Oak Fungus

Oak Root Fungus, Armillaria species (also known as oak root fungus, armillaria root rot, honey fungus) is a naturally occurring soil fungus that helps decay or decompose woody material.

Armillaria is not a soil born pathogen; it is a wood decay fungus. Wood is both its energy source and its survival structure and it can survive in old root pieces for decades when buried in undisturbed in soil. The fungus prefers dead tissue over living tissue and once the host plant dies, the fungus moves through the dead wood more rapidly than through the living wood.

Unfortunately, armillaria can attack living trees, shrubs and other plants. A white mycelial fan (a mass of branching fungal filaments) can usually be found between the bark and the wood near the root collar area of infected plants.

Insect infestations, disease pathogens, injuries, poor soil conditions, improper cultural and maintenance practices, or any other factor causing stress to landscape plantings can predispose a plant to an armillaria infestation.

Although there are no chemical treatments available to control armillaria, the following practices help promote conditions for landscape plants to thrive, and thereby resist the armillaria infection that could lead to plant losses.

For a more detailed account of its life history, see http://ceventura.ucdavis.edu/newsletterfiles/Landscape_Notes4445.pdf.

To reduce damage or loss from oak root fungus:

  • Keep the root crowns of oaks and other susceptible plants dry. Adjust irrigation emitters so they face away from the trunk. Manage irrigation so that the soil has a wet/dry cycle; not even ferns like to be kept at saturation point continuously.
  • Regardless of whether the pathogen is present in the soil or not, the addition of appropriate quantities of organic matter in the form of mulches, manures, and composted materials to and on the soil is beneficial if appropriate to the plant species.
  • Provide optimal environmental conditions. A healthy, actively growing plant can better resist the incursions of the pathogen. Proper nutrients, water, temperature, light, drainage, etc. can be provided and manipulated to promote plant health.
  • Ensure that drainage is adequate to prevent water logging, which promotes the incidence and severity of disease. Proper planting methods should minimize drainage problems. The installation of a subsurface drainage system prior to planting can be considered for sites with poor drainage.
  • Do not plant material that is pot bound, diseased or deformed. It will be more susceptible to disease. Never attempt to propagate diseased or even marginally healthy plant material.
  • Never move a plant from an infected site to an uninfected site. When removing an armillaria-infected plant, it is essential to remove as much of it, including roots, as possible. The pathogen may persist in living and dead tissue for many years.

Planting and watering underneath an established California live oak and/or raising the grade in the root zone just a few inches can be enough to cause the tree to decline and may shorten its life significantly.

In natural situations in Southern California, armillaria is rarely a fatal plant disease. Oaks have co-existed with armillaria for centuries, as long as they are not irrigated during the summer when armillaria is most active.