key cited is not inclusive. While it mentions a description of C. rhacodes, it is not included in the key, and therefore is exclusive for that species.
Many keys are that way. Take a look at the keys Arora offers in Mushrooms Demystified, for example. At least Pacific Key Council mentions both species, as well as C. agaricoides and C. aeruginosa, which many keys totally ignore. In their defense, at least the species are mentioned with descriptions.
I mention this because I believe from reading your notes that you may be becoming one of those few, Briney. I hope so.
It is possible that only C. brunneum is associated with S. giganteum. The only Chlorophyllum I find in my area with S. giganteum is C. brunneum. But C. brunneum I have also found with Pinus, Tsuga and Deodor cedar in a neighbor’s lawn. Such data makes key creation so difficult as to be unmanageable. Many key makers (and mushroom finders) ignore the plant species nearby. This is unfortunate.
Good notes for collections must include nearby trees and shrubs. If the fungus is obviously growing from leaf litter, such as S. giganteum, it should be noted in the observation. Just my 2 cents worth. Many many mushrooms collectors do not know what nearby tree and shrub species are, thus relegating species specificity to the few, the detailed, the … retentive. (Like me?)
Without such detailed information, knowledge that certain fungi are species specific would still be unknown.
An example: the original collection of Tuber gibbosum by H. H. Harkness noted it was found in a “mixed stand” of conifers and hardwoods a few miles from San Francisco. It wasn’t for another 70 years that someone noticed it seemed to be more common under Douglas-fir stands. Another 10 years passed before finding abundant collections under pure Douglas-fir. T. gibbosum is now known to be species specific with Douglas-fir. It’s only taken 130 years or so.
Why is that important? Other Tuber species are also found with oak, hazelnut, oceanspray, Cascara, chestnut, basswood, and other hardwoods; and still other species are commonly found with Pinus species. To rule out other mycorrhizal host species, multiple collections with detailed host species nearby have to be made over time, and the collections preserved for the future in herbaria.
Tedious and time consuming? You betcha! Scientific knowledge is often that way though. Curious too in that S. giganteum at the University of Washington is WAY out of it’s naturally occurring area. Maybe C. brunneum is being spread along with ornamental plants, in the same way Amanita phalloides seems to be spreading along with chestnut planting up the West Coast.