Thursday, August 11, 2016

Ganoderma On Our Roots

I generally like to see various fungi on the ground but this is not one of them, especially when it is in our front yard.  We had a large native thorn-less honeylocust tree when we moved into our new house in 1973.  Over the years it has provided shade and served as a perch for many different birds.  Now these are appearing on the exposed roots.

Mark Bower confirmed that this is Ganoderma sessile, a parasitic fungus on hardwoods.  Species names get dicey as they frequently change or get reassigned by DNA.  As some have suggested, naming a new species is frequently making a species hypothesis.  In this case a related species Ganoderma curtisii looks somewhat like it.

Ganoderma fungi are white-rot saprophytes living on dead organic material in the soil.  They produce enzymes that digest lignin and cellulose that provide the structural strength of wood.  When it encounters cut or damaged roots it can parasitize the living tree, its hyphae spreading through the roots.  When it gains a sufficient spread it is ready to reproduce, creating its "flower," the mushroom that will produce spores, its method of spreading to other areas.

Our tree has been struggling in its urban setting for years  It was growing on a thin layer of soil before we came, its roots extending well beyond the diameter of the crown.  First the street and then our house and driveway construction constrained the root expansion.  Next a sweetgum was planted to the south that slowly encroached on its sun.  It grew anyway, loyally providing afternoon shade to the house.

Ganoderma - Mole's eye view
The thin layer of added topsoil supported a lawn but provided no depth for growth of the feeder roots.  They reached the surface 20 years ago where they were occasionally scarred by lawnmowers.  This likely provided the entrance wounds for Ganoderma to attack the roots.

Shallow soil, uprooted trees
In recent years the tree started to lose branches.  Ganoderma will also spread into the structural roots that hold the tree upright.  Our locust tree is plagued with rocks and shallow, poorly drained clay soil that prevents deep root penetration any way, increasing the risk of uprooting in a storm.  Now our arborist, Chris, has delivered the "last rites."

The tree shaded us for over 40 years, reducing air conditioning bills and providing a refuge for squirrels.  Now with the tree gone, Ganoderma will continue its role of saprophyte, converting the remaining roots into soil nutrients to feed the tree we will be planting soon.

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