In subsistence farming and in a developing seed system where there is no official certification scheme operating, growers use the small tubers of the harvested crop as seed for the next season.
To reduce degeneration of the seed, potato producers mark the best-looking plants in the field, the so-called positive selection. These plants are harvested and stored separately to be used as seed. Others use a corner of the field in which they remove diseased plants (the so-called negative selection) to be used as seed for the next season.
Brown rot
Some countries with an effective production of minitubers and basic seed, supply the first or second generation grown from them at the research station site and distribute a limited amount to associated growers that plant them in ‘seed plots’ of one to a few tens of m2 at the high end of the ware potato field. Seed plot growers remove the top soil of the plot to avoid contamination with brown rot (Ralstonia solani) bacteria and grow the seed in such dedicated plots. Here aphids are controlled, suspected plants are removed and the crop is defoliated and harvested prematurely. The resulting seed tubers are planted in the next season and then a new seed plot is established. The system of purchasing a few hundred rooted plantlets from cuttings in the central highlands of Vietnam is not unlike that in the African highlands, where the seed plot technique is practiced.
Positive and negative selection in subsistence seed potato production. Stakes are placed at best looking plants when still green and growing. The tubers resulting from this positive selection are stored in a diffused light or other store and are used as seed or the next planting. Plants obviously affected by brown rot and other diseases are removed (negative selection) and surplus seed is sold at the local market, usually bought by growers living at lower altitudes.
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