27. 



potential of P. pratensis on cereals would not be 

 higher than 1 to 3 or U. Pot experiments with the 

 closely related P. penetrans showed that here an in- 

 crease of 1 to 20 was possible in about six weeks 

 and 1-200 in less tnan a year. So it must be con- 

 cluded that there are factors in the environment, 

 most probably in the soil, which keep tne nematode 

 population in check. 



The economical importance of P. pratensis has 

 so far not been demonstrated by reliable experiments. 

 However, as seems to be tne rule rather than the ex- 

 ception with plant nematodes, this species will, even 

 when crop rotation is most favorable, only reach dan- 

 gerous levels in part of its area of distribution. 

 It is therefore misleading to generalize from obser- 

 vations on experiments which are done on fielas where 

 damage occurs. Tnese fields may just as well be the 

 only ones in existence. Population research must 

 be done over a much wider range of fields including 

 those where less favorable conditions for tne nema- 

 todes might prevail. 



B, Pratylenchus penetrans 



Although this nematode is much more important 

 than P. pratensis no extensive studies on population 

 fluctuations have been done. OOSTEWBRINK (I9i>6) did 

 some experiments on fields which showed excessive 

 degrees of infestation and found that cereals were 

 good hosts of the nematode although they did not suf- 

 fer. Mangolds were poor hosts. As these high degrees 

 of infestation only occur in very small areas it is 

 more important to have information about the persis- 

 tence and increase of P. penetrans under more normal 

 conditions, especially so because some plants (e.g. 

 daffodils) already suffer at low initial degrees of 

 infestation of the soil. In some areas the nematode 

 seeins to be very persistent at low degrees of infes- 

 tation. 



