Callistosporium hesleri is an exceptionally rare mushroom. It was first collected and described in 1983, and wasn't found again until a soil DNA study found the mycelium in a Pine plantation in South Carolina in 2014. As of August 2024, has been found a total of three times on iNaturalist, twice in Indiana and once in Nebraska.

Hesler's Callistosporium grows on soil and decayed wood and can be found spring through late summer. It is a small, agaric with distant, notched, gray-colored gills. Not much is known about this mushroom due to its rarity.

August 17th, 2023 Field Notes

Growing alone on downed ash tree (well decayed) in open (but well shaded) mixed oak/hickory woodland. North-facing slope.

• Smell: faint
• Cap: convex to flattening
• Stipe: strong, flexible, fibrillose
• Lamellae: notched