28 BULLETIN 40, U. S. DEPARTMENT OF AGRICULTURE. 



the disease. Although these aphides were very common on the under 

 side of the ground leaves during early summer, it was interesting 

 to note that very few could be found a few weeks later. 



The occurrence of the mosaic disease in the tobacco seed beds and 

 later in the field at Arlington can not readily be explained on any 

 other grounds than that of transmission by insects. The seed beds 

 were prepared from a good soil, largely mixed with clean sand. This 

 prepared soil was uniform throughout and had never been used 

 before. Nor can the widespread occurrence of the disease through- 

 out the field during the season of 1912 be attributed to mosaic refuse 

 from any previous tobacco crop, since the only crop of tobacco pre- 

 viously produced by this field, at least in recent years, was grown in 

 1911, and during that season only a very small portion of the field 

 was devoted to tobacco. 



WINTER SURVIVAL OF MOSAIC INFECTION. 



It would seem to be pretty clearly established that specific infection 

 is wholly responsible for the occurrence of the mosaic disease of 

 tobacco. During an outbreak of the disease an infectious principle 

 in some manner must be carried from plant to plant. Since it has 

 been shown that aphides are sometimes responsible for the spread 

 of the disease, the question arises as to how this infectious principle 

 survives the winter. Whether it can be carried over the winter by 

 hibernating insects yet remains to be determined, but it is hoped that 

 through cooperation with the Bureau of Entomology definite evidence 

 may be obtained on the point. 



There is strong evidence in support of the conclusion that the mosaic 

 disease in an affected perennial, such as Solatium carolinense, persists 

 throughout the life of the plant. In the roots of such plants the 

 mosaic disease of tobacco doubtless is sometimes carried over the 

 winter. Although the writer has examined many hundreds of plants 

 of S. carolinense in various localities, with the exception of the 

 mosaic plant of S. carolinense previously referred to, every plant has 

 appeared to be free from the disease. It must be kept in mind, 

 however, that it is frequently very hard to discern external symptoms 

 of the disease in plants of this species. No other susceptible species 

 of the Solanacese has shown such slight symptoms of the disease as 

 S. carolinense. 



It is of some interest here to consider the mosaic disease which 

 affects the perennial pokeweed {Phytolacca decandra). The writer 

 has observed that the disease persists from year to year in affected 

 plants of this species. Although all aerial portions of these plants 

 perish during the winter, the infectious principle responsible for the 

 disease is preserved in the underground portions until the following 



