CHAPTER VII 



NON-PARASITIC DISEASES 



The term non-parasitic is now coming into use for the class of 

 diseases often called " physiological." Plant pathology has 

 developed almost entirely on a basis of parasitology, and the 

 newer expression is perhaps the most appropriate for what in 

 practice has been not so much a natural group as a limbo where 

 all affections not shown to have parasites as their inciting cause 

 were detained. It is not possible to have a strictly definitive 

 name for such an assembly, and there is in fact no strict dividing 

 line between parasitic and non-parasitic diseases. 



A cacao tree growing in a light soil and subjected to undue 

 exposure suffers repeated losses of its leafy twigs from dieback, 

 and eventually, if the adverse conditions continue, dwindles and 

 dies with all the evidences of inanition. The immediate inciting 

 cause of the dieback would appear to be the want of balance 

 between loss of water from the broad thin leaves and its absorp- 

 tion by the roots. If now, as frequently happens, the failing 

 twigs and branches are attacked by Diplodia, the essential 

 causation is not changed, since the fungus cannot attack healthy 

 twigs, but its presence may increase the severity of the original 

 disease. In a similar way, the production of disease even by 

 obligate parasites may depend on some effect of the physical 

 environment upon the host. 



It is convenient to include with parasitic diseases those 

 border-line cases in which a parasite typically appears, though 

 logically, having reference to their actual inciting causes, such 

 affections as root-disease of sugar-cane and dieback of limes 

 and cacao should probably be classed as non-parasitic. 



This policy leaves in the group to be considered at least one 

 apparently definite class — affections directly produced by im- 

 perfect adjustment between the plant and its physical environ- 

 ment, or, in other words, by unfavourable climatic and soil 

 conditions. These affections show gradations from simple 

 injuries which would hardly be called diseases to systemic 

 disturbances affecting the whole plant. It must be confessed 

 that of the more complex of these types we have very little 

 comprehension, and cases frequently occur in which diseases 

 believed to be of this nature are shown to be of parasitic origin. 

 There is also the difficulty already mentioned in connection 

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