DISEASES CAUSED BY INFECTIVE VIRUSES 49 



The investigation of virus diseases has in consequence been 

 greatly confused by dependence on symptoms as evidence of 

 identity. This has resulted in wide disparities between the 

 conclusions of investigators dealing with externally similar but 

 separate diseases. There is the further difficulty that wide 

 differences in resistance and response on the part of the affected 

 plants exist in relation to these as to bacterial and fungus 

 diseases. 



The Mosaic Diseases. 



The name mosaic disease, which has reference to irregularities 

 of distribution of the green colour of the leaves, has come to be 

 especially associated with a well-marked class of diseases which 

 have in common, in addition to this symptom, an infectious 

 virus, contained in the expressed juice of most or all parts of 

 the plant, capable by inoculation or (except in sugar-cane mosaic) 

 by contact with a wounded surface of setting up the disease in a 

 healthy plant of the same species. The name has been applied 

 to other types of disease, as, for example, the mosaic disease of 

 cotton, which is a deficiency disease, but will probably be re- 

 stricted in future to the class under discussion. 



The best-known mosaic diseases are those affecting tobacco, 

 tomato, potato, and some other Solanaceous plants ; there are 

 also similar diseases of bean, cucumber, lettuce and spinach. 

 Recently the mosaic disease of sugar-cane has assumed great 

 importance in the West Indies. It is not a typical member of 

 this class, not having been found to be transmissible by contact 

 or ordinary wounding, and only with great difficulty by artificial 

 inoculation. 



There is close correspondence in the general characteristics 

 of these diseases as developed on their appropriate hosts, indi- 

 cating a generic relationship between them. Such differences as 

 occur are of a minor and specific nature. 



The following summary of symptoms given by Allard for 

 tobacco mosaic includes the characters of the whole class. 



1. Partial or complete chlorosis (reduction of chlorophyll). 



2. Curling of the leaves. 



3. Dwarfing or distortion of the leaves. 



4. Blistered or " savoyed " appearance of the leaves. 



5. Mottling of the leaves with different shades of green. 



6. Dwarfing of the entire plant. 



7. Dwarfing and distortion of the blossoms. 



8. Blotched and bleached corollas, 

 g. Mosaic sucker growths. 



10. Death of tissues. 

 These symptoms may be present in part only or on part of 

 the plant only, and may be lightly or severely developed. In- 

 fection appears to be in all cases quite general, so that removal 

 of the distorted parts of partially affected plants is no remedy. 



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