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



the disease, there is little evidence to support this view. The fact 

 is well established, however, that the occurrence of mosaic in both 

 the seed bed and the field may bear no relation whatever to a previous 

 outbreak. It has frequently been observed that an epidemic of the 

 disease may occur on new land as well as on land long devoted to 

 tobacco. Both in seed beds and in the field the sporadic occurrence 

 of symptoms is one of the most striking facts connected with the 

 history of the disease. This feature of the disease, together with the 

 fact of its highly infectious nature, has always proved a most serious 

 obstacle confronting all theories which have attempted to account 

 for the mosaic disease on a purely physiological basis. 



METHODS OF CONTROL. 



While conducting various greenhouse experiments with tobacco 

 during the season of 1911-12, the writer found it almost impossible 

 to grow plants which did not sooner or later become mosaic. At this 

 time it was observed that practically all the plants were infested with 

 aphides and that these insects were responsible for the occurrence 

 of the disease. By careful experiment it was later shown that cer- 

 tain colonies of these insects were the active disseminators of the 

 disease. 



In order to secure conditions in every way favorable to the develop- 

 ment of mosaic in all unprotected plants, fumigations were with- 

 held until the entire house was overrun with aphides. Large num- 

 bers of young tobacco plants were then grown in isolation cages 

 screened with a fine cloth to exclude these insects. At frequent 

 intervals the inclosed plants were also fumigated with nicotine paper. 

 By observing such precautions it proved to be an easy matter to 

 grow plants free from the mosaic disease for any length of time. 



Since these preliminary experiments were made, the writer, by 

 keeping the greenhouses thoroughly fumigated against aphides, has 

 grown many thousands of healthy tobacco plants during a period of 

 six months without the occurrence of a single sporadic case of the 

 mosaic disease. Throughout this period mosaic plants were growing 

 on every side in connection with various inoculation studies, yet it 

 was found quite unnecessary to isolate or to screen healthy plants so 

 long as fumigation was continued. 



Although all evidence at hand indicates that the mosaic disease 

 can be readily controlled under greenhouse conditions, practical 

 means of preventing its occurrence in the seed bed by similar methods 

 of procedure have not been attempted. This phase of the question is 

 now under investigation in cooperation with the Bureau of Ento- 

 mology. 



In any consideration of the control of mosaic under practical grow- 

 ing conditions two phases of the question present themselves. One is 



