DISEASES CAUSED BY INFECTIVE VIRUSES 53 



be produced by the operation of external factors, and diseases 

 of similar type exist which appear to differ only in the absence 

 of known transmissibility. This being the case, most importance 

 must be attached to the facts regarding transmission. In the 

 diseases under discussion the occurrence of epidemics and the 

 infection of plants derived from material originating outside 

 their range afford sufficient evidence of the general fact, but 

 except in the case of curly-top little is known of the manner in 

 which transmission takes place. 



It is now well established with regard to curly-top of beet 

 that it is readily and regularly transmitted from diseased to 

 healthy plants by the small Jassid leaf-hopper Eutettix tenella, 

 and recent experiments have indicated that for this to take 

 place a period of at least 24 hours' " incubation " of the virus 

 in the body of the insect is necessary. Leaf-hoppers from 

 healthy beets or from other plants do not produce infection. 

 All attempts at transmission by inoculation or contact have failed. 

 The disease is continued by cuttings, but not, so far as experience 

 has gone, by seed ; it can be transferred by grafting. 



The remaining diseases are transferred from infested to neigh- 

 bouring healthy plants by some means as yet unknown. All 

 have shown themselves capable of rapid extension from a small 

 initially infected area. Potato leaf-roll and the peach diseases 

 are transferable by grafting. Leaf-roll and sereh are continued 

 by cuttings. Peach yellows is definitely stated not to be contracted 

 from soil in which diseased plants are grown, and no statement 

 has been seen that this can occur in curly-top. The infection 

 of leaf-roll has been regarded as persisting in the soil for two 

 or more years, but confirmatory evidence of this is lacking 

 and survival may admittedly have been due to deeply buried 

 tubers. Good evidence is produced, however, that transmission 

 takes place through the soil from one growing plant to another. 

 Whether soil infection occurs in sereh does not appear to be 

 known, and the manner of its occurrence would appear to be 

 equally capable of explanation by a theory of insect transmission. 



