Dityl:7 



until a level of below five nematodes in ^00 grains of soil is reached, 

 if only one does not grow onions, rye, or oats. This is a very impor- 

 tant difference. We found that when growing onions, rye, celery and 

 even potatoes, if the degree of infestation is over ten stem nematodes 

 per ^00 grams of soil, there is a good chance of serious damage to these 

 plants. That explains why onions grown on the heavy soil always have a 

 good chance of being attacked. 



Ten nematodes per 500 grams of soil so\inds like an unbelievably small 

 munber to produce serious damage, but it is a fact. Most likely, the 

 nematodes keep alive on weeds, although most of these onion soils are 

 rather weed free. Another point is that on heavy soils the stem nema- 

 tode population does not go down below ten in the winter time. VJhatever 

 the crop grown in the summer, susceptible or not, it does not matter, 

 the stem nematode population has a general tendency to rise. 



We have the impression that the popixLation declines on different soils 

 are not orQy different between the light and heavy soils but also show 

 differences in the various kinds of light soils. This accounts for the 

 possibility of growing rye once in two years on the very light soils. 

 In these two years, the stem nematode population dxops down from about 

 two hundred to below ten, whereas this decline takes a longer time on 

 the slightly heavier soils. Of course, all these studies do not help 

 the farmers very much. They are only very sure now that they cannot 

 grow onions on the heavy soils. Whatever they do, whatever rotation is 

 used, it is impossible to have a good crop of onions in most years on 

 these heavy soils if they are infested with the stem nematode^ and it 

 seems that all of our heavy clay soils are infested. 



The next step we are trying forhorticultTiral areas which are in this 

 same condition is to completely eliminate the weeds and see if this 

 will improve the situation. However, there are difficulties both in 

 the expense of complete weed control and in the detection with accuracy 

 of population numbers less than ten per 500 grams of soil in reasonable 

 numbers of samples. At any rate, all this illustrates how much different 

 the situations are between Ditylenchus dipsaci and, for example, 

 Heterodera , where crop rotation is one of the answers for getting con- 

 trol. With Heterodera , the situation seems as though it can be simply 

 put like this: If a host crop is grown, there is an increase in the 

 nematodes; eliminate the host crops, and there is a very good chance 

 that tcLthin a certain period of years the degree of infestation goes 

 down below temporarily to a level which is dangerous to the plant. 

 Obviously, this is not the case with all kinds of nematodes. It is 

 likely that the situation with Heterodera is more the exception than 

 the rule. The nematode populations in the soil are influenced by crop 

 rotations, of course, but this influence is limited. There are these 

 loiiT infestation levels below which the populations may not drop. 



