Hetrr: 12 



The graph plots the initial cyst nematode nopulation levels of 

 the soil a-ainst th^ final nopulation levels. The diagonal strai-^ht 

 line represents the maintenance level, where the final populations equal 

 the initial populations. If the data are olotted in terms of locarithirriic 

 values for convenience, the effects of the plant on the populations are 

 shown by the remaining two curves. If the host is an efficient host and 

 the initial nematode population is not too great, there is a resulting 

 increase in the population, as shown by that portion of the curve to 

 the left of the maintenance level line. If the population is too high, 

 then there nay be a decrease in actual level, as shown by that portion ex- 

 tending to the right of the raaintenance level line. The curve is more 

 or less straight, though I arn not suggesting it always is so. It seems 

 there is some sort of a ceiling effect on the population level, so that 

 regardless of the initial population, the levels build up to this value. 

 I don't know that we have an explanation for this. 



The remaining curve illustrates what happens to the soil popula- 

 tion if the host is an inefficient one. At low initial population levels 

 there may be a slight increase, but at higher population levels there is 

 a decrease in nematodes to below maintenance levels. How much the de- 

 crease is varies m.th the kind of plant. 



ROOT DIFFUSATSS 



I would like to deal with one other matter which I tnink of very 

 great importance. I shall talk of some of the work of Dr, Fenwick, who 

 is really doing pioneer work on the standardization of root diffusates. 

 Eliminating a lot of the details, whicn I really out to give you, the 

 essence of his work is this. Cairying out of hatching tests is done by 

 using batches of cysts of not less tiian 100 in five-fola replications, 

 making sure the cysts are drawn from randomized plots and are thoroughly 

 mixed to assure a unifoim standard cyst product to begin with. Also 

 needed is a standard batch of root diffusate of hign potency, which is 

 kept in the refrigerator except when in use, A series of replicate cyst 

 batches are set up in the usual manner in watch glasses, diffusate is 

 added, and these cysts are incubated usually at 25°C, For example, set 

 this up as a dilution test using the diffusate at full strength, 1/10, 

 1/100, 1/1000, ana with water as a check. The results of this hatching 

 test can then be plotted (Figure U.) 



AliOUiMT OF HATQi 



point of intersection 

 expressed in logs is 

 the log Activity Value 



I/IO I//O0 



COWCENTPATION 



Figure U, Determination of log activity values for root diffusates, 



