The demarcation of Globodera rostochiensis and Globodera pallida infestations in fields for seed potatoes, using a Monte Carlo approach C. H. Schomaker and T. H. Been Plant Research International, Wageningen University and Research Centre, Plant Sciences Group, P.O. Box 16, 6700 Wageningen (The Netherlands); e-mail: corrie.schomaker@wur.nl; thomas.been@wur.nl A Monte Carlo approach was utilized to calculate the size of the demarcation unit, necessary to cover infestations with Globodera rostochiensis and Globodera pallida (PCN), detected with the new EU-sampling method. Results from 90 sampled fields were used to obtain frequency distribu- tions of the central population density (CPD), the length and width gradients of infestation foci and the distance between ‘primary’ and ‘secondary’ foci within one field section in the direction of culti- vation. With a simulation model written in R, a free software environment for statistical computing, the dimensions of the demarcation unit were calculated, based on the dimensions of the sampling unit, the position of the ‘primary’ infestation within the sampling unit and the size of the infestation focus. It is concluded that a demarcation of 90% of possible foci positions and dimension, analogous to the specified detection probability of the EU sampling method, provides sufficient security to demarcate any infestation. With the preferred sampling unit of 11 m width and 300 m length (1 / 3 ha sampled), used in The Netherlands, only 0.61% of the cyst numbers will be situated outside the demarcated unit. These cyst numbers compare to 0.17% of total amount of cysts in the EU infes- tation model: 1 focus with a CPD of 100 cysts kg )1 and three foci with CPDs of 50 cysts kg )1 . The required demarcation area has a width of 36 m and a (maximum) length of 402 m and covers an unit of 1.5 ha. The methodology described can be applied to all proposed sampling methods and sam- pling unit dimensions. Introduction Both potato growers and phytosanitary authorities want to know the exact location of infestation foci of potato cyst nematodes (PCN) and the infested units, although for different reasons. One or a few infestation foci do not cause large yield losses, but their detection results in prohibition of potato cultivation or, in the case sampling is executed after a potato crop, a total loss of the crop as seed potatoes for export. The grower wants to detect infestations, by way of voluntary sampling with a high pre- cision sampling method (Been & Schomaker, 2000) to control it and to ensure compliance with the phytosanitary requirements of the EU internal market or third countries. The phytosanitary authorities want to prevent the spread of PCN within and between countries and have to comply with phy- tosanitary legislation. In the past, eradication of PCN was the goal pursued by European countries. However, during the last 40 years PCN became established in Europe; eradication is con- sidered unobtainable. PCN is present in all countries of the Euro- pean Community (Anonymous, 2009). Therefore a less stringent approach could be adopted. From an EU risk assessment with respect to detection of foci (Hallman & Niere, 2004b; Hallman et al., 2004a) it was concluded that for PCN, a 90% confidence of detection is acceptable. This level is the basis of the EU sam- pling method (Council Directive 2007 / 33 / EC of 11 June 2007). Under these circumstances, pre-crop soil sampling is rou- tinely applied in all potato production areas and PCN can effectively be managed. The costs for the potato industry are then primarily the costs of the soil testing program plus the costs of the control measures, e.g. the costs associated with the use of longer rotations, chemical treatments, resistant varieties, etc. For seed potato production, the adoption of a harmonized pre-crop soil sampling rate, coupled with a protocol covering the actions to be taken when PCN are found, provides an equivalence in terms of assurances of PCN freedom and is valuable in permitting the free trade of seed potatoes. This approach provides access to most of the world markets for seed potatoes. The main challenge for the farmers is to ensure seed potato fields to be tested ‘free’ from PCN, taking into account the EU sampling method. Therefore, farmers must be equipped with appropriate tools, preferably other than the preventive use of nematicides. Dutch farmers have sampling methods with high precision and the decision support system NemaDecide at their disposal (Been et al. , 2005). After detection the infested area has to be demarcated. In the Netherlands up to now a 6 m wide strip is added to all sides of the area found infested and thought to encompass the full extent of the infestations. Other countries, e.g. Sweden, take the whole field out of pro- duction when a sampling unit is found infested. Neither of these methods are science based. ª 2010 The Authors. Journal compilation ª 2010 OEPP/EPPO, Bulletin OEPP/EPPO Bulletin 40, 147–157 147