michael adams wrote:
http://www.mushroomexpert.com/fairy_rings.html

Thank you - wonderful site. I was interested with the quote "What
triggers mushroom production is rather up for debate. Studies indicate
that many fungi--the morels, for example--produce fruit bodies when
they face nutrient deprivation, sort of as a survival strategy: "I'm
done for, so I'll make a mushroom that is covered with spores, lift it
up above the ground, and let the wind take my babies to a new home."
This explanation doesn't seem to fit the mushrooms that make fairy
rings, however, since they have not run out of food." Could it be that
in some very poor lawn (urban gardens), the survival instinct kicks in?

All this made me terribly hungry ....