DitylO 



cavse of onions, the visual condition is stunting of the plants which also 

 have too many short and thickened leaves. Curling of the leaves also 

 occurs, but not so much. Potato plant stems are attacked, and sometimes 

 rather often one finds stem nematodes in the main vein of the leaves 

 vdth a resulting abnormal bending of the leaves. The foliar parts of 

 carrots when attacked may stop growing and finally rot. Edtylenchus 

 dipsaci , when in potato tubers, spreads through them gradually, making 

 the tissue spongy and yellowish to brox^ in color. Only when there is 

 a very heavy attack of the whole tuber does a rot caused by fungi or 

 bacteria set in. 



This is quite different from the results of attack on potato tubers by 

 D. destructor. The infection sites, instead of being yellow or brown, 

 are whitish and surrounded by tissue where the starch grains have disap- 

 peared. The initial invasion is soon followed by secondary rots. Once 

 you have seen potatoes parasitized by D. dipsaci or D. destructo r, no 

 further difficulty will be had in distinguishing between the t-tro, as the 

 symptoms are quite different. (The symptoms of stem eelworm on cucumbers 

 are due to the fact that the middle lamellae of tissues between the vas- 

 cular bundles has been dissolved.) 



We are now approaching a discussion of what we call irregular symptoms. 

 Stem nematodes, when entering plants which are unsuitable as food for the 

 nematodes, will cause one of a series of abnormal symptoms. Of course, 

 in some cases no symptoms are produced at all. The most simple, perhaps, 

 of these irregular symptoms is necrosis instead of swelling. For example 

 consider stem nematode resistant clover attacked by the red clover race 

 or any red clover attacked by one of the other stem eelworm races. When 

 the nematodes enter the seedling the tissues show necrosis, the cells 

 dying quite soon after the entrance of the nematodes. There is no devel- 

 opment or multiplication of the nematodes in that tissue. Sometimes 

 there appears to be a swelling, but it is due mostly to stunting of the 

 center of the seedling. The cotyledons may bend down, but there is 

 nothing like a real swelling. 



Another type of symptom is found in flax. Tears ago Ritzema-Bos noted 

 that in flax there may occur symptoms which look like attack by stem 

 nematodes, however, no stem eelworms were found in the plants, at most 

 only a very few. We came across this same thing a few years ago. Again, 

 no Ditylenchus dipsaci nematodes were found in the tissues although the 

 symptoms at first sight certainly looked like the result of stem nematode 

 attack. We performed an inoculation experiment and found that these 

 symptoms did result from the presence of stem nematodes. Hox^^ever, even 

 after inoculation with a few hundred nematodes per seedling, no nematodes 

 or only very few entered the plants. So, one part of the symptom complex 

 may be stuntingj stunting that has been caused by these nematodes, per- 

 haps, while trying to enter the plant while it is still in the seedling 

 stage. The other part of the complex, the dissolution of the middle 

 lamella, does not occur; the tissue remains firm, and there is no feeding, 

 development, or multiplication of the nematodes. After some weeks, the 

 plant outgrows the initial effect of the nematodes to the top of the 



