While it is true that you've got to be very careful to avoid Chlorophyllum
molybdites when eating large lepiotas in most of the world, the original
question came from the UK (if I remember right). Though Chlorophyllum has
been found in association with exotic potted plants in Scotland (R.
Watling. Mycologist 1991;5;78-9) and under a palm tree in a health spa in
the Netherlands (E. Vellinga. Coolia 1990; 33:78-9), it has yet to be
reported as part of normal British (or W. European) flora. If you have a
Macrolepiota with a green sporeprint in the UK, write a paper about it!
By the way, according to A Bresinsky and H Besl, Colour Atlas of Poisonous
Fungi, there is a poisonous look alike which seems to be a "garden form"
of L. rhacodes. This was described as L. rhacodes var. hortensis which
might also have been misidentified as a a new species Macrolepiota
venenata. I don't know details of that literature and it seems there is a
bit of disagreement about relationships of these species. And: remember,
several smaller Lepiota species contain alpha-amanitin.
In my view, M. rhacodes is one of the most delicious of all mushrooms.
Medical College of Ohio at Toledo
lehmann at opus.mco.edu