when exposed to the stem nematodes from red clover. However , selection 

 based on necrosis is easier to use because if there is necrosis one can 

 be sure that the result of the inoculation test is positive. With plants 

 not showing symptoms, there is always the possibility that they are 

 merely escapes and are not resistant. Our experiments show that in our 

 inocvilation tests plants which do not show symptoms are mostly escapes. 



In rye the resistant plants only exhibit very slow development of the 

 nematodes in the tissues and dissolution of the middle lamallae, but to 

 only a low degree. On the other hand, there are resistant rye plants 

 which show necrosis, but as these are not as common in our material as 

 those shoring just the very slow development of the nematodes, we do not 

 use them in our breeding program. 



Knowledge of the host ranges of different races of the stem nematodes 

 might be of some importance for devising cultural control methods, such 

 as crop rotation. However, it is fotind that they have hardly any value 

 at all except in such cases where you know that red clover is not 

 attacked Toy the alfalfa stem nematode, or that rye is not attacked by 

 the red clover race of the nematode. Thus, you know that you can have 

 lye following red clover, or red clover following infested rye, or even 

 red clover in the infested rye without having any difficulty. However, 

 for devising crop rotations, all information of this type is almost 

 useless. That is because the occurrence of stem nematodes in all places 

 largely depends upon the soil type. We can divide the Netherlands into 

 a few regions based on soil types. Heavy soils having stem nematodes 

 are places where serious attacks of some crops occur. On the light, 

 black sandy soil we do not find the stem eelworms. light clay soils and 

 loaay sand soils are intermediate, crops being attacked under certain 

 conditions. A survey of one of the islands in the southwestern part of 

 the country showed that onions would always have a good chance of being 

 attacked ty the stem nematodes when grown on soils containing more than 

 305S clay. Even if there had been a rotation to a pasture for a long 

 time, the first year back in onions might result in a very heavy attack 

 by the nematodes. However, soils lighter than this, having less than 

 305S clay, are mostly good onion soils, and there will be nematode trouble 

 if onions are grown more than once in three or four years. Keeping the 

 rotation to onions not more frequently than this assures not having the 

 trouble. The same is more or less the case in growing rye on lighter 

 soils. The farmer can grow rye once in two years and not have much 

 trouble, but if it is grown two times in three years or three years out 

 of four, the whole field may become heavily infested. In slightly 

 heavier soils with less huimis, the situation may be that the farmer can 

 grow rye once in four years or so and still have very small patches of 

 plants which are attacked by the stem nemiatode. 



VJhat was the cause or the main reason for the fact that in some places 

 the stem nematodes Just do not disappear whatever was done and in other 

 places they were not so harmful as in the heavy soils investigated? We 

 decided to investigate the nematode population fluctuations in a number 

 of fields. We chose the following system for doing this work. It must 

 be realized that, in the cose of stem nematodes, which generally are 



