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. Copper is used to combat PLB, but its reduction as a plant protection product is an increasing priority in Europe. Alternative control strategies, including botanicals, are a potential avenue to reduce copper and control PLB. We investigated the application of Frangula alnus bark, its pairing with a reduced copper application, and a reduced copper application alone in field and lab experiments. The effect of different dosages and preparations on efficacy, and the posited active ingredients were examined. Frangula alnus treatments decreased disease severity compared to the untreated control but showed differences in efficacy depending on dosage and disease pressure. No difference in disease severity between the full and reduced copper treatments was observed. 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 extract’s efficacy in planta. It is unclear whether extract-associated microorganisms contributed to PLB control and would require additional experimentation to assess. The presence of measured anthraquinones likely contributed to the effect of F. alnus . Yields from the field experiments within each year remained stable despite different treatments and dosages, suggesting that treatment effectiveness is intertwined with the timing of disease development and ambient environmental conditions.