Description from Flora of China
Rhododendron rex H. Léveillé subsp. arizelum (I. B. Balfour & Forrest) D. F. Chamberlain.
Shrubs or small trees, 3–7 m tall; branchlets stout, rough, usually ca. 10 mm in diam.; young shoots pale brown tomentose, glabrescent. Petiole terete, 20–30 mm, tomentose, glabrescent; leaf blade leathery, obovate or obovate-elliptic, broadest above middle of blade, 9–19 × 4–8 cm; base cuneate; apex rounded, emarginate; abaxial surface with pale brown or rufous, 2-layered indumentum, upper layer brown, hairs narrowly cup-shaped, strongly fimbriate, lower layer thin, compacted, gray; adaxial surface deep green, glabrous, midrib prominent abaxially, flat or slightly impressed adaxially, lateral veins 10–13-paired, prominent abaxially. Inflorescence 15–20-flowered; rachis 15–25 mm, densely tomentose. Pedicel 1.2–2 cm, densely tomentose; calyx oblique; teeth 8, ca. 1 mm; corolla campanulate, white, pale yellow or pink, with crimson basal blotch, 3.5–4.5 cm, lobes 8, suborbicular, ca. 1 × 1.5 cm, emarginate; stamens 16, 1.5–2.5 cm, filaments glabrous or puberulent at base; ovary terete, densely tomentose; style glabrous. Capsule 30–40 × ca. 10 mm, tomentose. Fl. May–Jun, fr. Aug–Sep.
Rhododendron arizelum var. rubicosum Cowan & Davidian (Rhododendron Year book 8: 79. 1953), described from NW Yunnan, is a rich red- to purple-flowered form, which may be intermediate between R. arizelum and R. rex subsp. fictolacteum.
Pinus forests, Rhododendron thickets; 2500–4000 m. SE Xizang, W Yunnan [NE Myanmar].