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Abstract: Today over 90% of the potatoes for the fresh market are washed; therefore consumers already object to minor external quality deficiencies. The quality assessment performed by potato traders does not distinguish in detail between quality deficiencies and potential links to the farming systems and site parameters are unknown. Thus, the cause for the observed increase of external quality problems is unknown. From 2001 to 2003 the extent and the potential causes of quality deficiencies were studied on totally 278 fields on conventional, integrated and organic farms in the wet and cool climate of Switzerland. Just before harvest samples consisting of 810 tubers were collected on each field in a standardised sampling pattern. Wireworms, slugs and drycore were responsible for important economic losses in all farming systems. In the organic farming system the quality damage was higher for all three deficiencies. While wireworm and slug damage were of general importance, drycore was significantly most severe in the organic farming system in all three years. Grass clover ley in the years preceding potatoes increased the risk for all three quality deficiencies. Slug damage increased with soil cover (catch crops) and with the percentage of crops favourable to slugs in the crop rotation (e.g. vegetables). Seed tubers without black scurf infestation reduced the occurrence of black scurf and drycore on harvested tubers. Fungicide seed treatment reduced black scurf significantly if seed tubers were infested. Insecticide seed treatment for cereals preceding potatoes as well as molluscicide treatments in the potato crop had a beneficial but not always sufficient effect. [Copyright &y& Elsevier]