In indoor trials with potted potato plants and in micro- and small plot field trials, prepa-rations of Frangula alnus controlled Phytophthora infestans leaf blight significantly bet-ter than other antifungal plant preparations and the copper free reference Mycosin®. However, under field conditions the efficacy of the buckthorn bark preparation (BBP) was drastically lower than in the indoor trials. Therefore and in order to avoid environ-mental pollution with solvent of the plant extract, we developed a procedure to apply finely ground plant material as suspension directly on the potato plants without any ex-traction. The efficacy of BBP applied as suspension was significantly higher than those of ethanolic extracts. To improve the weather stability or the rainfastness, BBP suspensions were amended with wood protection oils in a model trial to demonstrate the effect of a good formulation. The efficacy of these formulations proved to be as ef-ficient as copper with a dosage of 200 g ha-1 per treatment. This was also the case for a BBP with the additive Nu-Film® in a small plot field trial at Zürich-Reckenholz. How-ever, in this trial the disease pressure was low
