Sept. 17. I9I7 Mosaic Disease of Tobacco 619 



point in a single leaf. Fifty similar plants were also inoculated at 20 

 points in several of the largest growing leaves. A very small percentage 

 of the plants receiving a single inoculation became infected with the 

 mosaic disease. In the series receiving 20 inoculations a few plants 

 failed to become infected with the disease. Two plants which became 

 infected from a single inoculation and five plants which remained free 

 from the disease until maturity after 20 inoculations were allowed to 

 produce seed. Seed of each plant was sowed separately, and a series 

 of young plants from each mother was inoculated once in a single leaf. 

 In these tests progenies of the mother plants which escaped infection after 

 20 inoculations did not give any evidence of greater immunity to the 

 mosaic disease than progenies from the mother plants which became dis- 

 eased from a single inoculation. It is difficult to explain why some plants 

 fail to become infected with the virus even after many thorough needle 

 inoculations have been made. Other experiments have shown that even 

 these apparently resistant plants may readily succumb to later inocula- 

 tions, for if they are more resistant than others, it seems to be a resistance 

 of a more or less temporary nature. Even in the field, where the mosaic 

 disease is very widespread, a few plants may be found which escape the 

 disease. Whether or not such plants have escaped infection, or are 

 actually more resistant than others, can not be determined until experi- 

 mental inoculations have been made. Investigations dealing with large 

 numbers of plants along this line would perhaps throw light on the 

 question of relative resistance of different individuals. 



Different individuals of the same strain may show different degrees of 

 expression of the disease. One plant may show the mottled phase 

 particularly well developed. A sister plant may show extreme stunting 

 and the development of depauperate leaves and blossoms. These differ- 

 ences appear to depend upon constitutional differences in individual 

 plants rather than upon differences in the degree of virulence of the virus. 

 Inoculation experiments indicate that the viius extracted from the 

 slightly mottled plant produces essentially the same type of disease in 

 young plants as is obtained from the virus of plants showing the most 

 extreme stunting and malformation of the leaves. Individual differences 

 in the expression of the disease in the same strain are probably analagous 

 to those differences observed when plants of different geneia of the 

 solanaceous family become infected with the mosaic disease. 



Certain characteristics associated with the expression of the mosaic 

 disease of tobacco are more or less dependent upon unfavorable condi- 

 tions of growth. Plants showing the mottled phase in the field when 

 transplanted to the greenhouse often develop depauperate leaves and 

 blossoms. Although the blossoms may be beautifully mottled, it is very 

 rare to find catacorolla showing in field plants, however badly they may 

 be affected with the disease. When these plants are transferred to pots 

 or beds in the greenhouse in the fall and are cut back severely, the most 



