This work is devoted to the experimental study of the transition to turbulence of a flow confined in a narrow gap between a rotating and a stationary disk. When the fluid layer thickness is of the same order of magnitude as the boundary layer depth, the azimuthal velocity axial gradient is nearly constant and this rotating disk flow is a torsional Couette flow. As in the plane Couette flow or the cylindrical Couette flow, transition to turbulence occurs via the appearance of turbulent domains inside a laminar background. Nevertheless, we show that in the rotating disk case, the nucleation of turbulent spirals is connected to the birth of structural defects in a periodic underlying spiral roll pattern.