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THE INFLUENCE OF SOIL FACTORS 

 ON DISEASE RESISTANCE. 



By ALBEET HOWARD, CLE. 



Imperial Economic Botanist, Pusa, India. 



(With 5 Text-figures.) 



I. Introduction. 



One of the features of the modern literature on agricultural science is 

 the number of papers dealing with the diseases of plants in which insects 

 and fungi are concerned. Much less attention, however, has been paid 

 to disease and to the factors on which disease-resistance depends. It is 

 now well understood that the unit species which go to make up the 

 varieties of cultivated crops differ greatly among themselves in re- 

 sistance to certain parasites and that in some cases at any rate this 

 character can be inherited. Further, it is a matter of common experience 

 that the various diseases only become epidemic where conditions assist 

 the spread of the parasite and at the same time injuriously affect the 

 condition of the crop which is being attacked. Briefly stated, the 

 ravages of any parasite are found to depend on the kind of plant grown 

 and on the conditions under which it is cultivated. We know practically 

 nothing however of the reasons why one unit species is resistant to 

 disease and why others, belonging to the same variety, do not possess 

 this character to any great degree. Further, little has been done to trace 

 the influence of separate soil factors on disease-resistance. 



During the last 15 years, opportunities of growing the same crop in 

 India under widely different conditions of soil and climate and of 

 studying the distribution of the crops of this continent have been 

 utilised. At the same time, many observations on the incidence of 

 disease have been made and of the general conditions which appear to 

 precede serious damage by parasites. In some instances, the matter has 

 been carried further and an attempt has been made to ascertain what 

 soil factors are responsible in lowering the natural resistance of a crop 

 to attack. In practically every case, light has been thrown on the 



