Name and species circumscription
Austrocactus diamantinus is the name accepted in Plants of the World Online, checked on 10 October 2026. Elisabeth and Norbert Sarnes described the species in the 2025 revision of the genus. [2-WWW] [Böhnert et al. 2025, taxonomic record and species description in the 2025 revision]
Branched stems and areoles
The stems grow vertically, are shortly cylindrical to club-shaped and reach 20 cm in length with a diameter of 4–5 cm. They are dark green. Around ten ribs divide into blunt tubercles. Oval areoles, woolly on young growth, measure around 4 × 7 mm. [Böhnert et al. 2025, p. 397]
Four central spines are strongly hairy. One projects forwards and is curved, one points upwards and two spread horizontally. The remaining central spines are straight. Around nine smooth radial spines lie in pairs on either side of the areole, with one pointing downwards. The contrast in hairiness between the two spine groups is a useful descriptive feature. [Böhnert et al. 2025, p. 397]
The flower and dry fruit
The funnel-shaped flower measures around 4.5 cm long and 4 cm in diameter. The spatulate tepals end in a short point. On the inside, their base is pale yellow and their upper part brownish pink to red. The style is dark red to purple, while the stigma has 12–14 dark purple lobes. The ovary chamber is oval, around 8 × 6 mm. [Böhnert et al. 2025, p. 397, 399]
The pericarpel is dark green, with little white wool and long white bristles. The almost spherical fruit is around 15 mm in diameter. It remains dark green, dries and opens irregularly from the base. The almost black seeds are curved and flattened, around 2.5 × 2 mm. [Böhnert et al. 2025, p. 399]
Distribution and closest relatives
The sites described in the revision lie in Pampa del Diamante, in the San Carlos and San Rafael departments of Mendoza Province. The type material came from Cuadro Benegas. The species fell within a group of local, geographically separated endemics, together with A. ferrarii, A. philippii and A. praecox. [Böhnert et al. 2025, p. 397, 399]
In both tree-reconstruction methods used, A. diamantinus formed the closest pair with A. ferrarii, with high, though not maximum, support. The species differ in features including growth form. A. diamantinus has upright, branched stems, while A. ferrarii has small stems forming low clumps. They also differ in perianth shape and stigma-lobe number. [Böhnert et al. 2025, p. 397, 399]