rhizoctonia; potato; biological; seed; potatoes

Vegetable production: Seed potatoes – make an informed choice

Potatoes are one of the few annual crops for which vegetative tissue (tubers) are used instead of botanical seed. Tubers used as planting material are known as seed potatoes.

Edible tubers are called ware or table potatoes. Using them as seed can cause problems, as one does not know how healthy their source is – it’s easy for the soil to be infected with pests such as nematodes and diseases like fusarium, common scab, etc. These can multiply and attack the potatoes – as well as other crops susceptible to them.

  • Seed potatoes can be certified or uncertified.
  • Certified seed is grown by registered seed producers under very strict conditions, and have to be tested before they can be sold as such.
  • Uncertified seed (any tubers not labelled as certified) can be obtained from neighbours, a vendor, or at the market; or they could have been kept for planting.


A registered seed producer is a farmer who is registered at the relevant certification organisation in your country. In South Africa this is Potatoes Certification Service or PCS, a company that administers the certification scheme.

Farmers registered at PCS either sell their own seed or supply them to co-operatives, or both. The farmers’ fields are inspected regularly for diseases, and after harvesting some of the tubers are sent to a laboratory where they are tested for viruses and diseases.

  • A few tubers in each generation of seed (G number) are allowed to have some diseases, but none may have golden nematode or bacterial wilt.
  • The lower the G (generation) number, the fewer the number of diseases the tubers are allowed to have.
  • This means G3 potatoes are better than G4, G5, G6 and G7 potatoes and so the lower G numbers are also more expensive.
  • Diseased seed usually produce a lower yield, so the fewer the diseases in the seed, the higher your yield potential can be.
  • The G number is given on the certification tag tied to the seed bag.


  • All the varieties (UTD, BP1, Vanderplank, etc.) planted these days were developed at a research station.
  • The potato certification scheme in South Africa starts at ARC-Roodeplaat where disease- and pest-free potatoes are multiplied in a laboratory and kept in vitro.
  • These in vitro plantlets are sold to interested certified seed producers, who then transplant them and keep them in greenhouses where the first tubers (known as minitubers) are produced.
  • These minitubers are G0 (if they pass the strict test) and are very expensive.
  • Other seed producers who buy mini¬tubers plant them out in a field.
  • The seed tubers harvested from this first field planting are called G1 – but only if they do not have more diseases than the G1 rules allow.
  • If they have too many diseases, they will get the G number (G2 to G7) that allows the number they have.
  • If, however, they have too many for certification, they are sold as table potatoes.


  • G1 tubers will produce G2 seed the next time a seed producer plants them.
  • Each time the seed is planted, the seed it produces has to get a higher number than the seed from which it was produced.
  • As the G numbers become higher, the cost of producing seed becomes lower.
  • G8 is the highest G number one can buy, and the potatoes harvested from them must be sold as table potatoes.
  • Some seed producers could have seed that have a few diseases (for example as allowed for a G2) but they have to be classified as a higher G number because they were planted out in the field for more years than a G2 rating allows.
  • If you look at the quality of the plants during the growing season, you can identify these farmers/producers and buy very good quality seed at a lower price.


  • Certified seed bags have a tag attached to them that gives a lot of information.
  • If bags do not have a certification tag, the seed they contain cannot be sold as certified seed. Bags that are open are not certified, even if the tags are still attached.
  • So it is important to look for sealed bags, with an unbroken certification tag and seal, of the variety you want to plant.

Keep the tags for reference. If severe disease problems can be traced to the tuber, the information on the tags will make it possible for PCS to determine who produced the seed. And if you are satisfied with the seed, you will also have information about which seed was bought where, and what the quality was.

  • You can try different G numbers from the same seed producer, and then decide which generation of seed you prefer.
  • Remember to make sure that you use the right cultivar for the area where you are going to grow your potatoes.


  • The variety being sold is printed on the grower’s tag. Make sure it is the one you want to buy.
  • The generation (G) number. The higher the number, the lower the seed quality.

The class (or quality) of the seed:

Elite means the tubers are very high quality, and there are almost no diseases in and on the tubers. A round red sticker bearing the word elite is stuck onto the tag.

Class 1 means there are slightly more diseases in or on these tubers than for Elite. The quality is still good, however. A round green sticker bearing the words ‘Klas 1 Class’ is stuck onto the tag.

Standard means these tubers have more diseases on the outside of the tubers than either Elite or Class 1. The yield cannot be certified and must be sold as table potatoes. The standard seed has a bi-coloured tag, half blue and half white.

  • A number that identifies the farmer from whom the seed came.
  • This number tells PCS who produced the seed.
  • A number, different for each bag, that tells PCS where the bag came from.

Also read:
Vegetable production: Grow your own potatoes in bags
Potato production: Biology kicks rhizoctonia in the teeth
Potato bags good business for Zambian photographer turned farmer

  • This article was written by Ineke Vorster and Chris Du Toit and first appeared in Farming SA.

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