BY

a new scheme is proposed for naming pathotypes of potato cyst-nematodes. Pathotypes of Globodera ( = Heterodera) rostochiensis and G. pallida are designated separately using a simple nomenclature Ro1 to Ron for G. rostochiensis and Pa1 to Pan for G. pallida. Currently seven clones are used to differentiate five pathotypes of G. rostochiensis and three of G. pallida. Several of the pathotypes recognised in the British, Dutch and German National Schemes are the same. Results of pathotyping tests may show considerable variation; adequate replication and standardisation of method are essential. With an internationally used scheme it is highly desirable that new pathotypes or differential clones are added only after testing at Institutes in several countries. After resistance to potato cyst-nematodes was found in European collections of Andean potatoes and included in potato breeding programmes, populations of potato cyst-nematode were found which multiplied on otherwise resistant clones. As a result two systems of identifying and naming these populations were adopted, one in the United Kingdom and one in the Netherlands. Three kinds of popu-lation have been recognised in the United Kingdom, pathotypes A, B and E, an