Dityl:! 



DITYLENCHUS 

 Races, Pathogenicity, and Ecology 



J. W. Seinhorst 



The attacks of this nematode on various plajits have been known since 

 the 17th centuiy. Unfortiinately, in the earlier works iidth the stem 

 nematode new species were made on the basis of new hosts in which the 

 nematodes were found iidthout careful study of the nematodes' iiiorj.:iholog3'- . 

 About 1880 Ritzema-Bos studied the morphology of the stem nematode from 

 several of these different hosts and found that morphologically the 

 nematodes were all the same. So he brought them together ujider the 

 name Tylenchus deva statr ix. At the same time, however, he noticed that 

 biologically these nematodes were not the same. He found that he could 

 not always successfully transfer the stem nematodes of one kind of plant 

 to another kind vrhich was also known to be a host of the stem nematode 

 and which was morphologically like the other population. However, 

 because he was thinking along the lines of adaptation theories he never 

 becajne quite clear about the situation in this species. 



Another worker did a good piece of work but unfortunately held to the 

 wrong theory about the biological races. He thought that if you only 

 tried hard enough, eventually you would be able to transfer sxij stem 

 eelworm from any host plant to any other host plant. In Holland, we 

 have worked with stem nematodes in oats and in red clover and, after a 

 number of years of ejqjerimentation, are convinced that the stem nematode 

 of clover will not attack oats. VJe are also of about the same opinion 

 concerning stem nematodes from hyacinths and from daffodils. That is, 

 one cannot transfer stem nematodes from hyacinths to daffodils or from 

 daffodils to hyacinths. In the clovers there is exactly the same 

 situation* Stem nematodes from red clover do not attack white clover 

 or alfalfa. Those from white clover do not attack red clover, or at 

 least only to a very small extent, and those from alfalfa do not 

 attack white clover and red clover. 



This situation in which there are different forms of the nematodes 

 which could not be distinguished, or at least was thought to be 

 indistinguishable on morphological grounds, has existed in other 

 genera, too. For example, this has been the case in the Hete rodera 

 and root-knot nematodes. In some of these cases it has been found, 

 after some investigation, that distinctions could be made on morpho- 

 logical grounds. The same has been tried for Pi tylenchus dipsaci and 

 so far the only result has been to split off a form called Pi tylenchus 

 destructor which had been considered as P. d ipsa ci. Tliis nematode not 

 only differs morphologically from P. d ipsaci , but differs also in the 

 type of symptoms it causes in potaTo tubers and in other plants which 

 are quite different than those produced by the stem nematode. 



The actual stem nematode, P. di psaci , has not yet been split into 

 different species on morphological grounds, as far ns I cvn determine. 



