We see all sorts of “strictly MR” mushrooms growing on well-rotted wood here in CA. Still, it is surprising when you find one. Darned mushrooms don’t play by our rules!

I’ve seen this a couple of times and for several different species of Amanita. I think that when wood is very rotten, it is essentially very carbon-rich soil. The mycelium of an amanita or a russula, etc. spreads into very rotten wood and primordia form and grow just as in soil with higher mineral content.

Very best,

R