Armillaria Root Rot

Armillaria root rot is a soil borne that attacks the roots and trunks of many fruit and nut trees. It is also the largest living fungi in the world.

In Oregon’s Malheur National Forest, there is a mushroom colony that covers 2.200 acres. That colony is believed to be a single entity, all growing from the same network of fungal mycelium.

​By itself, that's impressive. In your tree, it's a deadly fungal disease.

‘Armillaria mellea’ (Wikipedia) CC BY-SA 3.0

Trees vulnerable to Armillaria root rot

Also known as honey fungus, shoestring fungus, or oak root fungus, Armillaria root rot (Armillaria mellea) is a deadly disease that infects avocado, cherimoya, cherry, chestnuts, conifers, kiwifruit, kumquat, lemons and other citrus, pomegranate, stone pine, and walnuts, along with the mighty oak. If that weren’t bad enough, trees weakened by Armillaria root rot become more susceptible to serious pests, such as Pacific flathead borers​.

Armillaria root rot symptoms

Everything starts out looking fine. Your tree is growing nicely and you suspect nothing. Suddenly, you notice downward cupping leaves, chlorosis (yellowing), dieback of upper limbs, and leaf drop. You may also see a variety of mushrooms growing nearby.

Honey fungus (Wikipedia) CC BY-SA 3.0

Your tree is dying. Young trees die quickly, while older trees make take longer, but the end result is nearly always the same.

Armillaria root rot diagnosis

If your tree shows the above mentioned symptoms, take a closer look at the base of your tree. You may be able to see fan-shaped fungal growth rising up the trunk from the soil level. Use a sharp knife and cut away a section of bark at the base of the tree, so you can see the cambium layer. If you see white fungal threads (mycelia) and can smell a strong mushroom odor, your tree is in serious trouble. If you see reddish brown streaks or patches, or water-soaked areas, the infection is more likely to be Phytophthora root and crown rot, rather than Armillaria, though that isn't any better news for your tree.

White Armillaria mycelial ‘felts’ under the bark of an infected tree (USDA) Public Domain

If you have easy access to the tree’s roots, cut one open. Infected roots are darker than normal and have a cottony center. You may also see black rhizomorphs, called ‘shoestrings’, on the surface of infected roots.

Armillaria rhizomorphs, or “shoe-strings” may be seen on the surface of exposed roots (USDA) Public Domain

How Armillaria spreads

As a soil borne disease, Armillaria mycelia can remain viable in the soil for many years. It isn’t until a healthy root comes into contact with infected wood, roots, stumps, or other wood fragments. Then, the fungus enters the healthy tree and begins to populate the cambium layer, eventually killing the tree. Infected nursery stock can also carry this disease, so always quarantine new plants.

Preventing Armillaria root rot

Good drainage, sunburn protection, and proper (not excessive) irrigation can all help protect your trees against Armillaria root rot. Once infection occurs, the tree should be completely removed and the area should only be planted with crops that are not vulnerable to Alternaria root rot.