Final-state interaction phase difference in $J\mathrm{/}\psi \to \rho \eta$ and $\omega \eta$ decays

It is shown that the study of the $\omega -{\rho }^0$ interference pattern in the $J\mathrm{/}\psi \to ({\rho }^0+\omega )\eta \to {\pi }^{+}{\pi }^{-}\eta$ decay provides the evidence for the large (nearly ${90}^{\circ }$) relative phase between the one-photon and the three-gluon decay amplitudes.