  It has been pointed out that there exists a tension in $\sigma_8-\Omega_m$
measurement between CMB and LSS observation. In this paper we show that
$\sigma_8-\Omega_m$ observations can be used to test the dark energy theories.
We study two models, (1) Hu-Sawicki(HS) Model of $f(R)$ gravity and (2)
Chavallier-Polarski-Linder(CPL) parametrization of dynamical dark energy (DDE),
both of which satisfy the constraints from supernovae. We compute $\sigma_8$
consistent with the parameters of these models. We find that the well known
tension in $\sigma_8$ between Planck CMB and large scale structure (LSS)
observations is (1) exacerbated in the HS model and (2) somewhat alleviated in
the DDE model. We illustrate the importance of the $\sigma_8$ measurements for
testing modified gravity models. Modified gravity models change the matter
power spectrum at cluster scale which also depends upon the neutrino mass. We
present the bound on neutrino mass in the HS and DDE model.
