On a recent visit examining trees and making recommendations for care, the property owner pointed out several eruptions in the asphalt surrounding a large old blue oak where strange growths have been emerging. The asphalt covers the entire root zone of the tree at a radius of only two feet from the trunk.
I recognized the pattern, scouted around the yard for a good example and found one in a gravelly area nearby: the fruiting body of Pisolithus tinctorius, the Dead Man’s Foot. Notoriously, it is found on barren, disturbed and compacted soils and is known, in part, for its ability to lift stones, bricks and pavement.
It is one of many fungi that form a symbiotic relationship with plants. Together, the fungus and the plant form mycorrhizae, which means fungus-root. The fungus greatly enhances mineral absorption, sharing with the plant while receiving carbohydrates in return. It is a neat arrangement because the fungi do not have the ability of photosynthesis, but they are great at absorbing minerals from the soil. Further, these fungi may protect the host plant from pathogens.
Not readily recognizable as a mushroom, Pisolithus has been described in various ways. In “Mushrooms Demystified” David Arora says: “This dusty monstrosity is among the most distinctive and memorable of all fleshy fungi.” When young it resembles a puffball type of mushroom, developing just under the soil surface but as it matures, the outer covering disintegrates revealing a mass of brown spores. With a bit of imagination, one can see its resemblance to its common namesake.
My first recollection of this distinctive fungus is from a tree nursery, transplanting seedlings into the ground and inoculating them with it to enhance growth. One of the many mycorrhizal fungi, Pisolithus is included in some fertilizers commonly available in garden centers and various formulations of root zone inoculants that have come into widespread use. There is some uncertainty as to the effectiveness of artificial inoculations.
The relationship of plants and fungi is an old one. I once heard a mycologist say fungi made it possible for plants to become established on land about 400 million years ago.
These symbiotic fungi form their relationship with plant roots in two ways, depending on the species of fungus and tree:
1. Ectomycorrhizae infect the host plants between cells in the roots and form a visible sheath of short, clubby branched structures around the roots.
2. Endomycorrhizae infect individual cells. They do not affect the appearance of the roots.
“Arboriculture 4th Edition” (Harris, Clark Matheny, 2004), a main reference book for arborists, discusses the subject at considerable length. Briefly summarizing, it says most plants form this relationship naturally. The relationship is suppressed by heavy irrigation and fertilizer applications. Highly disturbed soils may lack the fungi, but they can return naturally over time. Applications have not shown significant increases in growth except for seedlings in highly disturbed soils and where the fungal species naturally associated with the plant has been isolated, grown and introduced into new plantings.
Here in summer-dry mid California, where fungi are the dominant beneficial microorganisms, one simple practice can effectively support the symbiotic relationship of fungi and trees: Allow the natural litterfall of leaves and twigs to remain in tree root zones. Do not pile it deeply or leave it up against tree trunks, or wooden structures.
As for the tree owner with the seasonal eruption of Dead Man’s Foot mushrooms in the asphalt, I could not offer a practical solution except to remove some of the asphalt and give the tree and the fungus some room to do what comes naturally.
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