CHAPTER XL 



CAUSES OF DISEASE. 



A. External causes /. Non-living environment: soil, 

 atmosphere, temperature //. Living environment: 

 plants, animals Complex interactions Predisposing 

 causes -No one factor works alone Tajigled problems 

 of natural selection involved. B. So-called internal 

 causes. 



It is customary to classify the causes of disease 

 in plants into two principal groups (i) those 

 due to the action of the non-living environment 

 soil, atmosphere, physical conditions such as 

 temperature, light, etc.; and (2) those brought 

 about by the activities of living organisms plants 

 and animals of various species. Before passing 

 to further subdivisions under these two heads, 

 however, it is necessary to observe that no disease 

 can be efficiently caused by an organism alone, 

 since its powers for injury as a parasite, or other- 

 wise, are affected by its non-living environment 

 as well as by the host-plant. For instance, the 



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