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The Yellow Stainer: A Mushroom That Visibly Changes Colors!

Updated: Oct 17, 2023

A few days ago I was in my kitchen washing dishes when I glanced out my window and spied several curiously familiar looking white spheres poking out of the grass two stories below. We keep our garbage cans in that particular spot, where the gravel from our driveway meets a perimeter line of lawn. It is an area that gets a lot of moisture (it is often very wet here on the coast!) and it rests just a few feet from a small grove of trees. I knew right away what they must be--we've had an increase in precipitation lately, and with the fall months well underway, it's safe to say that mushroom season has officially begun!

Could they be shrooms?

When I finished with dishes I ran down to check.

YEP, they were shrooms!

It took a few days for them to develop to a size large enough for me to feel that I had an adequate selection to experiment upon (for science, of course!), but once they were ready for my research session, it didn't take me long to detect what they were...

There are several different types of mushrooms that will change color surprisingly abruptly when the flesh is scratched, cut, or outwardly marred in any way. These mushrooms include some boletes, psychedelics, and members of the Agaricus genus--the latter of which happened to be the very mushrooms we had growing in our yard!

They were Agaricus mushrooms: Agaricus xanthodermus, or the "Yellow Stainer."

Why is it called this mushroom called the yellow-stainer?

Well, this particular mushroom turns YELLOW when the surface is compromised! Check it out:

From left to right:

Photo 1:

Two "Yellow-Stainer" mushrooms, untouched.

Photo 2:

The same two "Yellow-Stainer" mushrooms after being scratched by a rock (see the video, below).

Photo 3:

A maturing "Yellow-Stainer" mushroom, untouched, with gills showing.

Here is a video of the "staining" phenomenon in action (this is how I created photo #2, above)!:

Identifying these initially unremarkable-looking white mushrooms may seem challenging at first glance--but the yellowing of flesh that occurs with scarring is quite the definitive give away. Often growing in clusters and large troops, they start as pure to dull white rounded buttons and develop 3-15cm caps (with pink gills) that become convex to flat with age. These mushrooms have a fragile partial veil ("skirt") and a dark chocolate brown spore deposit. They are most abundant in fall and spring and can be found in a variety of environments, but most commonly in urban habitats such as grass lawns, gardens, wood chips and even gravel. Prior to the 1960s, this species was not well documented in California, suggesting that it was introduced and is now invasive.

Unfortunately, this fungus is toxic to dogs (and humans!)--and since a fuzzy friend lives with us on our property, we decided to pluck these little lovelies from the ground for safety purposes. Most of the mushies were still closed up, with their gills tightly enclosed, but a select few really showcased how gorgeous these fungi can become underneath. Check out the gill structure! Aren't they just lovely? I'm a sucker for thick pink gills under a white cap any day! The gills of this particular fungus will slowly age into a more brown color.


Mushrooms of the Redwood Coast by Noah Siegel and Christian Schwartz (2016):

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