Potato Late Blight Control with a Botanical Product and Reduced Copper Applications

Potato late blight (PLB), due to the pathogenic oomycete, Phytophthora infestans, can cause extensive economic damage, particularly in organic potato production. Although copper is used to combat PLB in organic production, it is banned in some countries, and its reduction or elimination as a plant protection product is an increasing priority in Europe. Alternative control strategies, including botanicals, could potentially reduce copper and control PLB. We investigated the application of Frangula alnus bark, its sequential use with a reduced copper application, and a reduced copper application alone in field and lab experiments. The influence of different dosages and preparations on efficacy and the quantity of the posited active ingredients were examined. Frangula alnus treatments decreased disease severity compared to a water control but showed differences in efficacy depending on dosage and disease pressure. Through in vitro and in planta experiments, we investigated whether F. alnus directly or indirectly controlled PLB. A bacterium (Erwinia spp.), originating from the F. alnus extract, colonized the media and accounted for most of the direct inhibition in vitro, but removing microorganisms through filtration had no effect on the extracts efficacy in planta. The contribution of extract-associated microorganisms to PLB control is unclear and requires additional experimentation to assess. The presence of measured anthraquinones likely contributed to the effect of F. alnus. In field experiments, copper consistently and F. alnus generally (except for one year) reduced disease severity compared to a water control. No difference was observed in disease severity between the full and reduced copper treatments. Potato variety more consistently drove differences in total and marketable yields compared to the applied treatment. The relative stability of the yield suggests that treatment effectiveness is intertwined with the timing of disease development and environmental conditions.