DISEASES CAUSED BY INFECTIVE VIRUSES 51 



principle does not reside in the soil. Even buried diseased 

 material does not as a rule produce many infections, and such 

 cases as do occur are believed to depend on root injuries which 

 give access to the virus. Steam sterilisation is effective in remov- 

 ing any risk of infection from diseased material retained in the 

 soU. 



The transmission of mosaic diseases in the field or green- 

 house has been repeatedly shown to be effected by aphides 

 (" plant lice " or " green fly.") Most of the experiments have 

 only gone so far as the transfer of the insects from diseased to 

 healthy plants, with others from healthy plants as controls, but 

 in spinach blight McClintock and Smith have demonstrated that 

 the offspring of virus-bearing aphides, as far as the fourth 

 generation, may transmit the disease without themselves having 

 had access to infected plants. 



The distribution of mosaic diseases suggests the probability of 

 the occurrence, at least occasionally, of transmission by seed. In 

 several cases, as in tobacco and spinach, the results of attempts 

 to demonstrate this have been uniformly negative ; in others, as 

 in the mosaic diseases of bean (Phaseolus) and cucumber, the 

 occurrence of seed transmission has been established. 



The viruses of mosaic diseases exhibit specific limitations 

 similar to those of organic parasites. The infectivity of a virus 

 ma}' be confined to one host or it may produce disease, of varying 

 severity, in related species. The disparities in the results ob- 

 tained by different groups of investigators even suggest the 

 occurrence of different " strains " of virus in the same host 

 species ; in fact, it may be said that in respect of susceptibility, 

 resistance and immunity the mosaic diseases exhibit the same 

 phenomena as do those originating from fungoid or bacterial 

 parasites. 



Virus Diseases with Restricted Transmission. 



Under this heading it is proposed to include for comparison 

 the following diseases : curly-top of beet, leaf-roll of potato, 

 sereh disease of sugar-cane, peach yellows, and peach rosette. 

 These have all shown themselves to be capable of transmission 

 to healthy plants, while affording no evidence of causation by 

 any visible parasite. This leads to the assumption, as in the 

 first group, that an infectious virus is concerned in their pro- 

 duction, but in distinction from these it has not been found pos- 

 sible to produce infection by artificial means, if budding and 

 grafting, which involve organic union, be excluded from this 

 category. 



The affections mentioned have great economic importance, 

 and include some of the most widely discussed plant diseases, 

 but further experimental information, especially regarding 

 sereh disease, is required before a really definite comparison xan 

 be made. 



