Collection location: Howarth Park, Santa Rosa, Sonoma Co., California, USA [Click for map]
I’m totally stumped on these guys. The first one pictured has a pileus width of 11 cm. The stipe is 8 cm from apex to ground. They’re growing under douglas fir, a three needle pine and madrone and there’s a group of them all in a concentrated area. They start out a coppery brown and were viscid. Now they’re slightly sticky, mostly dry. On the newest ones the gills and stipe are a warm grey. As they age the pileus becomes a mahogany brown and then black. The stipe and gills seem to get more grey and the gills eventually become black, but it takes longer than the pileus.
Given the staining reaction you described, it would be one of R. nigricans, R. densifolia or R. dissmulans. The photos are good match to the one in Agaricales of California for R. nigricans. It also looks like it has a viscid cap which eliminates R. dissimulans. R. densifolia has a thick cap cuticle (> 150µm) and close or crowded lamellae. From the photos I wouldn’t call the lamellae close or crowded, so without looking at it with a scope, I’d say R. nigricans.
Nathan, the pileus stained red instantly. I checked it two hours later, at the end of my walk, and it was black.
This is one of the blackening Russulas. I’d have to know how it stains to put a species name on it. Some go straight to black or gray, others stain reddish at first and then later turn to black. The stain reactions can be very slow (10-20mins).