PLANT VIRUS ASSAY 25 



between numbers of infections and virus concentration is that an in- 

 fection cannot be caused by a single virus particle, but that more than 

 one is needed to cause each infection. Lauffer and Price have shown 

 that lesion- virus concentration curves cannot be accurately fitted on this 

 assumption. 



Equation No. i above is essentially the same as the equation that 

 has been used to describe the relation between concentration of an 

 animal virus and the number of lesions on inoculated egg membranes, 

 with this difference. Animal viruses may cause infections at extreme 

 dilution, where numbers of lesions are approximately proportional to 

 concentration. Plant viruses produce countable numbers of lesions only 

 at much higher concentrations because so few virus particles of those 

 applied enter the host plant and cause infection. Plant virus concen- 

 tration and numbers of lesions are generally not proportional. The part 

 of the dilution curve over which measurements are often taken has a 

 changing slope. The relationship between numbers of lesions and con- 

 centration of unknow^n samples of a virus that one may wish to com- 

 pare cannot then be predicted. There is the additional possibility of 

 complications due to aggregation of virus particles, which adds to the 

 uncertainty of deductions from lesion counts that one virus sample is 

 more concentrated than another. Strictly speaking, it has seldom been 

 possible to assume wdthout further evidence that one sample of virus is 

 more concentrated than another because it produces more local lesions. 

 In practice this assumption is often made, and may often be correct. If 

 there is a wide difference in numbers of local lesions, its correctness is 

 very probable. 



This uncertainty rests on the difficulty of obtaining an estimate of 

 A^, the maximum possible number of lesions, i.e., the number of entry 

 points or susceptible regions on the inoculated leaves. If A^ can be de- 

 termined and is the same for both samples it seems reasonable to assume 

 that the sample of virus producing more lesions contains more infectious 

 units. However, if the virus under test is subject to aggregation of its 

 suspended particles, the infectious units in the two samples may repre- 

 sent different degrees of aggregation, and the sample producing more 

 lesions may not contain the greater number of infectious virus particles. 

 Obtaining a true estimate of relative virus concentration in aggregated 

 samples would involve the solution of equation 3 above for each sample. 

 To make direct comparison of two samples on the basis of lesion pro- 

 duction, the constant K, which indicates the degree of concentration, 

 should be zero (no aggregation) or have approximately the same value 

 for the different samples. 



The equality of the two constants A^ and K is the basis for a direct 



