132 C. A. KNIGHT 



showing predominantly yeUow symptoms. Somewhat less frequently, a 

 necrotic brown lesion may appear on leaves showing mosaic patterns. Each 

 of these distinctive spots represents an island of virus different from that in 

 the major portion of the leaf, and if the virus is one transmissable mechanic- 

 ally, it can often be isolated by needle puncture of the spot, followed by 

 puncture of a leaf on a healthy plant. There is e\ddence that similar transfers 

 of new strains can be made by insects which by chance feed on a mutant spot 

 and then on a normal plant (Hoggan, 1935). 



An unusual instance of strain isolation from distinctive spots was reported 

 by Best and GaUus (1955) working with strains of tomato spotted wilt virus. 

 Mixed infection with two strains of this virus resulted in the production of 

 pigmented necrotic lesions, from which a new strain was isolated having 

 properties not coinciding with those of either of the strains used in the 

 mixture causing infection. 



3. Passage in a Different Host 



While virus strains are usually characterized by possession of a common 

 host range, it is equally well known that a strain which does well in one host 

 may do poorly in another, and the reverse. Hence, if a given strain is passed 

 in a host in which it does not flourish, a new strain may gain the ascendancy. 

 One passage, or, more often, several passages are required to secure a new 

 strain by this technique. In any case, the origin of such a new strain is diffi- 

 cult to decide. A mutation may have occurred as a result of the altered en- 

 vironment to which the old strain was subjected, and the new strain may 

 then have been selected by repeated passage in the new host whose environ- 

 ment favored it over the old strain. On the other hand, a mutant arising 

 spontaneously in the first host may simply have been selected by passage in 

 the new host. It is difficult to exclude the presence of such a mutant in the 

 inoculum used on the second host, because the multiplication of mutants in 

 any host is apt to be circumscribed owing to the interference phenomenon 

 shown by related plant viruses (see Section II, C, 2). Hence, the mutant 

 could constitute so low a percentage of the inoculum that it would never be 

 detected save by some selective device, such as passage in a different host. 

 The end result is, of course, the same, whether the new strain was already 

 present in small amounts in the inoculum, or whether it arose by mutation 

 of the first strain during multiphcation in the second host. 



4. Exposure to Elevated Temperatures 



Strains possess similar, although not identical, thermal stabilities. How- 

 ever, they multiply quite unequally at elevated temperatures. Hence, strains 

 may be obtained by holding infected plant tissues or virus-containing insect 



