gS WALLACE 



through numerous different plant hosts, involving both vector and graft 

 transfer, has revealed no evidence of strain mixtures. Furthermore the 

 vectors pick up two or more strains of virus from known mixtures and 

 when used for inoculation of plants in repeated, short-time feedings may 

 infect those plants with individual strains or with various combinations 

 of the strains carried.^ 



There is much experimental evidence against the suggestion that the 

 behavior of the plants in this instance results from strain interference. 

 If it is accepted that the vector, by selection, acquires only the virulent 

 strain from the immunized tobacco or tomato plants then it must be 

 agreed that the vector screens out the protecting strain. The facts are 

 that healthy tobacco plants infected by insect transfer of the virus from 

 immunized plants of either tobacco or tomato repeat the process of re- 

 covery and again confer protection to tomato plants that are infected by 

 graft transfer from the tobacco. Thus it seems that if these phenomena 

 are to be explained on the basis of strain interference it is necessary to 

 accept firstly, that the vectors are capable of strain selectivity when 

 there is much experimental evidence that such is not the case, and sec- 

 ondly, that when any single strain is introduced into a tobacco plant a 

 second strain always arises by some process which confers protection to 

 tomato plants graft-infected from that plant but which cannot be trans- 

 ferred by the insect vectors. 



Recovery of plants from the symptoms of virus diseases, that is, the 

 gradual passage of the plants from a severe stage to a chronically mild 

 or symptomless stage, and their resistance to reinfection thereafter 

 could arise it seems by non-immunologic processes. Up to that same 

 point the recovery of tobacco plants from curly top provides no good 

 evidence of immunologic responses. However, the passage of the ac- 

 quired condition of tolerance of the virus and resistance to reinoculation 

 in recovered tobacco plants to tomato plants which very rarely initiate 

 the recovery reaction, and the further transfer of these properties from 

 tomato to tomato indicate that these responses differ from other reported 

 instances of recovery from plant viruses. The question still remains as 

 to whether or not protective or antibody-like substances are involved in 

 these phenomena. 



COMMENT 



S. E. LURIA 



Dr. Wallace's results on transmission of curly-top infection in 

 tobacco and tomato (mild disease by graft transmission, severe disease 



*See footnote 2. 



