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Caption: Armillaria root rot infecting trunk
Photo by: C.R. Foss
  
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Oak : Armillaria root rot
(revision date: 4/28/2014)


Biology
Armillaria root rot is a fungal disease often found in newly cleared soils or soils which have been flooded. Above-ground symptoms typically include production of smaller-than-normal leaves, leaf yellowing, premature leaf drop, and branch dieback, often on only a portion of the tree. White thread-like masses of the fungus may be found beneath the bark near the crown of infected trees, and/or as shoestring-like rhizomorphs, which are dark strands of the fungus growing on or just beneath the soil surface. Honey-colored mushrooms often grow near the base of infected trees in the fall. Infected trees may also exhibit a dark black line in the infected area encircling the base of the plant. Native oak trees usually tolerate infection easily, unless damaged by human activity such as construction or improper irrigation.
Management Options

Non-Chemical Management
  • Avoid watering native oaks during the warm months. When irrigation is necessary, deep watering is recommended. Avoid wetting crown or main roots.
  • Plant resistant species in infected areas. Lists are available in the PNW Disease Management Handbook or contact your county Extension agent or WSU Master Gardeners.
  • Provide proper culture. Healthy plants are often extremely resistant to this disease.
  • Remove damaged trees if necessary. When possible, remove roots 1" or more in diameter and air-dry soil before replanting.
  • Avoid injury to roots and trunk.
Select non-chemical management options as your first choice!

Chemical Management


None recommended

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Caption: Armillaria root rot infecting trunk
Photo by: C.R. Foss
Caption: Armillaria root rot infecting trunk
Photo by: R.S. Byther
Caption: Armillaria rhizomorphs
Photo by: R.S. Byther