The present invention relates to a rotor control system for actuating pitch of pitch adjustable rotor blades of a wind turbine in order to reduce edgewise blade vibrations. The system comprises a pitch actuation unit being arranged to receive a torsional vibration signal and apply a filter to the torsional vibration signal to couple the torsional vibration signal to an edgewise vibration of the rotor blades. An m-blade coordinate transformation, such as the Coleman transformation, is applied to the filtered torsional vibrational signal to obtain pitch modification signals.