A short review of differentiation into pathotypes is given. Use of the word ≪ pathotype ≫ is recommended when a very clear difference is established between virulence of nematode populations. Our present knowledge makes it possible to differentiate between 10 pathotypes of Heterodera avenae. It is suggested that the pathotypes are given numbers, and corresponding terms should be used for genes for resistance in plants, e.g. a gene Hal on the barley chromosome gives resistance to nematode pathotypes 11,21,31,41 etc. Some proposals for improvement of pathotype identification are given.

A close linkage is shown in barley between a gene for resistance to race 1 and 2 of Heterodera avenae (Ha2) and lack of purple colour at nodes, ligules and awns (pau). A number of recombination values between genes are given and a likely order is suggested for the following genes: Ha2, pau, cer gl0, v and e, and the translocation break T,-,f. A survey is given of the varieties used and the problems encountered are briefly outlined. The importance for plant breeding is discussed.

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Differentiation of barley plants resistant to Heterodera avenae is simple when they are completely without cysts and the number of cysts is high on a susceptible control cultivar. Problems with classification are discussed.