Tree Diseases: Armillaria Root Rot


Armillaria root rot, also called fungal root rot, or Armillaria root disease, is a plant disease caused by several members of the genus Armillaria. The fungus most often identified as the causal agent is Armillaria mellea. Several closely related species may also infect susceptible plants. Fungi are natural components of forests, where they reside on the coarse roots, and lower stems of conifers and broad-leaved trees. Armillaria fungi act as parasites, infecting trees, and causing the inner wood tissue to decay. Infected trees often decline, with many eventually succumbing to the disease.

Distribution & Habitat

Armillaria root rot is found throughout temperate and tropical regions of the world. In the United States, the disease has been reported in nearly every state.


Armillaria root rot infects hundreds of species of trees, shrubs, vines, and forbs growing in forests, along roadsides, and in landscape settings. Larch and birch are the only trees that exhibit a resistance to the disease.

Disease Cycle

Armillaria fungi may reside for decades in coarse woody materials, from which they spread to living hosts. The fungi spread when rhizomorphs growing through the soil contact healthy roots, or when healthy roots become entwined with diseased roots. Rhizomorphs can extend up to ten feet in the upper soil layers. They penetrate the roots through a combination of mechanical pressure, and enzyme activity. The ability of the rhizomorphs to penetrate the roots depends on the strength of the fungus, the soil environment, and the host species.

When healthy roots make contact with diseased roots, the fungal mycelium can invade the roots without forming rhizomorphs. This type of spread is most common in dense tree stands where root contact frequently occurs. Armillaria fungi reproduce sexually with the mating of hyphae. They produce a basidiocarp, a type of mushroom, at the base of the infected host. Basiocarps produce basidiospores, which are released into the air during moist periods. The basidiospores are disseminated by the wind to nearby plants, where they transmit the disease.

When infected, healthy trees are generally able to confine the fungi to localized lesions by secreting resin, and rapidly forming callus tissues. Trees that have been weakened, or are undergoing environmental stress will not readily contain the fungi, enabling it to spread through the roots. If the health of the tree improves, additional fungal growth is often stymied. When infected trees are removed, Armillaria fungi spread into the stump, as well as into any remaining roots. As a result, the fungi will remain in the soil, increasing the potential for new infections.


Armillaria fungi typically inhabit roots. Mushrooms may be found growing in clusters around the base of infected trees and stumps. Mushrooms are produced sporadically from late summer to fall. They are most abundant during moist periods. Mushrooms have yellow or brown stalks around two inches long. Some mushrooms form a ring around the stalk just below the gills. Stalks have yellow caps that are two to five inches in width. The upper side of the cap is laden with dark brown scales, and may be slightly sticky. The underside of the cap features light-colored gills, which produce millions of light yellow to white spores. During wet periods in late summer and fall, these spores are released into the air, where they are disseminated by the wind onto nearby plants.

Crown symptoms on conifers and broad-leaved trees vary. Generally, the foliage thins and becomes discolored, turning yellow before finally browning. Shoot and branch die back may occur, with foliar growth significantly reduced. On large, vigorous trees, crown symptoms develop gradually over several years. Broad-leaved trees may form sunken cankers on infected limbs and branches. These cankers are covered with loose bark, or bark infiltrated with resin. Conifers, particularly Douglas-fir and western larch, frequently produce an abnormally large crop of cones, referred to as stress cones, as they decline. On most conifers, the infected portions of the lower stem will enlarge, and exude resin. The infected portions of the roots often become encrusted with resin, soil, and fungal tissue. On stressed or weakened trees, crown symptoms develop rapidly, with the tree often succumbing to the disease within a year.

Removing the bark on infected trees will reveal white mycelial mats on the rhizomorphs that form between the wood and the bark. Rhizomorphs also grow through the soil, becoming entwined with healthy roots. Rhizomorphs are black or reddish brown, with a compact outer layer of dark mycelium, and an inner core of white mycelium. The thick mycelial mats on the rhizomorphs decompose, leaving impressions on the inner bark.

Stripping the bark away may also reveal a white rot of infected wood. When the decay process first begins, infected wood turns light brown, and looks faintly water soaked. As the decay advances, the infected wood turns light yellow or white, and may be marked by numerous black lines. In hardwoods and conifers, the texture of decayed wood varies. Decayed wood is spongy in hardwoods, but stringy in conifers.

Trees that are suffering from drought stress, or have incurred injuries through mechanical wounding, insects, or other fungi may produce symptoms associated with those caused by Armillaria fungi.


  • Armillaria root rot cannot be completely eradicated. Management should be directed towards limiting the buildup and spread of the disease.
  • Chemical fumigants, including chloropicrin, methyl bromide, and carbon disulfide may be applied to reduce infection levels. Fumigants should be applied in and around the base of infected stems, or in perforations created in the soil after trees have been removed.
  • When planting, select a mixture of species that exhibit a resistance to Armillaria fungi.
  • Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during dry periods. Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and retain soil moisture.
  • Uproot infected or susceptible root systems and stumps to reduce the number of food sources available to Armillaria fungi.

Photo courtesy of Eric Steinert / CC-BY-3.0-US