What is Armillaria Root Rot?

Serving Families Throughout Portland & the Pacific Northwest

It’s also called oak root fungus or shoestring, which refers to the black shoestrings caused by the fungus called rhizomorphs. It’s hardy and can survive even during unfavorable conditions.

There are several species of this soil-borne fungus, which can be found everywhere. It’s especially common in the Pacific Northwest where it thrives in our heavy clay soils and cool climate.

What trees are most affected?

Armillaria root rot attacks over 700 species of plants and most are woody plants. Oak and maple trees are the most common but it also infects birches, beeches, dogwoods, Douglas fir, elms, hemlocks, poplars, willows, fruit trees and even rhododendrons.


There are several tell-tale signs.  They include premature fall coloration, stunting and poor grown as well as general health decline of the plant, twig, and branch dieback. The tree usually declines slowly over several years but can progress faster with advanced infection and dies not long afterwards.

Infected trees often blow over in windstorms because its roots have decayed.  The most obvious sign is an unusually small root ball with stubbed off roots. In conifer trees (like a Douglas fir), you can spot infected tree by looking for resin or pitch seeping from the lower trunk.


More signs of the fungus can be found at the base of the trunk (or root collar) and the main roots.  White, creamy, paper thick fan-shaped sheets of the fungus strands called mycelium grow over sapwood when the bark is removed for inspection.

You’ll also see black root-like shoestrings or rhizomorphs under the bark or in the soil around the root collar.  These cord-like formations grow from the infected roots (under the soil). That’s how it’s able to spread to other trees.

Therefore, we recommend having infected trees removed before it can spread to other trees.  Another sign is small clusters of yellow or honey-colored gilled mushrooms growing in late autumn in rainy weather.


Trees are an asset to your property and good for the environment. You certainly don’t want to wait for the disease to worsen before doing something about it.

If you suspect Armillaria root rot or have any tree that’s in decline or generally looking unhealthy, contact General Tree to have one of our certified arborists inspect it.

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