Aqueous fermentation extracts of composted microbiologically active substrates reduced significantly the infestation of grapevine leaves with Plasmopara viticola under growth chamber conditions. Enhancement of efficacy was available through the enrichment of the extracts with nutrients - like sucrose and brewer's yeast - or with a natural surfactant (methylized cellulose). Studies on the microbial composition of the phyllosphere showed that the quantity of the total number of colony forming units, of yeasts/filametous fungi, of enterobacteria, of pseudomonads and of aerobic bacilli depends on the specific amendments to the extracts and on the microclimatic conditions. They ace responsible for the differences in efficiency of the various extracts against P. viticola. Appropriate investigations on the phyllosphere in a field experiment resulted in a different composition of the microflora, though the same combinations of extract amendments were used. This is caused by the varying environmental conditions. In spite of the changed microflora, the disease suppressing effects against Urcinula necator were preserved
