Ite3.(Dit,yl.):4 



no multiplication of the nematodes In these resistant plants. Because 



the nematodes do cause this necrosis in the resistant plants, it is 



necessary to be careful vrith the dosage of stem nematode. If the number 



of nematodes is too high, the resistant plants may be killed. Plants 

 wliich do not show necrosis nor show normal stem, nematode disease symptoms 



are only escapes. If they are kept for a few weeks, they may later 

 develope svjellings if infection occurs later. 



It is not accepted that if a plant is more or less resistant, all of its 

 tissues have the same degree of resistance. Some tissues may orove to be 

 more or less suceptible even in resistant plants. Beets, for instance, 

 when infected with the rye stem nematode show two types of syTnptoms. In 

 beet seedlings the normal symptor- is damage to young tissues causing 

 crinkling. There are hardly any nematodes in these irrei-^lar tissues. 

 By the time the beet has developed its swollen root, there may be a heavy 

 nematode attack in the top of the root. In Holland I have never seen any 

 attack by nematodes in the beet leaves even when there have been enough 

 chances for the leaves to contract the nematode infection. I think this 

 must mean that the leaves are not susceptible, but the top of the root, 

 or beet itself, is susceptible. In England the leaves of the sugar beets 

 are infected. This may mean that the English races of the stem nematode 

 are different from ours. 



Fortunately, red clover plants which show the necrosis type of resistance 

 occur in all the red clover varieties. It is not a high percentage, but 

 it is enough to start a breeding program. Thus it is not necessary for 

 us to cross local varieties with the higlily resistant varieties from 

 other countries. To have to do that would cause difficulty in ^lurope, as 

 we vjould have to go to the Swedish varieties. These are late varieties 

 and this character of lateness in the Swedish varieties is a very persis- 

 tent character not easily rid of in br<>eding work. Now, we just take the 

 local varieties and selnct the resistant plants. 



In the work with rye plants, v;e use the absence of stem eelworm disease 

 symptoms as the characteristic for resistance. Unfortunately, in rj-e 

 there are all sorts or degrees of resistance. There is a full series, 

 ranging from highly susceptible to highly resistant^ and one has to draw 

 the line somewhere. That was one of the difficulties that people doing 

 breeding of resistance of rye in the field ran into. They could onl,\ 

 find and elirainate the heavily infected plants and go on with the rcrain- 

 ing material which contained a rather high percentage of susceptible 

 plants. The result of their work at the end of about five years was t.hat 

 they had not obtained more than 2^ percent of really resistant plants; 25 

 percent were just as susceptible as the usual susceptible vari titles; -nd 

 the remaining ^0 percent viere susceptible, but not very susceptible. ]n 

 the heavier inoculation tests on the filter paper, we could tlirow out so 

 many of these half susceptible plants that in three years of breeding; wo 

 had material which shovjod hardly any susceptibility, and we , had ovi^r ['^0 

 percent of highly resistant plants. This took only three years, boginuinr' 

 with material which consiatcnj only at most of 5/' or, perhaps, only 2 or 1 

 percent of resistatjt plants, '''hur, with tliis crop, work in the laboj-atoi'y 



