620 Journal of Agricultural Research voi. x, no. 12 



expression of symptoms, it is known that the infective principle does 

 not necessarily produce e\'ident symptoms wherever it is found. If 

 half-grown plants become infected with the virus of the mosaic disease, 

 the younger leaves show symptoms of disease, while the lowermost 

 mature leaves remain healthy in appearance. Such leaves may contain 

 the infective principle, however, if sufficient time has elapsed for general 

 or systemic infection to take place. 



Likewise, if nearly mature plants become infected, the corolla alone 

 may show symptoms of disease. Subsequently all secondary branches 

 become mosaic diseased, showing that the virus has migrated to all 

 parts of the main stem, finally reaching and affecting new growing cen- 

 ters. If such dormant plants are severely cut back, even to the ground, 

 all subsequent sucker growth becomes mosaic diseased. 



In large, vigorous plants grown in the field systemic infection some- 

 times proceeds gradually from branch to branch. This has been shown 

 with large plants producing many suckers. In some instances the upper- 

 most suckers were badly diseased, while others lower down on the stalk 

 appeared normal in all respects. 



Although inoculation tests have shown that such suckers in some 

 instances did not contain the active, infective principle at the time the 

 sap was extracted, such suckers invariably become diseased sooner or 

 later. 



TRANSMISSION OF THE MOSAIC DISEASE OF TOBACCO BY PLANT LICE 



It was first obser\^ed in the greenhouses at Arlington, Va., during the 

 winter and spring of 191 2 that plant lice were in some manner associated 

 with outbreaks of the mosaic disease of tobacco. Experiments have 

 since shown that the common and well-known green peach aphis, or 

 spinach aphis {Myzus persicae Sulz.) may become an active carrier of 

 the infective principle of the disease. These plant lice are very common 

 in the greenhouse and may be found on a great variety of plants. In 

 winter they multiply in great numbers upon tobacco plants and may 

 even produce serious injury to these plants if allowed to multiply 

 unchecked. 



Many colonies of these plant lice obtained from different sources have 

 been studied in their relation to the transmission of the mosaic disease. 

 Colonies have been found which have failed to produce infection in 

 healthy tobacco plants. Such colonies have been obtained from bella- 

 donna and cabbage plants, which are naturally immune to the disease. 

 Colonies free from infection have also been obtained from healthy 

 tobacco plants. On the other hand, experiments have shown that 

 colonies obtained from mosaic-diseased tobacco plants may become 

 very efficient disseminators of the infective principle of the disease. 



In one of the earlier experiments carried out during the winter of 

 1 91 2, a colony of Myzus persicae obtained from belladonna plants was 



