Heter: 8 



I have vrorking with me at Kothamsted a man who followed me from 

 Cambridge named VJallace. He is concentrating particularly on studies 

 of the various factors in hatching, emergence, and movement in the soil, 

 but hasn't yet gone on to effects of invasion. Personally, I think he 

 is doing some really fine pioneering work using techniques borrowed 

 from soil physics. Perhaps, there may be opportunity to say more about 

 that later; I regret I do not have the time right now. 



POPULATION STUDIES; Pathogenicity 



I would like to pass on to the effects of host populations and 

 initial He terodera populations on yield. This is being studied at two 

 or three different places using pots and microplots. In studies with H. 

 cruciferae and H. schachtii, and in the U.S.A. with H, tobacum , the con- 

 census of opinion is that low inoculum levels can increase plant yields 

 in pot experiments using sterilized soil. In one experiment which I did, 

 the difference was significant. One therefore supposes there can be a 

 stimulating effect from low population levels. As the nematode popiila- 

 tion levels rise, yield begins to decline in a manner that can be plotted, 

 Figure 1 shows this on a logarithmic basis and on an arithmetic scale. 



YIELD 

 lbs/pot, 

 stem length, 

 etc. 



a significant increase in yield 

 Logarithmic scale 



nemas I 



log IiMlTIAL iMliMATaDE POPULATION 



YIELD 



Linear scale 



INITIAL NF34ATO]")E POPULATION 



Fir^ure 1, Yield in relation to initial nematode population. 



