This name is a synonym of Rhododendron rex subsp. fictolacteum (Balf. f.) D.F. Chamb..
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 12305021) with original publication details: Bull. Soc. Bot. France 34: 280 1887.
This name was in version 1 of The Plant List, record tro-12305021, and has not been changed.