In field experiments carried out in 3 years with and without fumigation and irrigation 4 cultivars of potato differed in their tolerance of cyst nematodes (Globoderapallida). Ground cover, tuber and foliage fresh and dry yields were determined at regular intervals. From these measurements, intercepted radiation, radiation use efficiency for production of total and tuber dry matter and the apparent amount of intercepted radiation at tuber initiation were calculated and compared for all the treatments. The results indicate that cultivars which best maintained ground cover in the absence of nematodes also maintain it best in the presence of nematodes. Such cultivars gave the highest yields at all levels of nematode infestation because the radiation use efficiencies were far less affected by nematodes than was intercepted radiation. The similarity of the responses to nematode infection and to drought and the absence of interaction between these two stress factors facilitated crop simulation modelling. Th e relationships found here may well be wider used, though they will have to be validated for other cultivars and growing conditions.

SummaryIn three seasons, 21 potato cultivars, ranging in maturity from very late to mid-early, were grown on sandy and sandy peat soils naturally infested with potato cyst nematodes. The levels of infestation ranged from very low to very high. Between growing seasons, weather conditions differed greatly from very dry and hot to rather cool and wet.The data obtained were used to establish the effect of different growing seasons on the initial nematode density -tuber yield relationship and to evaluate the fit of the several equations describing this relationship. Over the three experimental years, the equations elaborated by Oostenbrink (1966) and Elston et al. (1991) described this relationship with the most accuracy and consistency.There were high year effects on yield, but as tolerance was not affected by the yearly fluctuations, the results suggest that testing for tolerance can be adequately done in any single year.

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SummaryGreenhouse experiments on the effects of white potato cyst nematode infestations (Globodera pallida Stone) on initial growth and development of a series of potato cultivars are compared with the results of field experiments on sandy and sandy-peat soils on the effect of nematode density on tuber yield. A simple greenhouse test, assessing root growth response to potato cyst nematode infection, provided a good insight into a cultivar's tolerance performance in the field early in the growing season. As a very limited number of plants is needed for the greenhouse test, screening for tolerance can be conducted in the early stages of a breeding programme.

SummaryData from five long-term field experiments on sandy and sandy peat soils were analyzed for the effect of soil type on damage caused by potato cyst nematodes to potato crops. It was shown that the water retention of the topsoil and the subsoil largely determined the potential yield level in the different fields. According to Oostenbrink's equation the slopes of the curves are steeper with increasing yield level, indicating that the higher the potential yield level, the higher the absolute yield loss. However, when yield was expressed as percentage of maximum yield, yield reduction related to potato cyst nematode density did not differ significantly, except when tolerant cultivars were grown. Increasing tolerance of cultivars to nematode damage was reflected in less steep slopes of the yield/nematode density curves.