#042: Mushroom Morphology: Earthballs
This group of mushrooms goes by a variety of common names, including “earthballs,” “earth balls,” and “false puffballs.” Additionally, all of these mushrooms belong to the family Sclerodermataceae and could casually be referred to as “sclerodermas.”* Using these terms to distinguish earthballs from puffballs is a fairly recent development, so many earthballs are still commonly called puffballs. Visually, immature earth balls can look similar to puffballs. If you are collecting puffballs to eat, make sure you know how to tell the difference between true puffballs and the usually poisonous earthballs. The main difference between earthballs and puffballs is that, unlike the puffballs, earthballs do not release their spores through a regular pore. Instead, the peridium (outer layer) cracks and tears irregularly. Like all gasteromycetes (fungi whose spores mature internally), earthballs have lost the ability to forcibly discharge their spores. The spores mature in the center of the earthball, enclosed by the peridium. At maturity, the peridium tears irregularly to reveal the mass of powdery spores. The spores are then slowly dispersed by the wind.
The earthballs also share a few other physical characteristics. If you slice an earthball in half, you will easily be able to tell that it is indeed an earthball. The gleba (interior, spore-forming tissue) starts out white but soon turns purple to purple-brown. If your mushroom has a solid white or olive-colored gleba, then you probably have a puffball. The gleba of earthballs is also divided up into small, spore-forming compartments called locules. Locules are not found in any true puffball species. Additionally, the gleba of earthballs remains firm until the peridium tears. Finally, earthballs have spores that feature spikes, ridges (reticulation), or a combination of the two.
Scleroderma citrinum, commonly known as the “poison pigskin puffball,” is a good example of the classic earthball. It makes small to medium mushrooms (3-6cm or sometimes -12cm in diameter) that have a brown to yellow-brown exterior with darker-colored warts. When you slice S. citrinum open, you reveal the firm, dark purple gleba. If you look closely, you can see that the gleba is not a solid color but is riddled with white lines separating the locules. Pisolithus arrhizus, commonly known as the “dyemaker’s puffball,” offers a better view of its locules. When you cut open the brown mushrooms you reveal a number of layers. The upper layer consists of a mass of brown spores. Below the brown spores are black locules containing immature spores. Near the base of the mushroom are newly-formed white/yellow locules. Although P. arrhizus starts out roughly spherical, the older mushrooms can become elongated and much larger. At this point the top of the mushroom becomes a powdery mess, making the entire thing look like dog poop, a giant, brown tooth, or a medium-sized tree stump.
There are two exceptions to the normal earthball morphology. The first of these is the stalked puffball genus Calostoma, which includes C. cinnabarinum. These odd-looking stalked puffballs have a gelatinous outer covering that falls off to allow spore dispersal through an ostiole as in traditional puffballs. Although it sounds like it belongs with the puffballs, recent genetic evidence has placed this genus in the Sclerodermataceae. The second exception is Astraeus hygrometricus, which is commonly known as the “false earthstar” or the “barometer earthstar.” This earthball has an exoperidium that tears into star-like “arms” or “points” and it retains its endoperidium, making it look just like an earth star. A ragged pore is formed at the center of the endoperidium, which can eventually tear further. The ragged pore and checkered arms are the main external features that separate this mushroom from the earth stars. Unlike the earth stars, A. hygrometricus has a peridium that consists of only two layers and a gleba that is less organized and which lacks a columella. The barometer earthstar is so named because its arms are hygroscopic, meaning that they easily absorb water from the air. As a result, they open when then get wet and close when they dry out. When the arms are closed the mushroom is protected from damage. When the arms are open, the mushroom is free to release its spores in the wet weather. The open arms also hold the mushroom up above the ground, which increases air flow around the mushroom and aids in spore dispersal.
All earthballs belong to the family Sclerodermataceae except for Astraeus hygrometricus, which belongs to the family Diplocystaceae. Both of these families are found in the phylum Basidiomycota, class Agaricomycetes, and order Boletales. The families Sclerodermataceae and Diplocystaceae are grouped together in the suborder Sclerodermatineae along with two other families that produce bolete mushrooms. The earthballs are, in fact, closely related to the boletes. Like most boletes, all earthballs are mycorrhizal. This is another factor that separates earthballs from puffballs, which are all saprobic. The earthball P. arrhizus is exceptionally good at forming mycorrhizae, especially in poor soil conditions. Because of this, P. arrhizus is often used in tree nurseries to help young saplings grow. It is also used during reforestation efforts to help the recently planted trees survive in their new environment. Believe it or not, some people also use the fungus’s ugly fruiting bodies as a source of dye for wool, hence the common name “dyemaker’s puffball.” Apparently it results in a reddish-brown to black color. As for edibility, no false puffballs should be eaten. All mushrooms in the genus Scleroderma are poisonous. They contain toxins that are gastrointestinal irritants, so they won’t kill you but they will give you the symptoms of food poisoning. Other earthballs are tough and considered inedible.
* Scleroderma mushrooms should not be confused with the medical condition also named scleroderma. Both the mushroom and the disease were named for the characteristic hard skin (sclero = hard and derma = skin). For mushrooms, this “hard skin” separates the earthballs from the softer-skinned puffballs. The disease, however, is an autoimmune disease characterized by a hardening of the skin and connective tissues.