Oak Root Fungus

By Mary Bernard, Master Gardener

Armillaria root rot, also known as oak root fungus, is one of the most widespread plant diseases in
California.  It is most prevalent in landscapes established in areas where oaks or other native trees once grew.  Oak root fungus thrives under moist conditions, for example when turf is planted around the roots of California native oaks.  Plants become infected through root contact with infected plants or rhizomorphs attached to infected roots.

Armillaria can develop slowly, and symptoms may not appear until the fungus is well established.  Affected trees usually show a general decline in vigor over many years.  Sometimes trees that look healthy will suddenly wilt and die in a matter of weeks.  The above ground symptoms are similar to other root problems, including too much water, Phytopthora root rot, or gopher damage.

The key symptom to look for is trees that are declining in patches and the patches seem to get larger each year.  Roots infected with oak root fungus have white to yellowish fan-shaped mycelium between the bark and the wood.  Dark brown to black structures that resemble shoestrings sometimes can be seen on the root surface.  Sometimes, large, densely packed, honey-colored mushrooms form at the base of infected trees in late fall/early winter after rains.

To confirm the presence of Armillaria, dig around the crown of the tree and scrape off the bark on small sections of the crown and main roots.  It should be easy to see the dense, felty tissue between the bark and the wood.  Infected wood will have a strong mushroom smell and feel slightly spongy.  Sometimes it may be difficult to locate the mycelium in the earlier stages if the infection has not yet moved up to the crown.

There are no effective fungicide treatments for the control of the disease in living trees.  The fungus can survive for many years in the dead or living tree roots.  Remove as many roots from infected soil as possible before replanting.

Armillaria is sensitive to drying and grows most rapidly under wet conditions.  For this reason, heavy watering should be avoided.  Air-dry the soil before replanting.  Physical barriers such as root collars can contain infection centers.  Prepare a new landscape site well before planting and provide proper cultural care.  Prevent healthy roots from coming in contact with diseased ones, thus avoiding spread of the disease.  Plant only Armillaria resistant species in locations where oak root fungus has been a problem.  Call the UC Cooperative Extension office for information on plants resistant or susceptible to Armillaria.

(Note: Although another oak killer - Sudden Oak Death - has received a lot of public attention this year, it has not yet been detected in
San Luis Obispo County.)

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request .Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM.  You may also call the Paso Robles office at 237-3100 on Wednesdays from 9 AM to 12 PM.  The San Luis Obispo Master Gardeners website is at http://groups.ucanr.org/slomg/.  Questions can be e-mailed to: mgsanluisobispo@ucdavis.edu.