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



The plants in this frame were considerably smaller than those in frame No. 7, 

 which had received the stems, etc. 



In addition to the above tests, which were made under greenhouse 

 conditions, the following were made in the same manner, out of doors, 

 in the experimental tobacco plat: 



Frame No. 9, 2 feet square ; 2,200 grams of mosaic material chopped into the 

 soil; seed sowed August 4. On October 9 the plants, between 100 and 200 in 

 number, were 10 to 11 inches high ; all were healthy but one, which had become 

 diseased and was carefully removed. On October 19 the plants were 12 to 15 

 inches high. One more plant had become diseased. The rest were fine, healthy 

 plants. 



Frame No. 10, control, 2 feet square; seed sowed August 4. On October 9 

 the plants, several hundred in number, were 10 to 11 inches high and all healthy. 

 On October 19, 2 plants in this control frame were diseased. It is interesting 

 to note here that the mosaic plants in both frames were infested with aphides. 

 Throughout the season these aphides were present on the tobacco plants grow- 

 ing immediately around frames Nos. 9 and 10. Practically all the plants in 

 this field became diseased following infestation. Since the screens protecting 

 these frames were removed on September 27, it is not surprising that these 

 aphides should appear on the young plants. 



Many other tests of growing plants in infected soils have been 

 made in addition to these mentioned. Although large quantities of 

 green material have in some instances been added to the soil, this 

 very quickly decomposes and incorporates itself with the soil mate- 

 rial. In no instance has an outbreak of mosaic disease appeared in 

 plants germinated and grown in such soils. Plants when young have 

 been pulled and transplanted from infected soils to healthy soils, 

 where they were allowed to blossom, in osder to simulate practical 

 methods of handling plants from soils supposedly infected. Out of 

 20 or 30 plants thus handled not a single plant developed the disease. 



ORIGIN OF THE DISEASE DEPENDENT UPON SPECIFIC INFECTION. 



Although earlier investigators have concluded that a true infectious 

 mosaic condition can be induced by simply cutting back the plants, 

 ihe writer's experiments do not support this conclusion. 



Many tobacco plants have been used to determine whether the 

 operation of severely cutting them back can in itself produce an in- 

 fectious mosaic disease. Good-sized individual plants were grown in 

 a screened inclosure, fumigated frequently to prevent infestation by 

 aphides, and cut back at frequent intervals. 



Tobacco plant "A," in a screened cage, was cut back on January 

 31, February 24, March 8. and March 25. This plant continued 

 healthy. 



In other experiments tobacco plants were allowed to germinate and 

 to grow thickly crowded in carefully screened cages, which were 

 fumigated from time to time. These plants were cut back at frequent 



