Kesistarice :?- 



^vxcn a susceptible variety there is a regression of grovrbh to root 

 population, and the coefficient of regression is highly significant. 

 Secondly, the population build-up of Pratylenchus in the roots of the 

 tobacco plant can be altered by soil temperature. Because there 

 appeared to be a linear relationship between growth of the tobacco 

 plant and the number of Pratylenchus within the roots, and since the 

 number of Pratylenchus which entered the roots could be controlled by 

 soil temperature, it appears feasible that by using these two charac- 

 teristics we might be able to construct a growth curve which would be 

 characteristic for each tobacco variety in relation to several species 

 of Pratylenchus . Theoretically, at least, by following along these 

 growth curves, it should be possible to express the tolerance in terms 

 of the root population of a certain Pratylenchus species required to 

 reduce growth to some arbitrary level; and we have chosen a reduction 

 of ^0%. This technique is not new, of course. It is the same tech- 

 nique as one uses to evaluate the tolerance of a fungus spore to a 

 fungicide, or an insect to an insecticide. 



This technique could only be carried out under rigidly controlled 

 environmental conditions. The various temperatures of the soil must 

 be held constant, and conditions for the growth of the plant must also 

 be constant, so that the growth response will not vary from one test to 

 another. 



The actual procedure we are following, at present, is this: The various 

 species of Pratylenchus are maintained the year around in the greenhouse 

 in the roots of selected host crops. The species include Pratylenchus 

 penetrans, P. minyus, and two other single-striated populations which 

 do not fit our present keys. At the time the test is to be carried out, 

 the roots of -he host crop are chopped and mixed into the soil, which 

 is then placed in steamed five-inch pots. These pots are then placed 

 in six special Wisconsin soil temperature tanks, each maintained at a 

 separate temperature within a range of 1° F. In each tank, a control 

 is set up consisting of the identical soil treated with D-D at a rate 

 corresponding to UO Imp. gallons per acre. Seedlings of the variety to 

 be tested are planted in the soil, one plant to each pot. The plants 

 are grown under constant light of 1,000 fc for a 17 hour day during a 

 period of exactly BO days. At the end of the test, the plants are 

 weighed to the nearest l/lOO of a gram, and the weight is then expressed 

 as a per cent of the potential weight at each temperature. The potential 

 weight, of course, is that in the absence of the nematode, i.e. in the 

 D-D treated controls. The roots are incubated, using a modification of 

 Young's technique, whereby the roots are placed in pint jars and stored 

 in the dark at 65° F. for 2 months. At weekly intervals, a solution of 

 ethoxyethyl mercury chloride and streptomycin sulfate is sprayed over 

 the roots in each jar by a compressed air sprayer. This treatment 

 inhibits bacteria and fungi. The numbers of Pratylenchus which emerge 

 are counted, and the populations are standarized on the basis of the 

 numbers per gram dry weight of root. Growth-population curves are 

 then plotted on log-normal paper, and probit analyses are carried out. 

 We then establish the population required to reduce growth ^0%, and 



