26. 



U» Population studies on root lesion nematod es 



The importance of soil type for the occurrence of root 

 lesion nematodes has already been mentioned. 



OOSTENBRIwK (1936) investigated population fluctuation 

 of Pratylenchus pratensis and P. penetrans in field experi- 

 ments and stresses tne importance of population build up on 

 host plants like cereals and consequently advises crop ro- 

 tation as a general practice on sandy soils. From the author's 

 investigations the conclusion may be drawn that the situation 

 is more complicated for both nematodes and because of tnat, 

 partly, not alarming. 



A. Pratylenchus pratensis 



Although this nematode is probably of little 

 economic importance its widespread occurrence made 

 it a good subject for population studies. These 

 ,' were done by the author in the same way and largely 



on the same fields as the work on stem nematodes. 



Importance of host crops . Although cereals 

 are hosts of tne nematodes they did not cause a 

 large increase in number in any of tne •^ields in- 

 vestigated. Their effect (comparing autumn degrees 

 of infestation) ranged from a distinct decrease to 

 a 2 fold increase of nematode numbers. The effect 

 of non-hosts like potatoes and mangolds ranged from 

 a marked decrease to a slight increase. In some of 

 the fields even continuous cropping with cereals for 

 three years did not increase the population, which 

 in all these cases (on light sandy soil) fluctuated 

 between 50 and 200 specimens per 500 g. of soil. 

 In other fields there was a fluctuation of about the 

 same magnitude but at higher mean levels. 



Occasional observations on other fields have 

 revealed that wider fluctuations both in increase 

 in summer and decrease during winter may occur. On 

 the other hand a sampling of ten small areas in each 

 of ten fields in one area revealed that the means of 

 the ten counts in each field showed less variation 

 than the separate counts in each field. As these 

 fields had only in common that they were destinod to 

 carry peas tnis is not very suggestive of a major im- 

 portance of crop rotation for levels of infestation 

 with P. pratensis in this area. 



It is not very probable that the multiplication 



