 

 influence the organism causing the disease. Considerable information 



