Oak Root Fungus

Oak root fungus is a common root problem of ornamental plants. It is a common cause for the death of trees and shrubs. The scientific name is Armillaria mellea and the visible signs are mushrooms growing around the base of the plants or on the main roots. In England and Europe this is called honey fungus because of the color.


IMG_3418The mushrooms appear in the late fall after several rain storms (however they came this year even when we had no rain.

They grow on the trunk and main roots of plants that are infested with the mycelium throughout the year.  Once the mycelium (fungus roots) take over the cambium layer and can support the mushrooms the mushrooms appear almost overnight and start to spread spores in the air. Notice the ring of white tissue around the stem which is common on Oak Root Fungus mushrooms.

IMG_3581Here you can see the spores are like a white powder that rain down on the top of the mushroom from the mushroom gills above.  The wind carries it long distances and all it needs to sprout is moisture and a host.

Every year I fill up garbage bags with Oak Root Fungus mushrooms and dispose of them in the garbage so they won’t be around to spread their spores.

The mycelium is the white filamenteous material growing just under the bark and sapping the energy from the host plant.  Once the mycelium grows completely around the trunk the sugars from the leaves can no longer reach down to the roots and the roots gradually die and then the plant dies.  If you can catch it before it completely girdles the trunk and peel off the mycelium and the rotten parts with a knife the plant may recover if you let everything dry out – no irrigation.

One of the symptoms of the disease is the discoloration of the leaves of the plant like it might need water.  Don’t water it if you see the mycelium on the trunk or root because that will encourage the disease.  If you don’t see any mycelium under the bark and you water the plant deeply and it recovers then lack of water was the problem.  If the leaf discoloration gets worse after watering then it is probably Oak Root Fungus deeper than you checked or it could be another root rotting disease such as Phytopthera.

Sometimes deciduous plants, such as this Japanese Maple, prematurely develop fall color. This is a clue that it might have Oak Root Fungus.
img_7879If you see a plant that is loosing leaves and doesn’t look healthy like this plant you should suspect some kind of root rot or lack of water.  Before you water it deeply check just under the bark with a knife to see if you can see any mycelium.

 If you see some white flecks under the bark the plant has Oak Root Fungus and you should let it dry out and cut any mycelium out that you can see.  If it goes all the way around the trunk then take the plant out, it is dead.

Here is a Japanese Cherry that has Oak Root Fungus because the emitters are too close to the trunk. The mycelium has not yet completely circled the trunk so it is possible to save the tree if the soil is dug out around the trunk and the emitters are moved 18-24″ away.

Variegated Dogwood with Oak Root Fungus

Here is a Rose that has died because of Oak Root Fungus.

Most plants that are affected by Oak Root Fungus have woody stems but some herbaceous plants like this Sedum ‘Autumn Joy’ also are susceptible. You can see the white flecks of the fungus above.

Oak root fungus on Heuchera root.

To prevent Oak Root Fungus always plant a new plant high above the soil level.  You do not want any soil touching the trunk.  Keep all mulch and leaf litter away from the trunk.  Keep standing water away from the trunk.  Don’t let the emitters touch the trunk.  Don’t water every day because the soil needs to dry out a little between waterings.  The spores of Oak Root Fungus need constant moisture to germinate.  Periodically every month or so go around all plants in your garden and rake away mulch and leaves that are touching the trunk. 

There are a few plants that are resistant to Oak Root Fungus:

Japanese Maple (some)











More resistant plants can be found on the web under articles written by Dr. Robert Raabe from the University of California, Berkeley.

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