A blade for a rotor of a wind turbine having a substantially horizontal rotor shaft, said rotor comprising a hub, from which the blade extends substantially in a radial direction when mounted to the hub, the blade having a longitudinal direction (r) with a tip end and a root end and a transverse direction, the blade further comprising: - a profiled contour (40, 42, 50) including a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord having a chord length extending there between, the profiled contour, when being impacted by an incident airflow, generating a lift, wherein the profiled contour is divided into: - a root region having a substantially circular or elliptical profile closest to the hub, - an airfoil region having a lift-generating profile furthest away from the hub, and - a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift-generating profile of the airfoil region, characterised in that - the blade is provided with a plurality of longitudinally extending flow guiding device parts, which are grouped together as modular parts to form a first flow guiding device group in the transition region of the blade, the first flow guiding device group extending along at least a longitudinal part of the transition region, wherein - each of the flow guiding device parts is added to the profiled contour (40, 42, 50) of the blade on the pressure side of the blade, and wherein - wherein the flow guiding device parts are shaped as planar elements protruding from the profiled contour (40, 42, 50) and are extending substantially in the longitudinal direction of the blade and comprise a first side nearest the leading edge of the blade, a second side nearest the trailing edge of the blade as well as a first longitudinal end nearest the root end of the blade and a second longitudinal end nearest the tip end of the blade, and wherein - the flow guiding device parts are arranged so as to generate a separation of airflow from the pressure side of the blade at a point between the flow guiding device parts and the trailing edge of the blade, when the blade is impacted by the incident airflow. - a profiled contour (40, 42, 50) including a pressure side and a suction side, as well as a leading edge and a trailing edge with a chord having a chord length extending there between, the profiled contour, when being impacted by an incident airflow, generating a lift, wherein the profiled contour is divided into: - a root region having a substantially circular or elliptical profile closest to the hub, - an airfoil region having a lift-generating profile furthest away from the hub, and - a transition region between the root region and the airfoil region, the transition region having a profile gradually changing in the radial direction from the circular or elliptical profile of the root region to the lift-generating profile of the airfoil region, characterised in that - the blade is provided with a plurality of longitudinally extending flow guiding device parts, which are grouped together as modular parts to form a first flow guiding device group in the transition region of the blade, the first flow guiding device group extending along at least a longitudinal part of the transition region, wherein - each of the flow guiding device parts is added to the profiled contour (40, 42, 50) of the blade on the pressure side of the blade, and wherein - wherein the flow guiding device parts are shaped as planar elements protruding from the profiled contour (40, 42, 50) and are extending substantially in the longitudinal direction of the blade and comprise a first side nearest the leading edge of the blade, a second side nearest the trailing edge of the blade as well as a first longitudinal end nearest the root end of the blade and a second longitudinal end nearest the tip end of the blade, and wherein - the flow guiding device parts are arranged so as to generate a separation of airflow from the pressure side of the blade at a point between the flow guiding device parts and the trailing edge of the blade, when the blade is impacted by the incident airflow.