19. 



ass-umed that 6-lU years should elapse between two 

 beet crops in case of beet sickness. As it is im- 

 possible to predict tne incidence of beet sickness 

 so far ahead, no fixed rotation is followed in the 

 Netherlands, Instead of tnat, the farmer can have 

 his soil investigated by the Institute for Sugar 

 Beet Research at Bergen op Zoom and get advice as 

 to whether or not it is suitable for growing sugar 

 beets. 



Danger Level . HELLINGA (19U2) found that re- 

 duction of yield could be expected at degrees of 

 infestation of 10 cysts or more per 75 grams of 

 soil. This figure agrees fairly well with the 

 danger level found by JONES (19U5) which is 10 

 viable eggs per gram of soil, 



Distinguisning Heterodera Species . As this 

 advisory work is based entirely upon cyst and egg 

 counts, it IS necessary to distinguish the differ- 

 ent Heterodera species by their cyst and larval 

 characters. In Western Europe it is important to 

 distinguish the beet cyst eelworm from H. major , 

 H. schachtii, H. trifolii and H. galeopsidis . 

 UlJum (HIJNril, OOSTSWBRINK and DEti OUDEI-I 1953) 

 worked out larval characters and (OOSTENriKINK and 

 DEN GUDEN 19bh) cyst characteristics, mainly based 

 on differences between the vulvar regions of the 

 different species. COOPER (1956) extended this 

 work and included more species. 



Top Level of Infestation . Experiments by 

 JONES showed that with different host plants there 

 is a top level to whicn population numbers can in- 

 crease. Generally such a top level is explained 

 as being the maximim number of nematodes which can 

 feed on the plant without serious reduction of the 

 root system. This could be the case in beets. It 

 is not necessarily due to damage also affecting 

 the above ground parts. JOwES (1956) found that 

 with cabbage the top level of tne nematode popu- 

 lation was reached without any signs of damage to 

 the plant. This agrees very well witn the impression 

 that cabbage, although heavily attacked by H. 

 schachtii , is not damaged in the field. It~is dam- 

 aged though by H. cruciferae . 



Catch Crops . idJHN (1882) tried to catch and 

 remove the larvae from the soil by growing a sus- 



