Polish forests: Distinguishing between ‘witch butter’ fungus and slime mold

The common name ‘witch butter’ (masło wiedźmy) found in Polish forests can refer to two distinct organisms: the orange jelly fungus (Tremella mesenterica) and the slime mold Fuligo septica. While both appear as bright yellow or orange masses on wood, they belong to different biological groups.

Tremella mesenterica is a fungus that forms gelatinous, folded fruiting bodies. It does not feed directly on wood but parasitizes the mycelium of other fungi growing within the wood. It can be found year-round, particularly during damp autumn and winter periods.

In contrast, Fuligo septica is a slime mold classified as a protist rather than a fungus. Its appearance changes during development, starting as a bright yellow, soft, and foamy mass before darkening and hardening into a spore-containing mass. Distinguishing between the two is important for safe mushroom foraging.