2U. 



b. Below this degree of infestation there is an in- 

 crease in summer with ail crops and a decrease in 

 winter. Increase and decrease balance each other 

 at a degree of infestation between and 5 stem 

 nematodes per 500 g. of soil. 



3» a. The mean decrease in number of stem nematodes dur- 

 ing winter on sandy soil is more than on clay soil 

 at the same initial degree of infestation, 



b. On some light sandy soils very low degrees of in- 

 festation are reached more rapidly than on more 

 loamy types of soil provided no rye is grown. 



Onions and many other host plants of the stem nematode 

 are already seriously damaged when some 5-10 stem eelworms 

 are present in 500 g. of soil. Therefore 1. a. explains why 

 on heavy clay soils there is always a serious risk of damage 

 to these crops, ■yrtiether host plants of the stem eelwonns have 

 been grown in previous years or not. 



On more sandy loams the persistence level of the stem 

 eelworms is much lower than on the heavy clay soils. There- 

 fore here onions can be grown in a suitable rotation. Once 

 the onions became moderately infested, the degree of infes- 

 tation of the soil rises to a few htmdred stem nematodes in 

 500 g, of soil and it takes several years before these have 

 died out sufficiently for safe onion growing. 



The damage in rye on sandy soil shows some relation to 

 the crop rotation. On the light sandy soil rye can be grown 

 much oftener than on loamy sands which is in accordance with 

 population data mentioned under 3«t). 



Population numbers and their fluctuations do not explain 

 ail peculiarities in attacks of crops by stem nematodes. So 

 on clay soils crops are attacked practically in all seasons. 

 On sandy soil susceptible cereals are heavily attacked in 

 winter, but only lightly or not at all in spring and summer. 

 Also voltinteer plants of rye growing where rye was attacked 

 heavily in the same year have nearly always been found healthy 

 at degrees of infestation of the soil which may have been 20 

 to 100 times as high as those causing severe damage to aut\imn 

 sown rye. 



Also the weather has been found important. A di*y spring 

 reduces the incidence of bloat in onions in Germany (nOLKE 

 1957) whereas also in the Netherlands dry springs, as the one 

 which occurred in 19U7, have this effect. 



