  The problem of the occurrence of an energy cascade for Alfv\'enic turbulence
in solar wind plasmas was hystorically addressed by using phenomenological
arguments based to the weakness of nonlinear interactions and the anisotropy of
the cascade in wave vectors space. Here, this paradox is reviewed through the
formal derivation of a Yaglom relation from anisotropic Magnetohydrodynamic
equation. The Yaglom relation involves a third-order moment calculated from
velocity and magnetic fields and involving both Els\"asser vector fields, and
is particularly useful to be used as far as spacecraft observations of
turbulence are concerned.
