THE MOSAIC DISEASE OF TOBACCO. 33 



The visible expression of the disease is only an incidental feature 

 of obscure pathogenic conditions within the plant and is considerably 

 modified by external conditions. 



Scions of Nicotiana glauca grafted upon mosaic stocks of N. tabac- 

 um show that the sap of TV. glauca carries the virus even though this 

 species does not manifest visible symptoms. Solarium carolinense fre- 

 quently shows very slight visible symptoms. The character and in- 

 tensity of the symptoms depend considerably upon the morphologi- 

 cal structure of the leaf. Symptoms are more marked in thin, tender- 

 leaved plants. 



The mosaic disease, once initiated, becomes a progressive, incurable 

 malady. Although many individuals escape the disease from a single 

 inoculation, no plant of a susceptible species appears to be perma- 

 nently immune. 



The sap of mosaic plants after passing through a filter still retains 

 its infectious properties. Mosaic material ground and dried retained 

 its virulence 1^ years. The virus preserved by ether, toluene, and 

 glycerin was virulent four months later, as was also the original 

 juice, which had been allowed to undergo natural fermentation during 

 this time. 



Certain species of aphides are active disseminators of the mosaic 

 disease. Not all sucking insects appear to act as carriers of infection. 

 In greenhouses, where the disease is frequently very troublesome, the 

 writer has kept it under complete control by frequent fumigations 

 with nicotine to exclude aphides. 



Observations in the field have shown that aphides of certain species 

 may sometimes be responsible for the occurrence of the disease in 

 the seed bed and its subsequent spread in the field. 



Methods of controlling the mosaic disease of tobacco by fumigation 

 have not been attempted for practical growing conditions. If the 

 disease originates in the field after the plants have been transplanted, 

 its practical control will be exceedingly difficult. 



The simple processes of cutting back, frequent repotting, or other- 

 wise subjecting plants to unfavorable conditions of growth are not 

 sufficient to produce infections mosaic symptoms. All facts indicate 

 that the development of the disease depends entirely upon specific 

 infection. The disease is not in any sense an air-borne contagion. 



Although unbalanced enzymatic activities and physiological toxins 

 have been considered responsible for the origin of the disease, these 

 theories do not harmonize with all the facts at hand. Although or- 

 ganisms responsible for the mosaic disease have not }^et been isolated, 

 parasitism offers the more plausible explanation of the ultimate cause 

 of the disease. 



o 



WASHINGTON : GOVERNMENT PRINTING OFFICE : 1913 



