Description from Flora of China
Rhododendron ioanthum I. B. Balfour; R. jahandiezii H. Léveillé; R. leucandrum H. Léveillé; R. obscurum Franchet ex I. B. Balfour (1922); R. rubropunctatum H. Léveillé & Vaniot (1911), not Hayata (1913), nor T. L. Ming (1984).
Shrubs, 1–2(–4) m tall; young shoots brown, densely scaly. Petiole 5–15 mm, densely scaly; leaf blade elliptic or elliptic-lanceolate, 3–7(–11) × 1.2–3.5 cm; base cuneate or rounded; apex acuminate, acute or nearly obtuse; abaxial surface scales 0.5–2 × their own diameter apart, or contiguous small to mid-sized, all similar or slightly unequal, brown, concave; adaxial surface scales dense, concave, glabrous or rarely pubescent along midrib. Inflorescence terminal or subterminal, shortly racemose, 3–5-flowered; rachis 2–4 mm. Pedicel 0.3–1.3 cm, scaly; calyx rimlike or undulate and 5-lobed, densely scaly; corolla tubular funnelform-campanulate, white, pale red, pale purple or rosy red, inside with yellowish green, pale red or pinkish spots or without spots, 1.6–3 cm, outer surface not scaly or sparsely scaly on lobes; stamens unequal, long ones longer than corolla; filaments pubescent or nearly glabrous at base; ovary 5-locular, densely scaly; style longer than corolla, usually glabrous, rarely pubescent at base. Capsule cylindric, 11–16 mm. Fl. Mar–Jun.
Rhododendron seguinii H. Léveillé (Repert. Spec. Nov. Regni Veg. 13: 148. 1914) was described from Guizhou. The identity of the type specimen, J. Cavalerie 1254, is in doubt as it totally lacks leaves. However, there is indirect evidence that it is related to, or synonymous with, R. siderophyllum (see Cullen, Notes Roy. Bot. Gard. Edinburgh 39: 65, 67. 1980).
Mixed forests, coniferous forests on slopes, thickets; (1200–) 1800–3000 m. Guizhou, Sichuan, Yunnan.