Plants of four potato (Solanum tuberosum L.) cultivars were grown in pots in a greenhouse at five densities of Globoderapallida between 0 and 300 eggs per gram of soil. Photosynthesis and transpiration of selected leaves were measured at 30, 37, 49 and 60 days after planting. Stem length was recorded at weekly intervals. Plants were harvested 70 days after planting and various plant variables were determined.At 30 days after planting, when second and third stage juveniles were present in roots, both photosynthesis and transpiration rates were severely reduced by G. pallida. In the course of time these effects became less pronounced. Water use efficiency was reduced by G. pallida between 30 and 49 days, but not at 60 days after planting. The results suggest independent effects of G.pallida on stomatal opening and on photosynthesis reactions. There were no consistent differences among cultivars in the response of leaf gas exchange rates and water use efficiency to nematode infection. Reduction of photosynthesis by G. pallida appeared additive to photosynthesis reduction due to leaf senescence.Total dry weight was reduced by 60 ~ at the highest G. pallida density. Weights of all plant organs were about proportionally affected. Shoot/root ratio was not affected and dry matter content was reduced. Stem length and leaf area were most strongly reduced during early stages of plant-nematode interaction. The number of leaves formed was only slightly reduced by G.pallida, but flowering was delayed or inhibited. Reduction of total dry weight correlated with reduction of both leaf area and photosynthesis rate. Leaf area reduction seems the main cause of reduction of dry matter production. Tolerance differences among cultivars were evident at 100 eggs per gram of soil only, where total dry weight of the intolerant partially resistant cv. Darwina was lower than that of the tolerant partially resistant cv. Elles and of the tolerant susceptible cv. Multa. The tolerance differences were not correlated with leaf photosynthesis and transpiration. Apparently these processes are not part of tolerance of plants.Additionallceywords: potato cyst nematodes, Solanum tuberosum L., host-parasite interaction, damage mechanism, tolerance, gas exchange, stomata, water use efficiency, total dry weight, leaf area, stem length.

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Three greenhouse experiments were carried out to examine differences in the number of cysts formed on susceptible potato cultivars, using various pathotypes and populations of Globodera rostochiensis and G.pallida . The occurrence of considerable differences in the number of cysts formed on susceptible potato cultivars was confirmed . These differences varied with different potato cyst nematode species, with different pathotypes and to some extent with different populations of the same pathotype .

In two experiments, using different testing methods, the number of newly formed cysts was determined on nine potato genotypes with resistance from various sources . Ten potato cyst nematode (PCN) populations were used in these experiments . Rank correlation between numbers of cysts over potato genotype-PCN population combinations for both experiments was high (r s = 0 .90) . Dendrograms for PCN populations and potato genotypes were constructed, based on a simultaneous hierarchical clustering procedure for potato genotype-PCN population interaction terms . Several virulence groups could be identified within Globodera rostochiensis as well as within G. pallida. Host genotypes, derived from the same sources of resistance, were clustered in different' resistance groups .

Hatching induced by root diffusate, obtained from various potato genotypes, and by standard potato root diffusate, was determined in uitro. The used potato genotypes differed considerably in tolerance to Globodera pallidu. A three parameter logistic model was used to describe the numbers of hatched juveniles in relation to time of exposure to root diffusate. Clear differences in hatching characteristics between genotypes were found. Some tolerant genotypes induced hatching of G. pallida juveniles relatively slowly, compared to intolerant genotypes. Other tolerant genotypes, however, induced hatching as fast as intolerant genotypes, and no significant correlation between hatching parameters and tolerance was found.