Copper free control of potato late blight in organic potato production?

Copper is used in organic potato production to control the late blight pathogen Phytophthora infestans. A disadvantage of copper is, that it accumulates in the soil and damages soil organisms. In Switzerland, the application of 4 kg of copper per hectare and year is allowed. In a six year crop rotation, the ecotoxicologically acceptable amount of 360 g per hectare and year is exceeded by a factor of around two by just one year of potato cultivation. Over the past few years, Agroscope has been testing different substances with the aim to reduce or replace copper in potato production. Satisfying results were achieved with potassium phosphonate under field conditions, however, residues are accumulated in the tubers depending on the input quantity. For this reason, phosphonate products will probably not be applied in organic potato farming. In the field trials, a partial effect comparable to 3 kg copper per hectare was achieved by a suspension of finely ground alder buckthorn bark (Frangulae cortex). In order not to exceed the permitted amount of copper, the potatoes could be protected from late blight infestation by using suitable formulations of plant substances with antimicrobial properties in the final treatments.