CHAPTER III 



DISEASES CAUSED BY INFECTIVE VIRUSES 



The diseases to be discussed in this chapter, which include 

 several of great economic importance, have been usually relegated 

 to the undefined category of " physiological " affections. Though 

 knowledge regarding them is very uneven, it would now seem 

 both possible and desirable to bring them together as a class, 

 distinguished by the possession of an infective principle which 

 can be transferred to a healthy plant and reproduce the disease, 

 but which within the range of our present technique gives no 

 evidence of association with an invading organism. It would 

 further appear that a sub-division of this class may be made by 

 distinguishing between affections in which the juice of the 

 diseased plant is generally and directly infective, as in the true 

 mosaic diseases, and those in which this property is absent and 

 infection is restricted to special means. In some of the latter 

 class transmission depends on contiguity in a manner which is 

 still obscure, in others it is regularly effected by insects. Usually 

 it may be effected artificially by budding or grafting where these 

 are possible, provided that organic union of stock and scion 

 takes place. 



The word virus used in connection with these diseases must 

 not be taken as more than a convenient expression of the idea of 

 an infective principle of unknown nature, recognised only by the 

 effects it produces. The validity of such a conception may be 

 more readily conceded if it is remembered that our knowledge 

 of enzymes, useful as it has proved, is of the same nature. 



The virus diseases are typically systemic, affecting all parts 

 of the plant. They have apparent analogues in animal and human 

 pathology, some highly infectious, others restricted in their 

 transmission. 



In their external symptoms these diseases are liable to be 

 closely simulated, on the same or on other plants, by appearances 

 produced in some cases by the action of external factors, as in 

 the dwarfing of shoots and mottling and crumpling of leaves 

 induced in cotton and ground-nuts by out-of-season planting, 

 in other cases by genetic factors, as in certain hybrids and loss- 

 mutations of cotton in which similar characters appear, in still 

 others by insects, as in froghopper blight of sugar-cane, which 

 closely reproduces many of the symptoms of sereh. 



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