In organic potato (Solanum tuberosum) production in Europe,solely copper-based fungicides allow to directly control lateblight (caused by Phytophthora infestans). Due to environmentalconcerns, the use of copper (Cu) fungicides has to beas efficiently as possible to reduce annual consumption.This can be achieved either by reducing the dose per applicationor by decreasing the numbers of sprays. Between2005 to 2009, six field trials were conducted at two sites(3 each) in northern Germany in order to (a) determine theefficacy of a copper fungicide (copper hydroxide) at reducedrates compared to the common practise of the most importantGerman organic farmers’ associations (3 kg Cu ha–1),and (b) to evaluate the copper reduction potential of an applicationstrategy based on a decision support system (DSS). Aclear dose-response relationship with reduced rates of copperhydroxide was not consistent over trials and strategiesalthough there was clear evidence for a considerable potentialto reduce Cu rates compared to the amount of 3 kg Cu ha–1.However, different degrees of late blight pressure stronglyaffected the efficacy of the Cu fungicide in individual years.Under less late blight pressure, a reduction to 1.25 kg Cu ha–1applied throughout the season was possible without significantloss in efficacy compared to the typical spraying of 2.5 or3.0 kg Cu ha–1. Due to a higher risk of decreasing efficacy,reducing Cu hydroxide supply was only possible to an amountof 2.0–2.5 kg Cu ha–1 under moderate disease pressure whilehigh late blight pressure in 2007 did not allow a Cu reductionto this extent. Even the highest amount of 3.0 kg Cu ha–1only decreased the disease by 18% in one single case. Consequently, yields were not consistent, but depended on year, site and late blight pressure. On average over rates and strategies, Cu application increased yields by 15%. In addition, the results revealed the clear tendency that reducing Cu amounts did not impair tuber yield. A spray schedule based on the new German DSS ÖKOSIMPHYT increased effectiveness of Cu sprays, reduced numbers of sprays and the total amount of Cu per year, even under moderate and high late blight pressure.Therefore, we conclude that the DSS has the potential for furtheroptimization. Knowledge of actual late blight pressure, asprovided by ÖKOSIMPHYT, is necessary to optimize the use ofCu fungicides allowing both to ensure the site-specific tuberyield potential and to reduce the CU application amounts.