Collection location: Madison Heights, Pasadena, California, USA [Click for map]
Based on all available info I could get access to in Arora’s book, mykoweb.com, etc.
This one’s been stumping me for ages. It can’t be A. campestris because of the K+y reaction, and the gills are white in button stage. For some reason no one wants to compare A. californicus with A. arvensis. The various descriptions just don’t make the differences clear enough for someone more comfortable with nice neat little flowers. But after watching them fruit all over Pasadena for half a year now, it is clear that this species rarely exceeds about 8 to 10 cm, suggesting A. californicus. How one interprets the alacrity of the K+ reaction must be a matter of experience. (Compared to lichen spot tests, it is downright faint and sluggish!) My spores are rather small, also suggesting A. californicus. The cap surface is highly variable, but again, after watching them for months, the “median” surface is very clearly brownish, appressed, radiating, long-fibrillose like the ones I’ve photographed here. (But note the almost scaly button growing in the exact same conditions.)
Short of actually finally seeing the other species, I doubt I will ever feel confident with this ID.
…the semi-crumpled stipe is typical of this species. The primary way to tell A. californicus from A. arvensis is the smell – unpleasant/phenolic in A. californicus, sweet/almondy in A. arvensis. In addition, A. arvensis will stain yellow and fade at a dark yellow/orange. A. californicus always goes to brown.