This name is a synonym of Rhododendron uniflorum var. imperator (Hutch. & Kingdon-Ward) Cullen.
The record derives from Tropicos
(data supplied on 2012-04-18)
which reports it as a synonym (record 50262788) with original publication details: Sp. Rhodod. 440 1930.
Full publication details for this name can be found in IPNI: http://ipni.org/urn:lsid:ipni.org:names:332619-1.
This name was in version 1 of The Plant List, record tro-50262788, and has not been changed.