23. 



3. Population dynamics of the stem nematode 



The importance of soil type for tne behaviour of stem 

 nematodes has been notices by several authors (DEWEZ I9U0, 

 VAN BEEKQ^l 19hO, MIGON and hlTTEK, 19U7, SEIwHURST 1930, 1956). 

 Their observations can be siunmed up as follows: 



1. Some regions are infested with stem nematode (clay 

 soils, loamy sand and lignt sand with low humus 

 content, others are not (sandy soils rich in organic 

 material) irrespective of crop rotation. 



2. In the infested areas the eelworm is more persistent 

 on the heavy than on the lignt soils. 



In the south western part of the Netherlands it is well 

 known by the famers tnat onions on soils containing more than 

 30% of clay always run a risk of being attacked and tnat crop 

 rotation seems to be of no importance. On lighter soil onions 

 can be grown, however, once every four years. A shorter ro- 

 tation carries tne risk of stem nematode attack again. 



Population studies on more than 30 fields in different 

 areas showed the relation between population dynamics and 

 the above observations (SEIWHORST 1956, 1957), 



Growing host plants appeared to be relatively unimportant 

 for population fluctuations. Most host crops have no other 

 effect on the population tnan non-hosts. Onions and rye may 

 cause a considerable increase of the number of stem nematodes 

 in tne soil on all soils infested witn the onion race or the 

 rye race. Occasionally also peas cause a marked increase in 

 number of the onion race. V/ith other crops, hosts or non- 

 hosts the fluctuations show the same trends; 



1. a. On clay soils there is nearly always an increase 



of population numbers during summer and a decrease 

 during winter except when the degree of infestation 

 is very high, 



b. On heavy clay soil increase in summer balances de- 

 crease in winter at degrees of infestation of 

 10-50 stem nematodes per 500 g. of soil. Higher 

 degrees of infestation nearly always decrease more 

 in winter than they increase in summer. 



2, a. On sandy soils generally degrees of infestation 



over 10 stem nematodes in ^uO g, of soil decrease 

 botn in summer and winter if no onions, rye or 

 oats are grown. 



