BRIEF DESCRIPTION: The cap of A. merxmuelleri is 65 - 150 mm wide, gray-brown to porphyry-brown, dull or subshiny, viscid in humid conditions, hemispheric to convex when young, later campanulate to planar with a rather depressed disc, with a nonstriate margin. The margin is decurved or slightly incurved at first and sometimes bears hanging parts of the volva. The flesh is whitish. The volva is present as irregularly distributed warts and small patches and are polygonal, clod-like, thicker over the disc, thinner near the margin, reddish-brown, cinnamon to ochre-brown, sometimes gray-brown.

The gills are free, crowded, whitish to cream, becoming pale leather color, with the edge colorless at first then becoming gray-brown to chestnut-brown. Short gills are present.

The stem is 75 - 150 × 18 - 45 mm, narrowing upward, exannulate,  with a mixture of lilac-tinted colors when quite young, later whitish at apex and having a whitish ground below decorated with gray-brown to porphyry-brown to lilac-brown to wine-colored streaks or squamules. The bulb is white, stuffed rather firmly, becoming slightly brownish after exposure to air, pronounced in young material, but reduced to a swollen base of then-clavate stem or only slightly broader than the stem in mature material. The flesh is whitish, mostly unchanging on exposure to air, but faintly browning in the base. The volva is present as a single ring of fine warts at the meeting point of stem and bulb and is concolorous with volva on the cap.

The odor is similar that that of raw potatoes, and the taste is reported as similar to asparagus.

The spores measure (10.1-) 10.6 - 14.0 (-15.0) × (6.5-) 6.8 - 9.0 (-10.1) µm and are ellipsoid to elongate, infrequently broadly ellipsoid and inamyloid. Clamps are common at the bases of basidia.

The following have all been shown to be absent from A. merxmuelleri: amanitins, phalloidin, muscimol, muscarin.

A. merxmuelleri is present in association with Nothofagus pumilio.

The present species seems so close to the following that the names may be taxonomic synonyms: Amanita ushuaiensis (Raithelh.) Raithelh. and Amanita grauiana Garrido.

According to Dr. Rolf Singer's notebooks (Field Natur. Hist. Mus.) he collected material probably of this species on which crassospores were plentiful. These unusual, malformed spores with a surface suggesting the dimple on a golf ball have been noted in other taxa collected in certain regions of the Argentine Andes (A. morenoi Raithelh. and A. pseudospreta Raithelh.).

This species may have been reported under various other names in the past including A. umbrinolutea in the sense of Raithelhuber.