Paxillus rubicundulus

Also known as Paxillus filamentosus, strictly associated with alders (Alnus) along riverbanks and riparian zones, with a convex to flattened, occasionally slightly depressed pileus 4 to 14 cm across, yellowish-brown, mottled or scaly-fibrillose, decurrent, forked yellowish-brown lamellae that bruise darker brown, a solid, cylindrical pale brown stipe with whitish fibrils, and flesh with a slightly bitter taste.

Pileipellis composed of cylindrical hyphae arranged more or less parallel to slightly interwoven, 4 to 8 µm wide, containing brown intracellular pigment, septate with clamp connections.

Cheilocystidia and pleurocystidia fusiform to lageniform with an elongated neck, 35 to 60 µm long, 5 to 10 µm wide, occasionally containing brown intracellular pigment.

Lamellar trama regular to parallel.

Basidia clavate, 30 to 40 µm long, 8 to 10 µm wide, clamped at the base, 2- or 4-spored.

Spores ellipsoidal, thick- and smooth-walled, pale yellowish-brown under the microscope, 5.5 to 7.5 µm long, 3.8 to 4.5 µm wide, with an eccentric hyaline apiculus at one pole, inamyloid in Melzer's reagent.