Pisolithus arrhizus: The Fungus That Signals Autumn

Late summer brings with it the unmistakable sight of crumbly dog turds along the sidewalk, a sure sign that autumn is on its way. Yes, you read that right – dog turds as harbingers of our favorite season! In the midst of California’s rainless summer, when moisture is scarce, a fascinating phenomenon takes place. A mushroom called Pisolithus arrhizus manages to grow and form firm, juicy fruit bodies that push through the pavement and pop up in unexpected places to release their spores.

Ugly but Fascinating

These mushrooms may not win any beauty contests, but they certainly capture our attention. I recall a foray in Denmark where these peculiar fungi were sticking out of a dry sandy pine plantation. We all admired their ugliness, but no one wanted to be photographed with them. Officially known as Pisolithus arrhizus, this fungus goes by many names. It gets its peculiar moniker, “rootless pea-stone,” from its Greek origin. Other common names include Dead man’s foot and Dye ball. The genus itself has had various names over the years, like Polysaccum, which translates to “the mushroom with many bags.” Its species name, tinctorius, alludes to its historical use as a dye for wool, dating back to at least 1729.

A Mushroom of Many Phases

Pisolithus starts as a club-shaped dark brown object, revealing compartments full of spores when cut open. Arora fittingly described them as “rice crispies in tar.” At this stage, the mushroom is firm, wet, and stains our hands. As it matures, the outer wall disintegrates, giving way to a dark chocolate brown, dry spore mass. These spores are perfectly adapted for airborne dispersal, with pigmented hydrophobic walls and spiny surfaces.

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A Diverse Species

Pisolithus exhibits a wide range of shapes and sizes. From small, round ones to enormous, amorphous lumps, the question arises: do these forms represent different stages of one species or multiple species with their own unique hosts? Recent molecular research suggests the latter. What was once thought to be just one species has now been identified as at least eleven distinct types, with several additional species described. Some of these types are found exclusively with specific host plants, such as Cistus in the Mediterranean basin or Afzelia in Africa. Part of the genus is native to Australia, growing with Eucalyptus and Acacia, but it can now be found far from its original habitat wherever Eucalyptus has been planted. Pisolithus albus, one such Eucalyptus symbiont, has been discovered in Spain and likely occurs in California as well.

Fascinating Associations

Interestingly, the Pisolithus found in different parts of the world have unique associations with their host plants. In the Northern Hemisphere, P. arrhizus does not associate with Eucalyptus, whereas in the southern Hemisphere, it grows alongside introduced pine species. Australian researchers were surprised to find that Eucalyptus planted in China formed mutualistic associations with a local Pisolithus species that proved detrimental to the tree’s growth. To address this, different Pisolithus species were introduced to promote faster and healthier tree growth.

A Partner for Tree Seedlings

Pisolithus is not just an ordinary mushroom; it plays a crucial role in the initial inoculation of tree seedlings, particularly in forestry. Its ability to thrive in drought conditions makes it an ideal candidate for this purpose. Fungi Perfecti, a renowned brand in mycology, sells a mycorrhizal mix that includes Pisolithus tinctorius. This mix, along with four species of Rhizopogon, aids in the establishment of young tree seedlings in areas such as mine tailings, dry sandy regions, and restoration projects. Pisolithus has proven to be a strong competitor, allowing it to coexist with fully grown trees without being rapidly displaced by other fungal mutualists.

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Common and Widespread

Pisolithus arrhizus is widespread and commonly found, both in urban areas and natural habitats under oak trees. Many people report finding it in their yards during the fungus fair. It is especially prevalent in the northern Sierras and Lassen area, particularly in disturbed areas, foothill woodlands, and open oak woods. As this species fruits during dry periods when other mushrooms are scarce, it can easily be overlooked during typical mushroom forays. Despite this, it has been recorded in 36 states across the United States. Many specimens have been found in dry and disturbed areas, ranging from open fields to sand dunes, often without any apparent tree hosts.

Uncovering the Family Tree

Pisolithus’s appearance doesn’t give away its closest relatives, but the presence of pulvinic acids and their derivatives hint at a connection to boletes, the pigments responsible for staining wool. Molecular comparisons have confirmed this link. Astraeus hygrometricus, another drought-adapted “bolete,” is believed to be the closest relative to Pisolithus. Scleroderma and true boletes like Gyroporus and Boletinellus merulioides are also distantly related.

So next time you come across a Pisolithus arrhizus mushroom, take a moment to appreciate the fascinating association it has with tree roots and the role it plays in the ecosystem. Autumn is on its way, and these unique fungi serve as a friendly reminder that the changing seasons bring new wonders to behold.

Further reading:

  • Binder, M., & A. Bresinsky, 2002. Derivation of a polymorphic lineage of Gasteromycetes from boletoid ancestors. Mycologia 94: 85-96.
  • Grand, L.F., 1976. Distribution, plant associates and variation in basidiocarps of Pisolithus tinctorius in the United States. Mycologia 68: 672-678.
  • Martin, F., J. Díez, B. Dell & C. Delaruelle, 2002. Phylogeography of the ectomycorrhizal Pisolithus species as inferred from nuclear ribosomal DNA ITS sequences. New Phytologist 153: 345-357.
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