18. 



OOSTEi^iRRINK'S figure is a mean caiculated from 

 figures which vary between reductions of less thrri 

 20% to more tnan 80%. Part of tnxs variation may- 

 be due to experimental errors. However, there is 

 a good chance that the actual decreases will differ 

 in different places. No data are known about the 

 reduction in the third and further years. Moreover, 

 these numbers may be different under different cli- 

 matic conditions. Thus crop rotations can only be 

 based on these figures tentatively. 



In the Netherlands the legally enforced rota- 

 tions with not more than one year of potatoes in 

 3 years is an optimistic interpretation of the 

 practical effect of a 10 fold increase of the nema- 

 tode on a crop of potatoes and a decrease of 50^ 

 in the absence of a host. It will at least slow 

 down the increase of the heavily infested area in 

 regions where before this compulsory rotation more 

 than 50% of the area might be under potatoes. 



In Germany 3 or U coxirse rotations have been 

 advised to the fanner or have been compulsory since 

 1933. On heavily infested land this did not pre- 

 vent damage. In England 3 to 6 years should elapse 

 between two potato crops on infested land to be 

 reasonably sure that no serious damage will occur. 

 GRAH^JGER even found a definite increase of the popu- 

 lation in these rotations. 



It is generally assumed, that the iarvae ai'e 

 released from the eggs gradually in the abaence of 

 a host crop. There is some evidence that this is 

 not the case, but that most of them hatch in a rather 

 short period in late spring (DEN GUDEN 1957). 



B. Heterodera schachtll 



In tne beet eelwoim, population dynamics in in- 

 fested fields are complicated by the occurrence of 

 more than one generation of the nematode (in the 

 case of H. rostochiensis one generation) and the 

 existence of a number of host plants, both field 

 crops and weeds. 



On one beet crop the population may increase 

 up to UO fold. The reduction in the absence of a 

 host is roughly estimated at 50%. It is generally 



