You may have learned this mushroom as Armillaria ponderosa, but it is certainly not an Armillaria. The genus Armillaria was once a taxonomic refugium for almost any white spored mushroom with attached gills and an annulus. Now typified by Armillaria mellea, the genus Armillaria is now restricted to white spored wood-decay species with attached gills and black rhizomorphs, regardless of whether there is a ring on the stalk. Any mushroom with deviating characters does not belong in Armillaria. The 263 or so species once placed in Armillaria are now better placed in 43 other modern genera. For a list of the 30 or so species currently accepted in Armillaria click here. The modern genera of mushrooms are meant to reflect a set of characteristics about the species in that genus. One important character that makes Armillaria ponderosa an incorrect name is that the fungus is mycorrhizal, not a wood-decay fungus, as Armillaria is defined. As you will recall, mycorrhizal fungi form a mutualistically beneficial relationship with the roots of photosynthetic plants. This particular species is near impossible to grow int he laboratory without trees and has not to my knowledge been successfully fruited in culture.