Potential suitability of various leaf treatment products as copper substitutes for the control of late blight (Phytophthora infestans) in organic potato farming

In organic potato production, late blight caused by Phytophthora infestans can cause severe losses in potato yield and quality. Currently, it can only be effectively controlled by the use of copper fungicides in some countries. In the course of a project aiming at the reduction of copper usage, potential copper-free products against late blight for organic farming were identified. Laboratory, greenhouse, and field tests were performed with a selection of commercial and non-commercial copper-free preparations as potential substitutes for copper. Data from lab tests with detached leaves are now available for more than 20 alternative products, some of which had efficiencies close to that of the copper fungicide control, including some degree of post-infectional effects. Climate chamber tests with whole potted plants confirmed these results for the most promising preparations, with almost no damage occurring on the treated plants. Data from field tests were less compelling but still highlighted some late blight reducing potential for some of the alternative products, in particular chitosan, a liquorice and a horsetail test product, and a copper fertilizer. In the future, these might possibly be used for applications mixed or alternating with copper, and thus might allow a further reduction in the use of copper fungicides in organic potato production.