Ertter (S.Watson) 1992 - Ertter B (1992) A Re-Evaluation of Potentilla drummondii and P. breweri (Rosaceae), with the New Species Potentilla morefieldii. Brittonia 44(4): 429-435. doi: 10.2307/2807193.
There is no corrections for Potentilla drummondii subsp. breweri yet.