26 BALD 



comparison of virus concentration on the basis of lesion counts. If the 

 values of TV and K as well as values for pv or pn can be estimated from 

 the data, it is easy to calculate the relative numbers of virus particles in 

 the tested sample. However, the absolute numbers of virus particles 

 are still not calculable. Although it may be possible to estimate the 

 volume of inoculum applied to a leaf surface, it is not possible to decide 

 how much of it makes contact with the scattered susceptible regions of 

 the leaf produced by wounding during inoculation. That is, the amount 

 of inoculum applied effectively is unknown. It is further impossible to 

 estimate the proportion of the unknown number of virus particles in 

 this amount of inoculum making contact with susceptible regions of a 

 leaf, that enters and causes infection. There is at present no way of 

 estimating absolutely the number of infectious virus particles in a 

 sample of a plant virus. It is possible to discover by physical means the 

 number of standard-size particles in a sample, say of a tobacco mosaic 

 inoculum, but this is not necessarily the same as the number of infec- 

 tious virus particles. 



Price and Spencer, in a series of four papers, attempted with some 

 success to by-pass the necessity for calculating values for A^ and K. 

 They conducted local lesion trials with two or more dilutions of each 

 virus sample. This allowed them to insert a factor for slope in the de- 

 termination of the effect of dilution on the numbers of lesions produced. 

 Slope was a term for the regression of lesions (log values) on concen- 

 tration and it automatically corrected the estimate of difference in the 

 ratio of lesion counts between two virus samples. 



The discovery in recent years of methods for keeping suspended 

 virus particles dispersed rather than aggregated, offers a chance largely 

 to eliminate aggregation as a source of error in estimating virus con- 

 centration. This being so, it becomes more feasible to make use of the 

 range of virus concentration where lesions and concentration are pro- 

 portional. The addition of inert substances to the virus inoculum to 

 increase contact between the inoculum and the surface of inoculated 

 leaves will increase the number of lesions and favor the use of dilute 

 virus suspensions. Celite has recently been used by several workers for 

 this purpose (personal communication from Dr. E. M. Hutton). The 

 effect of fine carborundum powder in the inoculum, which also aids in 

 wounding leaves with a tough cuticle or smooth hairless surface, may 

 be partly of this kind. Raising the susceptibility of test plants by cul- 

 tural treatment or submitting them to reduced light for some hours 

 before inoculation may also increase the numbers of lesions. Such 

 simple modifications of technique may eventually enable workers to 

 use virus preparations at dilutions high enough to be within the range 



