Phytophthora infestans, the cause of late blight, is the most devastating pathogen in potatoes world-wide. To replace copper fungicides in organic potato production, we examined preparations based on plant extracts, micro-organisms, and other natural compounds for their effect on late blight in field trials as well as in vitro and in vivo. Most of this work we realised as participants of the EU project Blight-MOP. The majority of the preparations effectively inhibited P. infestans in vitro or in vivo on tomato plants. However, under field conditions and with applications once a week, commercial and experimental copper-free preparations failed to sufficiently control late blight. In contrast, copper fungicides applied according to the decision support system Bio-PhytoPRE or in regular intervals consistently reduced foliar blight and prevented significant yield losses. Results from an in vitro test and from a detached leaf test indicate that a main cause of the failure of copper-free preparations could be low stability towards rain or dew
