Armillaria Root Rot – A Threat to Stressed Landscape Trees

Tree stress can come from numerous factors, including weather, mechanical damage, insects, or poor growing conditions. These stresses make plants more susceptible to the plant disease Armillaria root rot. This fungal disease is also known as shoestring root rot, mushroom root rot, and oak root rot. Once symptoms are observed, damage is often too severe to save infected trees, as no effective management strategies are available.

Armillaria Root Rot Facts

  • Symptoms include dieback and decline. Loose or decayed bark near the base of the tree is often observed. When bark is peeled back, creamy white fans of fungal mycelium (thread-like structures) or dark brown rhizomorphs (thick strands of fungal mycelia) (Figure 1) may be present. In fall, distinct “honey” mushrooms are produced at the base of the tree or along decaying roots (Figure 2).

Figure 1: Dark brown rhizomorphs (or shoestrings) may be observed under the bark of trees infected with Armillaria root rot. (Photo: Cheryl Kaiser, UK)

  • The fungal pathogen overwinters in decaying wood and can persist for many years on this plant material in soil.
  • Common hosts include oaks, maples, pines, hornbeams, taxus, and fruit trees.
  • Trees exposed to stressful growing conditions such as drought, winter injury, insect defoliation or borers, mechanical injuries, or construction damage are more likely to become infected.
  • Caused by multiple species of the fungus Armillaria.

Figure 2: “Honey” mushrooms may be present at the base of infected trees or along decaying roots, especially during rainy seasons. (Photo: Homeowner, Kenton County Kentucky)

Disease Prevention Options

  • Consider removal of infected trees, roots, and stumps.
  • Maintain plant health with proper nutrition.
  • Select well-drained planting sites that are high in organic matter.
  • Minimize stress from environmental factors.
  • If site has a history of Armillaria root rot, avoid susceptible tree species.

Additional Information

  • Shoestring Root Rot – A Cause of Tree and Shrub Decline (PPFS-OR-W-05)
  • University of Kentucky Plant Pathology Extension Publications (Website)

 

By Kim Leonberger, Plant Pathology Extension Associate, and Nicole Gauthier, Plant Pathology Extension Specialist

 

Posted in Landscape Trees & Shrubs