Potato late blight (PLB), due to the pathogenic oomycete, Phytophthora infestans, can cause extensive economic damage. Copper is used to combat PLB mainly in organic production, but its reduction or elimination as fungicide is a priority in Europe, and it is already banned in some countries. Alternative control strategies, including botanicals, could reduce copper and control PLB. We investigated the application of Frangula alnus bark, its sequential use with reduced copper, and reduced copper applications alone in field and lab experiments. Different dosages and preparations influenced F. alnus efficacy and the quantity of its posited active ingredients. 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 caused most of the direct inhibition in vitro, but filtering out microorganisms had no effect on the extract’s efficacy in planta. The contribution of extract-associated microorganisms to PLB control is unclear and requires additional experimentation to assess. The measured anthraquinones likely contributed to the effect of F. alnus. During four years of field experiments, the reduced copper and F. alnus treatments decreased disease severity in four and in three of the years, respectively, compared to the water control. No differences in disease severity or yield were observed between full and reduced copper treatments. Potato variety more consistently drove differences in total and marketable yields compared to the treatment. The yield was relatively stable within each year, suggesting that the treatment's effect on yield is intertwined with the timing of disease development and environmental conditions.
