Do you think all those little brown mushrooms on the ground look similar and are hard to identify? Well, mycologists agree with you! That's why they're referred to as "LBMs".
LBM isn't a scientific term for genus or species. Rather, it's a category used to lump together all small to medium brownish mushrooms.
Brownish is a relative term as some drab yellow, white, or gray mushrooms are considered LBMs. There are hundreds of species out there fitting this description, and many of them are extremely difficult to identify outside of a lab.
Little brown mushrooms are found everywhere, in all seasons, and in all habitats. If you look closely outdoors, you'll probably see more LBMs than you ever noticed before!
Many LBMs are poisonous mushrooms and will cause nausea or gastric distress. However, some contain the same deadly amatoxins as certain amanitas such as the death cap.
The most notable and deadly of these are species in the Galerina genus. They may not appear dangerous, but some of these little specimens can kill! Thus it's worth examining the deadly Galerina mushrooms in more detail.
Not all members of the Galerina genus are poisonous mushrooms, but many of them are. Within the past decade some of the more infamous galerina mushrooms were thought to be separate species. Genetic testing has since proved them all the same.
Thus old species such as Galerina autumnalis (the deadly skullcap), Galerina oregonensis, Galerina unicolor, and Galerina venenata now all have the same name: Galerina marginata.
So it's Galerina marginata that could ruin your day. It's a classic little brown mushroom, making it very difficult to identify. Let's learn a little more:
The deadly galerina is especially dangerous because it is sometimes mistaken for edible mushrooms. People have been known to confuse it with the honey mushroom (Armillaria mellea), enokitake (Flammulina velutipes), or the hallucinogenic Psilocybe cubensis.
You can see why it's extremely important to know your poisonous mushrooms when hunting for common species. A mistake when trying to identify a little brown mushroom could land you in the hospital, or at least make for a very unpleasant evening in the bathroom.
At the end of the day, it's easy to avoid LBM poisoning: never eat them! When mushroom hunting, stay away from all small to medium brownish species. If it's not easily identifiable, it shouldn't be on the table!
The second picture was taken by Lebrac and is published on Wikipedia under the GNU Free Documentation License.
The last picture was taken by Dan Molter and is published on Wikipedia under the Creative Commons Attribution-Share Alike 3.0 Unported License.