Ditylenchus destructor and Ditylenchus dipsaci are economically important plant-parasitic nematodes, affecting potato production mostly in temperate climates. Management through crop rotation is not feasible because of their wide host range. These nematodes are listed as quarantine pests in many countries. Limited information exists on the resistance and tolerance of currently cultivated potatoes to D. destructor and D. dipsaci. Two greenhouse experiments were conducted to screen 25 potato varieties for resistance to and tolerance for D. destructor and D. dipsaci infections. Reproduction factor (RF) and relative susceptibility (RS) were used to evaluate resistance, while potato tuber damage and tuber weight reduction was used to evaluate tolerance. Based on the RF, 16 varieties were evaluated as susceptible (S) while 5 varieties were evaluated as resistant (R) to D. destructor; varieties 'Innovator', 'Aveka' and 'Spunta' were resistant to D. dipsaci based on RF. 'Désirée' was observed to be highly susceptible to D. destructor and D. dipsaci in both experiments and was used as the standard susceptible control variety for the calculation of RS. A scale of 1-9 was used to classify RS of the potato varieties to D. destructor and D. dipsaci, where 9 indicated the highest level of resistance. All classes of resistance to D. destructor and D. dipsaci were observed in the potato varieties tested in the experiments. Six varieties had significantly lower RS to D. dipsaci than the standard susceptible control variety. Tolerant to highly sensitive potato varieties to both nematodes were also observed. RS and external potato tuber damage were identified as suitable methods for resistance and tolerance determination, respectively. This study provides essential information on the status of resistance and tolerance in potato varieties against D. destructor and D. dipsaci but needs to be confirmed under field conditions.

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Glasshouse experiments were conducted to evaluate the effect of initial population densities (P i ) of Ditylenchus destructor and D. dipsaci on potato tuber damage and nematode reproduction. Ditylenchus destructor did not influence tuber numbers but influenced tuber weight at high P i levels. Ditylenchus dipsaci influenced tuber numbers and weights at a P i level of 14.29 (g growing medium) −1 . Tolerance limit estimates according to the Seinhorst model were very low indicating both nematode species have a major impact on potato tuber weight. External and internal tuber rot caused by both species increased with P i levels. Ditylenchus destructor caused more tuber rot than D. dipsaci at all P i levels. Reproduction rates of D. destructor were higher at all P i levels studied compared to D. dipsaci. The equilibrium density of 1.3 and 0.6 for D. destructor and D. dipsaci, respectively, was observed at P i level of 14.29 (g growing medium) −1 .