Since lime sulphur was accepted in the annex 2b of the European Regulation 2091/92, more and more fruit growers use it to control apple scab in organic orchards. Field trials were carried out at the Research Centre Laimburg to evaluate stop and curative treatments with the aim to reduce the number of treatments and improving their efficacy. In this diploma thesis microscopic observations were performed for a better understanding of the mode of action of lime sulphur on apple-scab. The results showed that lime sulphur can stop the scab infection until the formation of the appressorium. If the infection proceeds to the formation of the primary stroma, the efficacy of the treatments with lime sulphur was insufficient
