Potato cyst nematodes: a critical review of the current pathotyping scheme1

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Abstract
The pathotype scheme proposed by Kort et al. (1977) is critically reviewed. It is proposed that many of their pathotypes are artifacts resulting from their using an arbitrary Pf/Pi value of ± 1 and differential clones with quantitatively based polygenic resistance to separate pathotypes. It is concluded that most European populations of potato cyst nematodes are heterogeneous for virulence genes and that Pf/Pi measurements are influenced too readily by several factors, including initial population density, the vigour of plant growth, and the viability of the nematode, to be used as either a measure of nematode virulence or plant resistance. Instead, reproduction should be related to that on appropriate non‐resistant or partially resistant controls and the aim should be to quantify the proportion of virulent individuals in the population. For greatest accuracy it will be necessary to use procedures which minimize genotype × environmental interactions.