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Other Causes of Plant Diseases. 



Not all the diseases of plants are produced by parasitic 

 fungi. Some diseases are physiological, due to drainage, 

 the character of the nutrition or the lack of it, and to many 

 other causes not well understood. Other diseases are caused 

 by insects of various kinds or by the ravages of other and 

 often more minute forms of animal life. 



In order to treat a disease succesfully we must know its 

 cause and if it is due to a parasitic organism we must know 

 its life-history, its mode of entrance to its host and its method 

 and time of reproducing itself, in order that we may attack 

 it at its weakest point, prevent its entrance to the host, and 

 prevent its spread by its many methods of reproduction. 



Not all diseases that are called by the same general name 

 are produced by the same cause. For instance the term 

 "rust" as applied in the State of Alabama to a disease of 

 cotton has nothing in common with the rust that appears on 

 the cereals. In fact the term "cotton rust" is a loose gen- 

 eral term that really means about as much as "cotton dis- 

 ease" for it is indiscriminately applied to several distinct 

 diseases, some of which are physiological and some of which 

 are caused by various parasitic fungi. Again the term 

 Uight has a very loose popular usage and has been the source 

 of much confusion. There is no such thing as a general 

 blight affecting various plants. The blight of the pear and 

 apple is due to a very definite organism, concerning which 

 much is already known. The blights of other plants, notably 

 various garden vegetables, is due to other entirely different 

 organisms whose character is far from being well known. 



Root galls, or swellings on the roots of plants, are due to 

 various causes and must be made a separate study before 

 the cause can be determined in any given case. In the grape 

 they may be due to the work of various insects, or produced 

 by minute thread worms {nematode.^); in many garden vege- 

 tables and field crops they may be due entirely to the latter 

 cause ; in the cabbage and other members of the mustard 

 family they are due to an internal plant parasite of a low 



