Virulence of Populations of Potato Cyst Nematodes (Globodera Spp.) From Europe and Bolivia Towards Differential Potato Clones Frequently Used for Pathotype Classification

European populations of Globodera pallida (ten from the UK, two from Iceland, one from the Netherlands) and G. rostochiensis (nine from the UK, two from Italy and one each from Germany, Iceland and the Netherlands) and a Bolivian population of the latter species were reared on three commercial potato cultivars and five differential potato clones. Using Principal Component Analysis and the results of two Hierarchical Cluster Analyses, even populations from the same place (e.g. two from the same farm at Woburn and two from Feltwell) were shown to differ in virulence. Differences were also found between a population selected on potato clone Solanum vernei (VT n)2 62.33.3 and its unselected parent population (Cadishead). Cultivar Morag is partially resistant to G. rn.stochiensis but showed less resistance to G. pallida than reported previously. The virulence bioassay clearly distinguished the two PCN species but failed to classify populations into 'pathotypes'. The arbitrarily designated Pf/Pi value of 1.5 was not necessarily appropriate for distinguishing virulent from avirulent populations. Since virulence appears to be very much a quantitative biological trait (particularly with respect to G. pallida populations), and therefore impossible to classify in a scheme with the logical alternatives of false (does not reproduce) or true (reproduces), its value as a concept probably resides in the future design of decision making software for forecasting population development based on recorded Pf/Pi values.