I will present first results from a galaxy population study in the five most distant clusters identified in the South Pole Telescope Sunyaev Zel’dovich survey (SPT-SZ), based on a dedicated 4-band HST and Spitzer follow-up of the complete z>1.4 tail of the SPT-SZ cluster sample (z~1.4-1.7). The wide survey volume of SPT-SZ, and the cluster selection to first order independent of cluster galaxies, make this sample ideally suited for studying galaxy populations in the rarest, most massive (>4x10^14 Msun) clusters at the cosmic epoch bridging the formation of massive cluster galaxies at z>~2 with their passive evolution regime at z<1. Based on photometric classification of quiescent and star-forming galaxies, we measure quiescent galaxy fractions at high stellar masses log(M/Msun)>10.85 of >~ 80% in the cluster core, yielding environmental quenching efficiencies in the range ~0.5-0.8 in the inner r<0.5xr500 cluster region. Our results thus suggest that, at least in the central regions of the most massive clusters, the suppression of star formation in massive galaxies with respect to their field analogs already effciently occurs earlier than z~1.5.

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