17. 



why sometimes a reasonable crop can be grown after 

 a complete failure. 



After CHITWOOD and FELHIESSER the increase on 

 one potato crop is about 10-to 20 fold until about 

 $0,000 cysts per plant are fomed. If more larvae 

 penetrate into the roots a portion of them die be- 

 fore maturity is reached. Now in the field and 

 even in pots the larvae are not evenly distributed 

 over all root tips. Therefore even at low degrees 

 of infestation the invasion may locally be heavier 

 than the roots can bear, and the maximum reproduc- 

 tion factor is found at very low degrees of infest- 

 ation. 



OOSTENBRINK (1950) came largely to the same 

 conclusions as CHITWOOD and FELQ^ESSER, He cal- 

 culated a factor of 10 for increase and assumed that 

 this factor would be applicable from very low initial 

 degrees of infestation to where damage to the plants 

 begins to occur. This picture is too simple however. 

 At very low degrees of infestation the rate of in- 

 crease may be well over lu times also in the field 

 (DEN UUUEN 19i»'8). 



There is only one generation of the nematode 

 on a potato crop. Cysts formed in summer do not re- 

 lease larvae before the beginning of December (DEN 

 OUDEN 1958). 



Decrease of the population in the absence of a 

 host plant . In the absence of a host plant the popu- 

 iation decreases gradually. Determinations of num- 

 bers of eggs per tinit of soil show that tnis number 

 decreases, wnereas only a few dead eggs are found. 

 So, although larvae of Heterodera rostochiensis do 

 not hatch easily in water which does not contain an 

 activating substance diffusing from the roots of 

 host plants, they do gradually leave the eggs in the 

 soil. If these larvae do not find a host plant they 

 die after a short time. This is the main cause of 

 the reduction of the popxilation in the absence of 

 a host plant. 



In field experiments GOFFART found a reduction 

 of the population to 1/3 - 1/6 in two years, OOSTEN- 

 BRINK (1950) estimated it at 50$g in one year. In 

 pot experiments FENWICK (1952) found the same figure. 



