Résumé
We consider the linear growth of matter perturbations on low redshifts inmodified gravity Dark Energy (DE) models where G_eff(z,k) is explicitlyscale-dependent. Dispersion in the growth today will only appear for scales ofthe order the critical scale ~ \lambda_{c,0}, the range of the fifth-forcetoday. We generalize the constraint equation satisfied by the parameters\gamma_0(k) and \gamma'_0(k) \equiv \frac{d\gamma(z,k)}{dz}(z=0) to models withG_{eff,0}(k) \ne G. Measurement of \gamma_0(k) and \gamma'_0(k) on severalscales can provide information about \lambda_{c,0}. In the absence ofdispersion when \lambda_{c,0} is large compared to the probed scales,measurement of \gamma_0 and \gamma'_0 provides a consistency check independentof \lambda_{c,0}. This applies in particular to results obtained earlier for aviable f(R) model.