The Rusty Pinwheel is a small decomposer that can be found in duff and leaf litter from summer through fall east of the Rocky Mountains. Macroscopically, it seems indistinguishable from Marasmius siccus and microscopic analysis is needed to separate the species.

The cap is pleated and shaped like a pinwheel.

The gills are white and distant.

The stems of Marasmius mushrooms are elastic. This one has a dark to black stem and white basal mycelium at the point of attachment to the substrate.

July 7th, 2023 Field Notes - Indian Cave State Park

Growing gregariously (abundant) among oak woodland duff in a low riparian woodland area near a creek. Oak trees on the slope above.

• Smell: faintly foul
• KOH: pileipellis: dull yellow (spicy or dijon mustard yellow) white mycelium; stipe: darkens black, whitens the orange.
• Taste: mushroomy phenolic aging into the sensation of back pepper.