Armillaria Root Rot Disease

Arborvitae damaged by Armillaria. Rot and mycelial fan visible in images.  Photo: Maine Forest Service Mycelial fan of Armillaria sp. on ash (Fraxinus sp.).  (Photo: Bill Ostrofsky, Maine Forest Service)

Armillaria root rot disease is caused by any of several species of fungi in the genus Armillaria.  The genus Armillaria is reserved for “facultatively parasitic” root and butt rot fungi that produce rhizomorphs.   A facultative parasite in this case is an organism which survives mostly on dead, woody tissues but can, under some conditions, act as a pathogen and infect living trees.  Rhizomorphs are large cord-like strands of fungus tissue that allows the fungus to grow some distance through soil, under bark, and along tree root systems.  The rhizomorphs give the common name of “shoestring root rot” to the disease.

Rhizomorphs or "shoe strings" of Armillaria sp.  (USFS photo)

The taxonomy of the group is complex and has gone through substantial changes through the years.  There are about ten species (depending on the accepted taxonomic criteria) that occur in North America, with seven species occurring in the Northeast. 

The fruiting stage of Armillaria fungi are gilled mushrooms. 

Fruiting structures (mushrooms) of Armillaria sp. (Photo Bill Ostrofsky, Maine Forest Service)

USDA Forest Service FIDLExternal Link


Maine Forest Service - Forest Health and Monitoring Division