An airfoil (26, 62, 64) is mounted on a chordwise stall fence (28) of a wind turbine blade (22). The airfoil may be an aerodynamic slat (26) positioned over a forward portion of the suction side (38) of the blade or a flap (63) disposed near a trailing edge (34) of the blade. The stall fence may be at least as high as the thickness of the boundary layer (39) of air passing over the blade. The slat or flap may be twisted between an inboard end (26A) and an outboard end (26B) to compensate for the radially varying angle of the relative air inflow (46A, 46B).