Research Institute for Agrobiology and Soil Fertility (AB-DLO), PO Box 14, 6700 AA Wageningen, The Netherlands

Background and objectives
Yield is determined by intercepted solar radiation, its efficient conversion into dry matter and the proportion of dry matter that is allocated to the tuber or seed. Potato cyst nematodes reduce both light interception and conversion efficiency. Damage mechanisms have given rise to several hypotheses. Nematodes reduce photosynthesis through either increased abscisic acid levels and closed stomata, or they reduce root growth leading to drought, wilting and closed stomata. Reduced concentrations of nutrients in the leaves, especially of phosphorus and potassium also contributes to reduced assimilate production. Supplemental nutrient application reduces part of the damage caused by nematodes. Increased nitrogen, phosphorus and potassium and reduced calcium applications have been shown to reduce the effect of potato cyst nematodes. Apparently, potato cyst nematodes affect nutrient uptake by the crop. This paper shows how nutrient management reduces the deleterious effect of potato cyst nematodes on crop growth and the uptake of which nutrient is affected most by them. The mechanism of yield reduction through nutrients is discussed in terms of yield-determining factors. Potato cyst nematode infection has been shown to reduce foliar concentrations of certain nutrients (e.g. N, P and K) [1] and to increase the concentration (hence uptake) of other minerals such as Ca [2]. Supplemental fertilizer application, notably of N and P has been shown to reduce damage caused by potato cyst nematodes [1]. Reduction of application of other elements, notably Ca, leading to reduced pH levels of the soil, has shown to reduce damage caused by potato cyst nematodes [3].

Interaction of nematode damage and nutrient acquisition
Increased pH levels increase nematode damage at similar or lower nematode densities. Nematode-induced ramification results in more root tips and Ca uptake takes place mainly in root tips. Too high internal levels of Ca in the plant may prohibit the uptake of other cations. It was found that at pH 5 potato plants grew better when supplied with nitrate and at pH 6 when supplied with ammonia [4].

Nematodes lead to high root densities in the topsoil and rapid depletion of N. Leaching following rain or irrigation accelerates this depletion. N uptake by the crop late in the season is also reduced because fewer roots are present in the subsoil [2]. There are different mechanisms of nutrient acquisition for the various elements: those that are mobile move to the roots (e.g. N) and for those that are not mobile (e.g. P), the roots have to be mobile; however, ramification leads to many roots in the topsoil and few in the subsoil.

A few weeks after emergence of the potato plant, the tuber is initiated. From then on, the demand for assimilates by the tuber increases. When tuber growth equals crop growth, no assimilates are left for the foliage that, as a consequence, dies after a predetermined time lapse expressed as a number of day-degrees. Cultivar lateness influences this competition as late cultivars form their tubers late and as a consequence are more tolerant of nematodes.

Future perspectives for remediation
From the above, it follows that several management practices are available and require further exploration. Increased lateness of the cultivar with increased nematode population densities is needed. Management of the soil pH should be aimed at reduction (depending at initial level) and formulation of N application may need attention. The application of temporal supplemental dressings of mobile minerals (e.g. N) and spatial supplemental dressings of immobile minerals (e.g. P) requires attention.

1. Trudgill DL, 1980. Nematologica 26, 234-254.
2. Haverkort AJ, De Ruijter FJ, Boerma M, van de Waard M, 1996. European Journal of Plant Pathology 102, 317-324.
3. De Ruijter FJ, 1998. Potato crop growth as affected by potato cyst nematodes (Globodera pallida) and abiotic factors. PhD Thesis Wageningen Agricultural University, The Netherlands.
4. Cao W, Tibbits TW, 1994. Journal of Plant Nutrition 17, 109-126.