Ah, the colors of autumn! Greens fading to yellow, vibrant oranges, rusty reds, tawny browns… and vibrant, deep blues.

That is, if you’re lucky enough to stumble on a specimen of Lactarius indigo, the “blue milk mushroom”.

We were back in Baker Creek, the same woods preserve where we sighted that amazing raspberry slime mold this summer, when suddenly the leaf litter flashed blue. I could hardly believe it – Lactarius indigo has been on my list for ages, and though they are native to eastern North America, it had never occurred to me that Tennessee might host some. (After going to college in Boston, I’m still getting used to the idea that Tennessee – where it almost never snows – is also in “the east”. Geography has never been my strong suit.) But there was an uprooted cap right there for me to pick up and verify – yep, bright blue mushroom!

As a general rule, I don’t harvest or cut mushrooms in nature preserves, so it was a lucky thing that some trick of weather or cyclists had snapped this Lactarius off its stalk and exposed its incredible color to the sunlight. Once we saw this baby on the path, it was an easy thing to find its home patch and the other specimens there.

The cut specimen had faded a bit by the time we found it, but you can still see its striking color on the outer edge of the cap. We can also see that the stem was starting to hollow out, which can happen to L. indigo as they develop. Here’s another young L. indigo, still growing. Note the vivid sky-blue color of the cap and the dark blue gills. As the specimen gets older, its cap will unfurl, making the mushroom roughly funnel-shaped and fully exposing the gill surface.

The cap is also “zonated”, which means you can clearly see some concentric rings coming out from the center of the cap. This is another characteristic of Lactarius indigo.

But was it really Lactarius indigo, or just another blue mushroom? They are rare, but they do exist, and I didn’t have a guide on me. Well, there’s one way to know for sure. You may recognize that the genus name “Lactarius” is similar to the word “lactose”, the sugar found in milk. That’s no accident – Lactarius mushrooms are so named because they produce a milky substance. That’s the most diagnostic feature of Lactarius indigo – when the gills are slashed, it produces a stunning indigo liquid. So I brought out my house key and sliced through a few gills…

There’s our confirmation – these were definitely Lactarius indigo! I get to check these fabulous blue mushrooms off my basket list (which is like a bucket list, but for mushrooms). But why do they have this milky trait?

Lactarius mushrooms are filled with lots of spherical, thin-walled cells called sphaerocysts (sfeer-o-sists) that make their flesh far more brittle than other mushrooms, which are made with cylindrical hyphae. Imagine breaking apart the cap of a button mushroom: the long hyphal filaments make it fibrous, so it tears in chunks. By contrast, a mushroom in the Lactarius genus (or the closely related Russula genus) will snap like chalk. I didn’t take a video doing this to the L. indigo I found, which I’m kicking myself for now. Next time I find a Lactarius mushroom I promise I will take a video showing how easily they snap!

The sphaerocysts make the flesh of the mushroom brittle, but they don’t contain the milky substance. That honor goes to thick, specialized hyphae called lactifers. I don’t think anyone really knows the purpose of the latex, but I’ve seen the theory that it deters herbivores by gumming up their mouths and making it hard to keep eating (1). Whether or not that’s the intended benefit, the latex (especially bright blue latex) makes Lactarius mushrooms easy to identify and extra exciting to find!

Cited sources

1. Roehl, Thomas. “#176. The Genus Lactarius.” Fungus Fact Friday. https://www.fungusfactfriday.com/176-lactarius/. Accessed 12 Oct 2021.