An important issue in turbulence theory is to understand what kinds of elementary flow structures are responsible for the part of the turbulent energy spectrum described by Kolmogorov's celebrated k(-5/3) law. A model for such structure has been proposed by Lundgren [Phys. Fluids 25, 2193-2203 (1982)]] in the form of a vortex with spiral structure subjected to an axially straining field. We report experimental results of a vortex burst in a laminar-flow environment showing that this structure is responsible for a k(-5/3) part in the energy spectrum. If there are many experimental evidences of the existence of vortices with spiral structures in turbulent flows, it is the first time that such an elementary structure is experimentally shown to be responsible for the turbulent energy cascade.