This slimy, purple mushroom belongs to the genus Cortinarius, the largest genus of mushrooms in the world. Mushrooms in this genus have partial veils, or cortinas – tissue that covers and protects the spore-producing gills, and they also have a rusty brown spore print and mature gills.
While it is relatively simple to determine that a mushroom is in this genus, identifying one down to species can be difficult. Two identical-looking species, C. iodes and C. iodeoides, can be found in the Northeast – both are purple and have slimy caps. Mycologists distinguish them by the size of their spores. For those more daring than I, there is a licking/taste test — the slime on C. iodeoides is said to be more bitter tasting than that of C. iodes.
Both species are mycorrhizal with oaks, in that both benefit from an association with each other. The mushroom helps the tree absorb water and nutrients while the tree provides sugars and amino acids to the mushroom. It is estimated that about 85% of plants depend on mycorrhizal relationships with fungi.