EFFECT OF DILUTION UPON THE INFECTIVITY OF 

 THE VIRUS OF THE MOSAIC DISEASE OF TOBACCO 



By H. A. Allard, 



Assistant Physiologist, Tobacco and Plant- Nutrition Investigations, 

 Bureau of Plant Industry 



In order to obtain some idea concerning the effect of dilution upon the 

 infective power of the virus of the mosaic disease of tobacco in subsequent 

 inoculations, the following experiments were made. A quantity of 

 expressed sap from mosaic-diseased leaves was first passed through filter 

 paper to remove the cell tissue, etc. Clean tap water was then used to 

 bring the filtered virus to the required degree of dilution. All dilutions 

 were accurately determined and inoculations immediately made from 

 these. Young, vigorous plants growing in 3-inch pots in the greenhouse 

 were used in all tests. In order to insure a thorough test of the infectivity 

 of the diluted virus, a drop of the solution carried on the point of the 

 needle was introduced with each puncture. Every leaf of any size on 

 the plants, usually four or five, was inoculated in this manner at several 

 points. 



The plants were kept under observation for a long period after the 

 first appearance of the disease in those groups treated with the original 

 undiluted virus and the lower dilutions. This is virtually a quarantine 

 period for the disease, since experience has shown that the incubation 

 period of the mosaic disease is very uniform for simultaneous inoculations 

 under any given set of conditions. A complete tabulation of all dilution 

 experiments is given in Table I. 



Table I. 



-Effect of dilution upon the infectivity of the virus of the mosaic disease of 

 tobacco 



(=95) 



