DISEASES DUE TO NEMATODES (EELWORMS) 63 



Satisfactory control of Heterodera is obtainable where the 

 soil can be heat-sterilised, as in small plots and seed beds. Control 

 by chemical agents has not been found to be feasible on any 

 adequate scale. The system for which the best results have been 

 claimed is that of trap crops, in which highly susceptible plants 

 are thickly sown in infested ground to attract the larvae and 

 pulled or ploughed out before these can reach maturity. Starving 

 out the worms by a complete fallow or the planting of an immune 

 crop is another well-recommended measure. Carbon bisulphide 

 has its advocates in dealing with permanent crops but it is 

 difficult to conceive of more than palliative effects resulting from 

 this treatment of an organism so well protected during much of 

 its life history by the tissues of the host. The worm has been 

 shown to have little power of resistance to desiccation and heat, 

 which perhaps accounts for its absence as a serious pest of arable 

 land in these islands. 



Tylenchus devastatrix, Kuhn. 



The eelworms of the genus Tylenchus, which includes many 

 serious plant pests, retain the normal worm-like form in both 

 sexes throughout the life cycle. 



T. devastatrix affords an example of a species which infests 

 tissues above ground and affects from within the development 

 of the organs of the plant. It causes dwarfing, crumpling and 

 other deformation of leaves and abortion of ears in rye and oats, 

 ring disease of the bulbs and yellow leaf-spots on hyacinths, a 

 seedling disease of onions resulting in abnormal forms, bud 

 proliferation and reduction in leaf size in clover, misshapen spots 

 on potato tubers, and the drying up of flowers in the teasel. 



The worms penetrate seedlings or young plants from the soil 

 and ascend to various locations in the stem by way of the 

 parenchyma. Here they breed, and are returned in the course 

 of time to the soil when the plants die down. 



Tylenchus tritici. 



The wheat eelworm similarly enters into the seedlings, ascends 

 the stem (where its presence gives rise to stunting and the curling 

 of leaves), and eventually bores into the ovaries, which develop 

 into gall-like bodies resembling somewhat the normal grains 

 among which they occur. The eggs are laid in this situation, 

 and each gall when ripe contains some 500-600 larvae, which after 

 hatching enter upon a resting stage. It was from this condition 

 that the frequently quoted example of revival after 27 years of 

 desiccation occurred. The worms resume activity when the 

 gall, in company with the grain, is resown, and make their way 

 through the soil to the seedlings of the new generation. 



Tylenchus angustus, Butler, 



The rice eelworm is one of the examples referred to above of 

 ectoparasitic nematodes. It is the cause of the destructive 



