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THE AGKICULTUKAL NEWS 



May 5, 1917. 



the sugar-caue is neither especially resistant nor 

 susceptible to root disease; but unfavourable conditions, 

 as for instance, water-]ogging or drought so weaken 

 the plant that it becomed susceptible owing to external 

 influences. In the case of root disease, also, it is to 

 be noted that the fungus is not an obligate parasite 

 and can in fact live as a saprophyte. Hence root 

 disease of the sugar-cane is essentially a 'conditional' 

 malad}-. 



As examples of the second group, we may take 

 'canker of cacao, the Rosellinia i-oot disease and the 

 disease of seedlings known as 'damping ofi'. Such 

 diseases as these are pi-evalent when conditions favour 

 the fungus, even though the conditions may be favour- 

 able to the health of the hosts. The fungus may or 

 may not be an obligate parasite. 



As to the exact meaning of resistance — constitu- 

 tional resistance — we know very little. In plants it 

 ma\' to some extent be mechanical. At any rate 

 resistance increases with age; but only up to a certain 

 point, when it tends to diminish. This looks as if it 

 were concerned to some extent with cellular activitv 

 and that constitutional resistance may be due to some 

 chemical property of the protoplasm. In the case of 

 the 'rust' fungi which are strictly parasitic on specific 

 grasses, the resistance of certain .strains of host plants 

 has been shown to be due to over-susceptibility. 

 When the leaf is first infected the cells of the surround- 

 ing tissue are killed anS the parasite then f;iils to 

 develop on account of its inability to live in dead tissue. 



In connexion with resistance to disease, it will 

 be realized that genetics affords an important line of 

 stud}'. IJesistance to rust in wheat was transferred by 

 genetic investigations, and it is of interest to record the 

 fact that resistance in cotton to leaf-blister mice has 

 been shown to be hereditary and a definite factor in 

 the gametic constitution of the plant. As to what 

 this factor is we know nothing. 



The mention of leaf-blister mite leads us to 

 a consideration of the resistance of plants towaids 

 insect pests. Observations seem to indicate that the 

 same principles govern the prevalence of infestations 

 as govern the prevalence of diseases. There is a good 

 deal that is parallel between the prevalence of scale 

 insects and the prevalence of, say, the root disease of 

 the sugar-cane. Both are due to the weakening of the 

 host by unfavourable conditions of environment. A pe.st 

 like the cotton worm, however, is more analagous to 

 canker of cacao. II' conditions favour their development 



they will attack their respective hosts without these 

 hosts being in any way weakened by unfavourable 

 conditions of environment. 



The discoveiy of the constitutional nature of 

 resistance to leaf-blister mite lends support to the idea 

 that there may be some organic explanation of resis- 

 tance to pests as well as to fungi. It maybe a ques- 

 tion of the production of enzymes or toxins. At any 

 rate it is a subject which deserves the attention of 

 entomologists and workers in bio-chemistr}-. 



In the higher animals we observe the same 

 fundamentals regarding resistance as amongst plants 

 and the lower animals. It is particularly interesting 

 to note the influence of condition in animals on the 

 degree of infestation by such parasites as worms and 

 ticks. A flick of sheep may be equally exposed to 

 infestation yet the tiirifty animals will resist the 

 parasites while those in poor condition will be found 

 to be infested. It is not merely that the healthier 

 animals show the eft'ects of infestation less, but that 

 they are actually free or almost free from parasites. 

 There is a good deal that is parallel between this 

 and the infestation of plants by scale insects. 



Amongst human beings we notice the same 

 relationship between condition and disease. There is 

 a class of diseases whose prevalence is largelj' influenced 

 by the effect of conditions on the subject, as for 

 instance pneumonia. On the other hand, a disease 

 like malaria is only prevalent when the conditions 

 favour the development of the parasite and its trans- 

 mitter. Natural or constitutional resistance also 

 occurs in man as well as in plants and the lower 

 animals. The Jews as a race have been shown to 

 be especially resistant to tuberculosis. This is 

 explained by the fact tliat for generations Jews 

 have led urban lives and often under conditions 

 favouring infection by tuberculosis. It is thought that 

 in this way the more susceptible individuals have 

 died off with the consetjuent production of a kind of 

 pure strain in regard to this particular characteristic. 



The whole question of the relationship between 

 the host and parasite and the influence of external 

 conditions, is one that should have very much greater 

 attention. From the agricultural standpoint in the 

 West Indies that part of the subject relating to the 

 pests and diseases of plants is of the first importance. 

 A little more oiiginal work of this kind in entomolocf\ 

 is particularly needed. 



