  We study the Yaglom law, which relates the mixed third order structure
function to the average dissipation rate of turbulence, in a uniformly
expanding solar wind by using the two scales expansion model of
magnetohydrodynamic (MHD) turbulence. We show that due to the expansion of the
solar wind two new terms appear in the Yaglom law. The first term is related to
the decay of the turbulent energy by nonlinear interactions, whereas the second
term is related to the non-zero cross-correlation of the Els\"asser fields.
Using magnetic field and plasma data from WIND and Helios 2 spacecrafts, we
show that at lower frequencies in the inertial range of MHD turbulence the new
terms become comparable to Yaglom's third order mixed moment, and therefore
they cannot be neglected in the evaluation of the energy cascade rate in the
solar wind.
