Dit7/1:6 



rather irrepulsrly spread throijrh the field in larf:e or small pstches, 

 it is difficult to find a good site for field ex}-ierir.ients. Moreover, I 

 think it is pretty clear that there is the iafluence of soil types. 

 Thus, we had to cover a large area to be siu'e to include at least a good 

 part of the possibilities^ we could not reach any conclusions en just a 

 fei^ fields. So instead of laying out experiments on a fex-r fields in.th 

 many different crops, we looked for infested spots on some thirty fields 

 and investigated populations in these spots two times a year on very 

 small areas. In the case of rye fields, the infested patches are always 

 more or less elongated with nothing at all in the area around. On the 

 heavier soils, if there \-i3.s an attack on onions, looking around one 

 finds infected onions spread throughout the field. This is in contrast 

 to the sharper boundaries of infestations in rye plantings. We used 

 sighting reference points to establish the location of the spots in the 

 fields and for refinding these spots on subsequent collecting trips. 

 Measuring to find all the spots would have talc en too long. 



We were interested in the nematode population fluctuations from autumn 

 to spring and from spring to autunin. To reduce the variation of counts 

 due to irregular distribution, we sampled a small area vrith a diameter 

 of about ten feet. Of course, what we found on such a small area would 

 not hold for the whole area, but by taking a few areas like this, one 

 gets a pretty good impression of v:hat is happening in the field. Had 

 we taken only a single sample, the variation would have been so large 

 that we could have never reached any conclusions. Novr, after having 

 done the sampling for a number of years on different fields and differ- 

 ent soils at different times, we compared the differences between the 

 degree of infestation of the stem nematodes in autumn and in spring with 

 the idea that if there is no host crop on the field, the stem nematodes, 

 being obligate parasites, would die. Thus, by measuring the differences 

 in populations between autumn and spring we might get an indication of 

 mortality in different soils when there is no crop present. On the 

 Other hand, we could compare the degree of infestation in the spring 

 with that in the autumn and so measure the influence of the different 

 Crops, 



V7e found that, generally, the reduction of the nematodes in the autumn, 

 •when there were over 100 stem nematodes per 500 grartis of soil, was three 

 to one to the spring counts. That is, the number in the spring are 

 about one-third those in the autiimn, if the numbers in the autumn were 

 high (above 100) j this is on light soils. On sandy soils, the reduction 

 ranged from ten to one to six to one. There is quite a lot of variation 

 because the light soils include different soil types. It was found that 

 when the degree of infestation in heavy soil in the autujnn was distinctly 

 below 100 per 500 grams of soil, there xras a good chance that there v;as 

 no further decrease. At least, we can say that in he?vy soils the 

 degree of infestation of a spot seems not to go down below ten stem 

 nematodes per 500 grams of soil x/hatcver happens on that soil. Mostly, 

 it remains soi.iCTjhere between twenty to sixty stem nematodes per 500 

 grams of soil. 



On the lir-ht r>oils tJip number of the nrm.'itodcs continues to decrease 



