15. 



Population studies of plant paragjtlc nematodes 



Quantitative research on nematode population Jias for a long time 

 been restricted to Heterodera - schachtli and H. rostochlensis* Both the 

 particular importance of crop rotation for the control of these nema- 

 todes and the relatively easy recovery of cysts from soil are the causes 

 of this situation. 



Population studies on other nematodes are of very recent date» 

 This is mainly due to the fact that no effective extraction methods 

 both as to percentage recovery and economic use of time and man power 

 were known in the past. 



Unfortunately the older work on Heterodera species is somevrtiat 

 less reliable because methods for extraction of cysts from soil were 

 not fully reliable and in both older and some of the newer work the 

 methods for estimating cyst content (number of eggs per cyst or oer 

 lanit weight or voltmie of soil) are open to criticism, 



1, Research on Heterodera species 



The close relation betvxeen intensive cropping with host 

 plants and severe damage both in beet cyst eelworm and potato 

 cyst eelworm was very suggestive of suitable crop rotation 

 as a remedy against "beet sickness" and "potato sickness"; 

 The farmers ask not only for a safe rotation but, as sugar 

 beets and potatoes are their most profitable crops, they also 

 want to follow the shortest possible safe rotation. Apart 

 from that, the idea of a rotation suitable to prevent the 

 build up of nematode populations to dangerous levels in areas 

 not yet found infested has come up in recent years, 



A, Heterodera rostochiensis 



Apparently the case is simplest in Heterodera 

 rostochiensis . Here in most cases only one host 

 plant occurs in the rotation, the potato. The 

 number of cases where also the tomato has to be 

 taken into account is small. In H, schachtli the 

 problem is complicated by the existence of more 

 host crops (cruciferous plants) and of susceptible 

 weeds. 



In the case of H. rostochiensis two or three 

 questions had to be answered before a rotation could 

 be devised? 



1, What is tne rate of increase of the nema- 

 tode when a crop of potatoes is grown? 



