Root of the problem: How to know if your tree has oak root fungus
Q: I have a pine tree on the edge of my lawn that has clusters of mushrooms coming up around the base of the trunk. My friend told me these mushrooms are a sign that the tree has rotting roots and the tree could potential topple onto my garage. Is this true? Is there some way to tell if the tree has root damage?
A: Clusters of light brown mushrooms can be a sign of Armillaria root rot. This is a fungal disease that attacks hundreds of species of trees and woody plants, but can also affect a range of other plants including herbaceous plants and palm trees.
There are several Armillaria species, but Armillaria mellea is the species of this fungi commonly found in home gardens in California. This fungal disease is also known as oak root fungus.
The University of California Integrated Pest Management pest note says: “Armillaria root rot is commonly recognized by the presence of light brown mushrooms — known as “honey mushrooms” due to their color; not their taste — which typically appear in a cluster of several to dozens of mushrooms at the base of infected trees or shrubs.” These clusters of mushrooms are somewhat distinctive in how they grow.
According to the UC IPM site, “oak root fungus attacks and kills the vascular cambium — the tissue that generates bark and wood — in woody roots, then spreads laterally to the main stem, which can girdle the base of the trunk and kill the entire tree.”
The Armillaria fungus also causes white rot wood decay, which destroys the strength of wood in roots and at the base of infected tree trunks. This is why trees that have been infected with this fungus can suddenly topple.
Usually trees or plants infected with Armillaria will show signs of die back, weeping cancers on the trunk or will die so suddenly that the leaves will turn brown without falling off the tree.
If you suspect that your tree is infected, you can dig down below the soil line at the base of the tree and peal back the bark. If the tree smells like mushrooms or you can see white fan-shaped mycelia — thin, flat sheets of fungal tissue — growing below the bark, the tree is infected with the Armillaria fungi.
If your tree is still looking healthy, the fungi growing in your yard may not be Armillaria, or the fungi may be growing on roots left from trees that have been removed previously.
Because Armillaria can be both a pathogen killing living tissues in a tree, and a saprobe that lives on dead or non-functional wood after the infected host dies, it can be difficult to eliminate from the soil.
The fugal mycelium of this disease can persist for decades below ground, living in partially-decayed woody roots long after the infected tree has died.
To keep your tree from succumbing to the fungi, limit stress by insuring it has regular deep irrigation, but allow the soil surface to dry between watering. Ensure good soil drainage and mulch around the tree to reduce water stress and provide nutrients.
The Shasta Master Gardeners Program can be reached by phone at 530-242-2219 or email firstname.lastname@example.org. The gardener office is staffed by volunteers trained by the University of California to answer gardeners' questions using information based on scientific research.