Abstract: Apple scab caused by Venturia inaequalis is the most important disease in apple production. Control of apple scab currently depends on the multiple applications of fungicides. The potential of the antagonist isolate Cladosporium cladosporioides H39, originating from a sporulating colony of V. inaequalis, to control apple scab development was tested in eight trials during two years in orchards in Eperjeske (Hungary), Dabrowice (Poland) and Bavendorf (Germany) planted with different varieties. Treatments were conducted as calendar sprays or after infection periods during the primary season or the summer season. Additional trials in an orchard in Randwijk (The Netherlands) focused on the effect of timing of the antagonist application before or after infection periods. The overall results of the field trials consistently showed for the first time that stand-alone applications of the antagonist can control apple scab in leaves and fruits. This was demonstrated in organic and integrated growing systems. In both systems, control levels as for common fungicide schedules could be reached. The antagonist also was effective if applied one or even several days after infection events. This has been found in several field trialsand has been confirmed by a trial with single spray applications at different intervals before or after infection events. The better understanding the biology of the antagonist will help to further exploit its use in apple scab control.