  We construct an updated and extended compilation of growth rate data
consisting of 34 points and including corrections for model dependence. In
order to maximize the independence of the datapoints we also construct a
subsample of this compilation (`Gold' growth dataset) which consists of 18
datapoints. We test the consistency of this dataset with the best fit
Planck15/$\Lambda$CDM parameters in the context of General Relativity (GR)
using the evolution equation for the growth factor $\delta(a)$ with a $w$CDM
background. We find tension at the $\sim 3 \sigma$ level between the best fit
parameters $w$ (the dark energy equation of state), $\Omega_{0m}$ (the matter
density parameter) and $\sigma_8$ (the matter power spectrum normalization on
scales $8h^{-1}$Mpc) and the corresponding Planck15/$\Lambda$CDM parameters. We
show that the tension disappears if we allow for evolution of the effective
Newton's constant, parametrized as $G_{eff}(a)/G_N = 1 +
g_a(1-a)^n-g_a(1-a)^{2n}$ with $n\ge2$ where $g_a$, $n$ are parameters of the
model, $a$ is the scale factor and $z = 1/a-1$ is the redshift. This
parametrization satisfies three criteria: a. $G_{eff} > 0$, b. Consistency with
Big Bang Nucleosynthesis ($G_{eff}(a\ll 1)/G_N=1$), c. Consistency with solar
system tests ($G_{eff}(a=1)/G_N=1$ and $G_{eff}'(a=1)/G_N=0$). We show that the
best fit form of $G_{eff}(z)$ obtained from the growth data corresponds to
weakening gravity at recent redshifts (decreasing function of $z$) and we
demonstrate that this behavior is not consistent with any scalar-tensor
Lagrangian with a real scalar field. Finally, we use MGCAMB to find the best
fit $G_{eff}(z)$ obtained from the Planck CMB power spectrum on large angular
scales and show that it is a mildly increasing function of $z$, in $3\sigma$
tension with the corresponding decreasing best fit $G_{eff}(z)$ obtained from
the growth data.
