b. 



The sten and bulb nematode, Ditylenchus dipsaci 



Contrary to Heterodera species, the occurrence of the stem and 

 bulb nematode shows a strong I'eiation to soil type, whereas crop ro- 

 tation is only of importance in areas where the soil is rather un- 

 favourable to this animal. 



It is a serious pest of flower bulbs, ornamental plants, oats 

 and teazel in England, of onions, shallots, vegetable crops, orna- 

 mental plants, rye, potatoes and mangolds in the Netherlands and Ger- 

 many and of red clover, white clover and alfalfa in Denmark and Sweden. 



In a number of cases the nematode is spread by seed (red clover, 

 alfalfa, teazel) or by bulbs (flower bulbs, shallots). Then control 

 sometimes is relatively easy, as seed can be treated with methyl bromide, 

 flower bulbs and shallots by hot water treatment. 



¥here the soil is the source of infestation not very much caii be 

 done against it. On light soil a suitable crop rotation may reduce 

 the damage caused by this nematode. On heavy clay soil crop rotation 

 has very little effect. Nematicides are either too expensive or the 

 soil is unsuitable for aoplication. 



Contrary to cyst forming nematodes, where damage to plants 

 starts at a degree of infestation of the soil of 2000-5000 eggs per 

 500 g. of soil, stem eelworm causes considerable damage in many crops 

 at degrees of infestations below 10 specimens per 500 g. of soil. 



At least eleven biological races of stem eelworm occur in Elurope. 

 These all have a wide host range, whicn gives ample opportunity for 

 survival on weeds. Breeding resistant rea clover, white clover, 

 alfalfa, and rye has been successful. 



