Co‐evolution of potato cyst nematodes and their hosts: implications for pathotypes and resistance1

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Abstract
The co‐evolutionary process is believed to have resulted, through interaction of wild populations of potato cyst nematodes and their hosts in geological time, in the resistant hosts now utilized in plant breeding programmes and in nematode populations with genes for resistance‐breaking or virulence. It is argued that all such interactions between highly adapted, truly parasitic plant nematodes and their hosts are likely to be governed by gene‐for‐gene interrelationships. Practical implications of this hypothesis are that only pathotypes (resistance‐breaking races) defined against identified resistance genes are scientifically sound and of practical value; that, in the case of potato cyst nematodes, other pathotypes (Ro2, Ro3, Ro5 and Pa2 and Pa3) should be abandoned; and that oligogenically based resistance to potato cyst nematodes, especially important in providing resistance to Globodera pallida, is non‐durable. Working definitions of the terms ‘pathotype’, ‘host‐race’ and ‘virulence’ are provided.