PLANT VIRUSES 95 



these immunologic reactions are of value in determining strain rela- 

 tionships is an important advance. 



Cross-protection — Cross-protection or strain interference has been 

 widely used in the identification of viruses. Because of the rather gen- 

 eral agreement between cross-protection and serologic identifications 

 of viruses, the former should be mentioned at least briefly here. 



Cross-protection involves two general types of behavior in virus- 

 affected plants. The first and most common is the protection afforded 

 plants against virulent strains of virus by a previous infection with 

 avirulent strains. It is perhaps sufficient to state that this phenomenon 

 is encountered quite often in the plant virus field and that it has been 

 of value in determining strain relationships. In general, strain identi- 

 fication by serologic and cross-protection methods have been in accord, 

 but at least in a few reported cases the results have not been in agree- 

 ment. Recent work with certain viruses that in earlier studies had not 

 shown the same relationship by the two methods has demonstrated that 

 disagreement resulted from the lack of a sufficiently sensitive serological 

 technique. 



The degree of protection afforded plants by one virus strain against 

 a second is influenced by numerous factors, — age of plant species, time 

 and method of infection, etc. Standardizing both serologic and cross- 

 protection techniques may prove successful for determining the anti- 

 genic relationships of other viruses that have shown relationships by 

 only the cross-protection test. 



Although it is generally true that plants previously invaded by an 

 avirulent strain of virus are protected against a virulent strain of the 

 same virus, there is at least one striking exception in the case of the 

 virus causing the curly top disease of sugar beets. This virus consists 

 of numerous strains which are without question closely related. In this 

 case it has been well established that in sugar beets avirulent strains do 

 not protect against virulent strains. Virulent strains superimposed on 

 avirulent strains induce infection and the host plant then shows symp- 

 toms of the virulent strain. Furthermore, plants already infected wdth 

 virulent strains are not protected from infection with avirulent strains. 

 The symptoms of the latter are of course masked, but the virus becomes 

 established in the plant and can be recovered separately from the plants 

 having mixed infections.^ 



The second type of behavior of virus-infected plants that has been 



"Giddings, N. J. Some interrelationships of virus strains in sugar beet curly top. 

 Phytopath., 40:4, 377-388. 1950. 



