Tolerance of the potato to stress, associated with potato cyst nematodes, drought and pH : an ecophysiological approach

The impact of Potato Cyst Nematode (PCN) soil infestations on the growth and development of potato cultivars was investigated in greenhouse and field trials. PCN effect on yield was almost totally explained by reduced integrated ground cover, and hence, less light interception. There was no indication for any specific PCN effect other than reduced ground cover. The damaging effect of PCN was also studied in combination with soil type, drought and soil pH. In addition to the considerable effects of the main factors PCN, drought and pH, interaction between PCN and soil pH was found, but none was found between PCN infection and drought. However, it should be considered that the last mentioned events occurred separately in time.With respect to the yield capacity of potato cultivars the following parameters appeared to be of major importance: PCN infestation level, water retention and pH of the soil. With soil pH _KCl values over 5.0, the vulnerability of the potato to PCN infection increases with decreasing acidity. The cultivar characteristics, resistance to PCN and tolerance to PCN, and the characteristics tolerance to PCN and tolerance to drought, were not correlated.In nematology, tolerance to PCN is presented as percentage yield under PCN infestation. Quantifying the yielding capacity by means of tuber production, large numbers of plants are needed for accurate assessment and hence can not be done in early stages of breedingprogrammes. Therefore, a screening method for tolerance to PCN was developed, allowing screening for this character to take place in a greenhouse at an early stage of a breeding programme. Also, integrated ground cover can be assessed with few plants, and offers a tool to assess tolerance under field conditions for breeding purposes.Four descriptive equations, relating PCN infestation and yield, were studied. Based on experimental data over a number of years, Oostenbrink's equation described this relation simply, accurately and consistently. A decision-making model was developed to assist farmers in making the best cultivar choice, with respect to yield, based on prevailing soil factors (PCN infestation, water retention of top and subsoil, and pH) and cultivar characteristics. The same model is able to predict final yield with increasing precision with time during the current growing season. For this purpose Tolerance Values of cultivars for these stress factors were introduced. A procedure for estimation of tuber number and tuber dry matter yield is discussed, based on their relationships with intercepted radiation in the course of the season. Such estimates are useful to seed growers in order to set harvest dates and to the potato industry to obtain yield estimates in order to adapt logistics, storing and marketing.