22. 



2 • Research on stem nematodes, root lesion npmatodp' s , and 

 Hoplolaimus uniformis. 



The potential of multiplication of many plant parasitic 

 nematoae species is much larger than that of Heterodera species. 

 This is due to a shorter life cycle and the succession of sev- 

 eral generations in one season. The offspring of one stem 

 nematode in one year on a suitable host may be several thou- 

 sands. In the Heterodera species discussed above the reproduc- 

 tion potential is of the same order of magnitude as the actual 

 population increase measured in the field (the latter gener- 

 ally is not lower than 1/2 to i/h of trie first) . In stem 

 nematodes, root lesion nematodes, and Hoplolaimus uniformis 

 the last value often is only a small fraction of the first one. 

 A high death rate of the nematodes in the soil and reduction 

 of tne activity of tne nematodes while in the soil are the 

 main causes of this. In these species population dynamics 

 can only be studied in the fields wnere they occur naturally, 

 otherwise the results m.ight not be applicable to agricultural 

 practise at all. Tiieiefore, for his population studies on 

 the species mentioned in the title of this lecture, the author 

 did not follow the practices of research on Heterodera species: 

 pot experiments, microplots and a small number of field experi- 

 ments with a large number of host and non-host crops. Instead 

 of that, a large number of areas about 3 ^^ in size, were 

 chosen on different fields and here population nvmibers were 

 measured mostly two times a year for several years in succes- 

 sion. The advantage of this method is that the normal farm- 

 ing practices are followed on the investigated fields, and 

 that as many soil types can be included in the investigations 

 as made possible by the capacity of investigation of the 

 samples. A weak point is that no influence can be exerted 

 on the crops grown, but if on the other hand the experiments 

 are done during a sufficient number of years many crops will 

 have been grown at least a few tjjr.es in most of the fields. 

 So, at least, the influence of the main crops on the population 

 can be measured adequately. This can be improved if this 

 method is combined with classical field plot experiments. 



