This report indicates that approximately 25% of all Rhododendron taxa are under threat of extinction in the wild. General habitat clearance is a major threat, as is (increasingly) the impacts of climate change.
Available to download as a PDF.
Gibbs, D., Chamberlain, D. and Argent, G. 2011. The Red List of Rhododendrons. Botanic Gardens Conservation International, Richmond, UK.