Resistance j3 



the standard deviation of this figure is calculated. Theoretically, 

 at least, we have reached the stage where we can make a critical coni- 

 parison of the tolerance of this variety with that of some other. 



I would like to emphasize, at this time, that there is still consid- 

 erable doubt as to whether this technique will work at all. We have 

 been able to get the standard deviations under control, although we 

 are getting a very good fit of the points to the line. We hope we can 

 reduce the standard deviations by some further refinements in our 

 techniques, which we will try this fall. However, there is a possibility 

 that the soil temperature is having an effect upon the dosage response 

 curve and that we are not getting true linearity. In any event, we 

 have only worked with this technique for a year, and that has not been 

 sufficient to eliminate all of the difficulties. Thus far, we have 

 tested three varieties of burley tobacco and five varieties of flue 

 tobacco against four populations of Pratylenchus . 



This work may appear to be quite academic, and we all realize it can 

 only be carried out where rather elaborate facilities are available. 

 I am the first to admit that the method may never have any practical 

 value. On the other hand, if one ever can define the relationship 

 between growth and the nematode with mathematical precision, one has 

 uncovered a very valuable research tool. We will actually be measur- 

 ing the interaction between the two organisms, and, therefore, one 

 could study the reaction of either organism by holding the other con- 

 stant. For instance, by testing different strains of the host against 

 one Pratylenchus species, one can determine variations in the tolerance 

 of these plant strains which, basically, is the reason for this work. 

 This, of course, should be of great benefit to the plant breeder. On 

 the other hand, if one were to test different populations of the nema- 

 tode against one variety of the plant, one should detect variations in 

 the pathogenicity of the nematode which, of course, reflect difference; 

 in the physiology of the two populations. A knowledge that such 

 physiological differences occur should be of benefit to the taxonomist 

 in his search for morphological differences between the populations. 



