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Armillaria

Armillaria mellea and Armillaria tabescens (Clitocybe tabescens)

Armillaria root rots, also known as mushroom root rots, are caused by two species of Armillaria, Armillaria mellea and A. tabescens (Clitocybe tabescens). Armillaria mellea is also known as the oak root rot or the shoestring root rot fungus.

Symptoms

Aboveground symptoms are similar to the other root rots, but early symptoms are often unilateral, affecting one or two scaffold limbs that may die back (Fig. 1). Symptoms on roots of trees affected by the two Armillaria species are similar. The fungi grow within and beneath the bark and appear as white to slightly yellow mycelial growth (Fig. 2). This characteristic differentiates the mushroom root rots from white root rot that is characterized by white mycelium visible on the outside of affected roots (Fig. 3). Fan-shaped mycelial mats are often produced between the bark and wood. Perforations may be observed in the mycelial mats produced by A. tabescens (Fig. 4). Armillaria mellea is distinguished from A. tabescens by the presence of dark brown to black, root like structures known as rhizomorphs (Fig. 5), which are often found on dead roots, on the surface of live roots, or growing out into the soil. In the fall following rain, yellow to yellow-brown to brown mushrooms are often produced in clusters at the base of affected trees (Fig. 6). The stems (stipes) of the mushrooms of A. mellea (Fig. 7) have a ring of tissue around them known as an annulus, which is absent in A. tabescens (Fig. 8).

Fig 1 - Scaffold dieback Fig 2 - Mycelial growth under bark Fig 3 - Mycelial growth on roots Fig 4 - A. tabescens mycelial mat
Fig. 1. Scaffold dieback Fig. 2. Mycelial growth under bark Fig. 3. Mycelial growth on roots Fig. 4. A. tabescens mycelial mat
Fig 5 - A. mellea rhizomorphs Fig 6 - Mushrooms on affected tree Fig 7 - A. mellea (with annulus) Fig 8 - A. tabescens (no annulus)
Fig. 5. A. mellea rhizomorphs Fig. 6. Mushrooms on affected tree Fig. 7. A. mellea (with annulus) Fig. 8. A. tabescens (no annulus)

Disease Cycle

Armillaria spp. are widely distributed, affect numerous native forest trees, and persist in the soil in infected roots for many years. Replanting sites where infected trees have been removed is often not successful because of the difficulty in removing all old roots serving as an inoculum source for the newly planted trees. (Soil fumigation has generally not been successful in the Southeast.) Still, they do not cause significant tree loss in most orchards. They may become more important in high-density orchards because of the increased potential for spread from tree to tree through root contact.

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Format updated March 29, 2011