Copper reducing strategies for late blight ( Phytophthora infestans ) control in organic potato (Solanum tuberosum ) production

In organic potato ( Solanum tuberosum ) production in Europe,solely copper-based fungicides allow to directly control late blight (caused by Phytophthora infestans ). Due to environmental concerns, the use of copper (Cu) fungicides has to be as efficiently as possible to reduce annual consumption. This can be achieved either by reducing the dose per application or by decreasing the numbers of sprays. Between 2005 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 impor­tant German organic farmers’ associations (3 kg Cu ha-1),and (b) to evaluate the copper reduction potential of anapplication strategy 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-1 applied throughout the season was possible without significantloss in efficacy compared to the typical spraying of 2.5 or 3.0 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-1 only 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 resultsrevealed the clear tendency that reducing Cu amounts did not impair tuber yield. A spray schedule based on the new GermanDSS ÖKOSIMPHYT increased effectiveness of Cu sprays,reduced numbers of sprays and the total amount of Cu peryear, even under moderate and high late blight pressure.Therefore, we conclude that the DSS has the potential for further optimization. Knowledge of actual late blight pressure, as provided by ÖKOSIMPHYT, is necessary to optimize the use of Cu fungicides allowing both to ensure the site-specific tuber yield potential and to reduce the CU application amounts.