Armillaria Root Rot

Armillaria root rot (Armillaria sp.) is a disease that affects hundreds of plant species throughout the United States.  It is commonly found on various plants in urban landscapes, which were previously transformed from naturally wooded areas.  Infections are usually sporadic within the landscape and typically occur on stressed plants.  Roots and plants of all sizes can be killed.  The fungus invades the bark and cambial regions of roots and root collars.  Fungal infection progresses from the cambium into the sapwood.

Aboveground symptoms of Armillaria root rot include a reduction in growth, chlorotic undersized foliage, premature defoliation, and branch dieback in the upper crown.  Wind throw of large trees has also been associated with Armillaria infection.  Infection in coniferous trees stimulates heavy resin flow that may saturate the bark.  The mushroom stage of the fungi develops annually in the fall.  These mushrooms are called "honey mushrooms" and appear on or near decaying wood.

Below the ground, fanlike, veined white fans (mycelial sheets) can be seen under the bark of the roots.  Black-brown fungal strands spread along root surfaces and also up the trunk of dead trees under the bark.  The interior of the strand is white.  These thick strands resemble shoestrings, leading to the name "shoestring root rot."  Armillaria is a decay fungus and depends on woody material as a food source.  Armillaria can remain active in the soil for decades if sufficient woody material is available for growth.

Currently, there are no fungicidal treatments that provide satisfactory control of Armillaria.  The fungus is not an aggressive pathogen, and does not usually cause widespread problems to plants in urban landscapes.  In general, plant susceptibility decreases as trees get older.  Therefore, it is most important to keep trees healthy when they are younger.  The most important strategies for the protection of shade and ornamental plants are to promote vigor, minimize stress, and to remove infected stumps and roots.  Even plants susceptible to Armillaria may survive infection if they are otherwise healthy and vigorous.