PLANT VIRUSES QJ 



G. The mildly-affected tomato plants carry a certain degree of 

 protection from additional infections. 



H. The "acquired immmiity" of tomato plants is retained through 

 many vegetative generations. 



I. The degree of protection in tomato plants varies with different 

 strains of the virus. 



J. Studies vdth 12 strains of the virus showed that plants im- 

 munized against one strain protected against certain strains but not 

 others. The degree of protection as shown by cross-inoculation ranged 

 from very slight to complete. 



The information presented above is sufficient to show that in the 

 case of curly top the responses differ from the usual plant-virus re- 

 covery phenomena. The reactions in this instance closely parallel 

 certain known immunologic processes in animals. The analogy may 

 be expressed simply as follows: 



a. "Active immunity" initiated in tobacco plants. 



b. "Passive immunization" of tomato plants that do not acquire 

 immunity actively. 



c. Strain specificity. 



In comparing these plant responses with known immunologic re- 

 actions in animals it is at least of interest to point out that the curly 

 top virus is known to be limited largely, if not entirely, to the phloem 

 sieve tubes which carry the elaborated plant food. Thus, a uniform 

 medium is provided for the virus at all times. Such a virus-tissue rela- 

 tionship in plants would, it seems, afford conditions somewhat similar to 

 those under which antibodies are produced in animals. 



It has been suggested by some virus investigators that these so-called 

 immunologic reactions of curly-top-affected plants result from the use 

 of undetected virus-strain mixtures. This presupposes ( 1 ) that the beet 

 leafhopper, the vector of the virus, can obtain avirulent strain A and 

 virulent strain B by feeding on a diseased sugar beet for example, (2) 

 that the vector can introduce the two strains into a tobacco plant, and 

 (3) that non-viruliferous vectors can then acquire only virulent strain 

 B even though strain A is present in the tobacco plant in sufficient 

 quantity to protect other tobacco and tomato plants infected by graft 

 transfer. 



Such an explanation is completely unacceptable to workers experi- 

 enced with the curly top virus strains and vector relationships. Over a 

 period of many years, repeated passage of the established virus strains 



