VARIATION AND ITS CHEMICAL CORRELATES 131 



cause different symptoms, a more or less severe disease, or different primary 

 or secondary lesions. The variant may also differ from its parent strain in 

 host range, in infectivity, in invasiveness, in specific insect vectors, and in 

 mutability. The variant may have different thermal stability, serological 

 properties, or chemical characteristics. However, in looking for mutants the 

 first evidence sought is some indication of a change in the disease character- 

 istics. 



The wide variety of symptoms that can be caused by different variants of 

 a plant virus in a given host is well exemphfied by the strains of TMV. Some 

 of these strains are so mild that in a given host no obvious symptoms are 

 observed, and these variants are commonly called "masked" or "symptom- 

 less" strains. On the other hand, many strains cause stunting of growth and 

 various kinds of distortion or malformation of the leaves; still others are dis- 

 tinguished by the different character or shades of their yellow or white mosaic 

 patterns. Some strains cause spotting or striping of fruits borne on the 

 infected plants, others do not. Some strains cause distinct necrotic effects, 

 such as brown or white lesions, ringspot patterns, necrotic lines along the 

 leaf veitis, or stem necrosis. In short, the symptoms associated with strains 

 of T]\rV can range from nothing discernible to complete annihilation by 

 necrosis (Figs. 1 and 2). 



B. Methods for Securing Strains 



Strains of a plant virus can be secured by a variety of methods. Some of 

 the most useful will be hsted and briefly described. 



1. Naturally Segregated Strains 



In any population of plants susceptible to virus disease and showing signs 

 of infection there will be occasional plants with distinctly different symptoms. 

 Thus, in a field of tobacco, it is possible to pick up new strains of tobacco 

 mosaic virus by examining and selecting plants showing obviously different 

 disease symptoms. Sometimes a population of plants may show a uniform 

 disease pattern which by subsequent testing proves to be attributable to a 

 virus not usually associated with that host. For example, there is a spinach 

 blight caused by a strain of the common cucumber mosaic wus. Likewise, 

 Holmes (1941) discovered an unusual strain of tobacco mosaic virus in a 

 roadside weed, Plantago. The identification of a strange virus with a known 

 one is accomphshed by a series of tests to be described in the next section. 



2. Isolation from Distinctive Spots 



Bright yellow spots occasionally appear on leaves of plants infected with 

 a virus that normally causes green mosaics or other symptoms (Fig. 3). 

 Similarly, distinctive green areas are sometimes observed on leaves of plants 



