Seed potatoes in the Netherlands

Potato is an important crop in the Netherlands. It would be impossible to grow good, healthy potatoes without high quality seed.

The Dutch seed potato sector is specialised in the production of high quality seed. Breeding companies continuously develop new varieties for different environments and purposes. Specialised farmers produce seed of over 300 varieties, and more than 900,000 tonnes are certified each year. Trade companies bring these to market worldwide.


The NAK (Dutch General Inspection Service for Agricultural Seeds and Seed Potatoes) is the competent authority responsible for the inspection and certification of seed potatoes in the Netherlands The NAK operates under the direction and supervision of the Ministry of Agriculture, Nature and Food Quality, and under supervision of the NPPO for the quarantine aspects of the inspection. Nevertheless, the NAK is an
independent foundation, founded in 1932. All costs for inspections are paid by farmers.

The board is composed of representatives of all organizations, active in the field of seed crops and seed potatoes: breeders, growers, merchants and users. The NAK organization employs a total staff of 220 people, of which 80 field staff.

Why inspection?

European law demands that before marketing all seed potatoes are officially inspected and certified. This is done to ensure that the seed potatoes meet the minimum EU requirements for quality and health. Member states of the European Union (EU) have to meet the requirements of the Plant Health Regulation (2016/2031/EU) and Directive 2002/56/EC for the marketing of seed potatoes.

Potatoes are susceptible to harmful organisms, of which many are passed on to the next generation
via the tuber. Fortunately, these harmful organisms are not all present at the same time, and also do not cause the same rate of harm. In Europe we distinguish two kinds of harmful organisms:

  • Regulated Non-Quarantine Pests (RNQP’s).
    These are allowed up to a certain threshold in seed potatoes and are in part used for classification. Examples are viruses that cause mosaic, or common scab.
  • Quarantine Pests
    There is a zero tolerance for these pests. Organisms with a quarantine status for seed potatoes are potato cyst nematode (PCN), brown rot, ring rot and wart disease.

NAK inspections also cover quality aspects like general appearance of plants, tuber size or external defects of tubers.

  • The main aspects of the Dutch inspection system
  • field inspection
  • post-harvest tests for viruses and bacterial diseases
  • lot inspection

Classification system

Seed potato production starts mainly with tissue culture material (minitubers), but clonal selection
is also still used.

In the Netherlands, minitubers are produced by companies that are authorised for this purpose by the NAK. To ensure that the minitubers produced are of good quality and free from pests, the
production facilities are audited regularly by the NAK.

During clonal selection, farmers select healthy looking, high yielding and true-to-type plants. These mother plants are harvested and propagated as separate clones.

Every year, seed crops are automatically downgraded one class. This stimulates the regular use of healthy seed. Depending on inspection results, further downgrading or rejection may occur. The classification system is as follows:

Field inspection

Prior to planting, each field intended for the production of seed potatoes must be sampled and tested for potato cyst nematode (PCN). Only if a field is found free from PCN can the production of seed potatoes be allowed.

The inspection season starts in spring with the online registration of the plots. Inspectors then check the original documents of the starting material.

Seed potato crops are inspected at least twice during the growing season, starting at the beginning of June. Inspectors pay attention to:

  • tuber born diseases
  • general appearance and developtment
  • trueness to variety/varietal purity
  • Off-types

The Dutch tolerances are more strict than the European tolerances. See the table below.

Field inspection tolerances (%)
Mosaic/leafroll Blackleg Off-types
PB/S 0,025 0,000 0,000
SE 0,050 0,000 0,000
E 0,010 0* 0,025
A 0,025 0,030 0,100
B 2,000 0,100 0,500

* sporadic: 1 plant per ha

Varietal identity is monitored at the central clone field of the NAK, where samples of 2nd year crops of all varieties are planted. Clones not true-to-type are rejected. More than 20.000 clones are inspected each year.

Roguing and aphids

To enhance the quality of the growing crop, farmers rogue diseased plants, or plants that are not true to type.

Roguing is also important to prevent the spread of viral diseases. Aphids can be notorious carriers of
viral diseases and populations are closely monitored during the growing season. Yellow water traps and suction traps are used to catch aphids, and these are further identified and counted by our staff.

Farmers use the information about aphid flights to manage the quality of their crops, or to decide when to kill the haulm.

For haulm killing, farmers usually apply a combination of mechanical and chemical methods. After haulm killing, the plants must not exhibit any re-growth, because the younger leaves attract more aphids and thus give a high risk for virus infection. Two to four weeks after haulm killing the
seed crops are harvested.

Post-harvest testing

All crops are sampled for a test on ring rot and brown rot (200 tubers), using EU protocols. This test is carried out under the responsibility and supervision of the NPPO. Since brown rot and ring rot have a quarantine status in the EU, a zero tolerance is applied. The same tuber sample is used for a virus test, using PCR. For this test, 50 tubers are bundled in one reaction. This means classification is based on 4 reactions, as indicated in the table below. Re-testing for virus is possible.

# Positive reactions        Maximum class
0 (0%) PB/S
1 (0.6%) SE
2 (1.4%) E
3 (2.7%) A
4 (not determinable)

Lot inspection

During lot inspection, each seed lot is checked for:

  • tuber diseases
  • defects
  • weight (of the container unit)
  • tuber size
  • contamination (e.g. adhering soil).

In assessing the potatoes for defects, the inspector looks for (quarantine) diseases, sprouting, bruising, damage, misshapen tubers, etc. The inspector visits the farm every day during the grading process.

After a seed lot has been approved, it is certified. Under supervision a NAK certificate is attached to each package or container. The certificate states the details of the lot and is combined with the plant passport. This document is the evidence that all national quality and European phytosanitary requirements are met.

Export certification

For export to countries outside the European Union, an additional inspection is often required. The export inspection is carried out by NAK inspectors under supervision of the NPPO, and if possible, is combined with the lot inspection. As proof of meeting the phytosanitary requirements of the importing country, a phytosanitary certificate is issued.

The Dutch standards have to comply with the requirements laid down in the EU marketing directive for seed potatoes and in the EU phytosanitary directive.

Lot inspection tolerances

This table shows the Dutch tolerances for lot inspection and the European standards. Dutch tolerances are more strict.

Disease or defect  Tolerance the Netherlands (NAK)  Tolerance European Union  
Rhizoctonia PB/S/SE: max. 10% (weight) has max. 10% surface cover.

E/A: max. 25% has max 10% surface cover

PB: max 1% (weight) with > 10% surface cover

Other: max. 5% (weight) with > 1/3 surface cover

Scab Scab scale 2.5 (max. 1/8 surface cover) Max. 5% (weight) with > 1/3 surface cover
Wet rot Practically none¹) 0.2% (weight)
Dry rot Practically none¹)


PB: 0.2% (weight)

0.5% (weight)

External defects 4 to 12 tubers per 50 kg2) 3% by weight
Soil etc. 1% 2%
1)  1 tuber per 250 kg
2)  depending on size