THE MOSAIC DISEASE OF TOBACCO. 27 



intervals from the time the plants were 2 or 3 inches high. The oper- 

 ation of cutting back was continued as long as the plants were 

 capable of making growth. 



In cage No. 1, 35 plants were cut back on March 8, March 21, March 

 28, April 10, April 23, May 11, and May 28. These plants remained 

 free from disease. 



In cage No. 2, 25 plants were cut back on March 21, March 28, April 

 10, April 23, May 11, May 28, and June 25. These plants were still 

 free from disease on July 5. 



All experiments indicate that plants may be cut back indefinitely 

 if they are free from the mosaic disease at the time of cutting and the 

 infectious principle of the disease is not accidentally introduced dur- 

 ing the progress of the experiments. Certain precautions, however, 

 must be very carefully observed. The plants must at all times be kept 

 screened and fumigated to exclude aphides and other insects. All 

 instruments must also be very thoroughly sterilized before cutting 

 back each plant. A flame has afforded the quickest and surest means 

 of sterilizing these instruments. 



INSECTS AS CARRIERS OF THE DISEASE. 



In the course of the writer's greenhouse experiments with tobacco 

 it was observed that the occurrence of the mosaic disease was asso- 

 ciated with aphid infestation. When certain colonies of these aphides 

 were introduced into cages containing healthy plants it was found 

 that the disease usually developed in every infested plant. (PL VI, 

 fig. 2.) On the other hand, control tobacco plants never developed 

 the disease if they were grown in screened cages fumigated at short 

 intervals to prevent aphid infestation. 



Whether other insects than aphides disseminate mosaic infection 

 to healthy plants yet remains to be determined. This phase of the 

 question has been taken up by the Bureau of Entomology. 



During the season of 1912 the occurrence of the mosaic disease 

 in tobacco plants in the seed bed and in the field at Arlington, 

 Va., was unquestionably associated with infestation of the plants 

 by aphides of the species Macrosiphum tabaci Perg. A small colony 

 of these insects early established itself at a single point in the 

 seed bed. Somewhat later the disease developed in plants at this 

 point. Shortly after plants from this bed were transplanted to 

 the field one or two sporadic cases of the mosaic disease came into 

 evidence. The lower leaves of these diseased plants were infested 

 with the aphides. These insects sooner or later began to appear 

 beneath the lowermost leaves of practically every healthy plant in 

 the field. At the same time mosaic infection spread rapidly through- 

 out the field until the greater portion of all the plants had developed 



