Armillaria Root Rot

What Is Armillaria Root Rot?

Armillaria Root Rot—commonly referred to as oak root fungus or simply root rot—is a destructive fungal disease caused by various species of the Armillaria genus. This soil-borne pathogen attacks the roots and lower trunk of both hardwood and coniferous trees, leading to decay, structural weakness, and eventual tree death if left untreated.

The fungus can lie dormant in the soil for years and is often spread through root-to-root contact or via underground fungal structures known as rhizomorphs. Once established, it compromises the tree’s ability to absorb water and nutrients, slowly killing the root system and internal wood tissues. Armillaria is particularly dangerous because it can infect healthy trees—especially those under environmental stress from drought, construction, poor soil conditions, or previous wounds.

Bur Oak Blight is more than just a seasonal leaf issue. It can cause significant canopy loss, dieback of major limbs, and increased vulnerability to other pests and diseases like the two-lined chestnut borer or root rot. In some cases, untreated trees may eventually die.

Signs Your Tree Might Have Root Rot (Armillaria)

Armillaria Root Rot often begins below the soil surface, making early detection difficult. However, there are several visible signs that can indicate your tree is infected. Watch for the following symptoms:

• Mushrooms at the Base
  Honey-colored mushrooms may appear at the base of the trunk or on nearby stumps in late summer to fall. These are the fruiting bodies of the Armillaria fungus.
  
  
• White Mycelial Mats Under Bark
  If you peel back the bark near the base of the tree, you may see white, fan-shaped fungal growth between the bark and wood.
  
  
• Black Rhizomorphs in the Soil
  These root-like structures, often resembling dark shoestrings, can be found growing on roots or in the surrounding soil. They allow the fungus to spread from tree to tree.
  
  
• Crown Dieback and Thinning Canopy
  A gradual decline in foliage density, yellowing or browning of leaves, and dieback of upper branches are common symptoms as the roots lose their ability to nourish the tree.
  
  
• Slow Growth or Early Leaf Drop
  Affected trees may have reduced annual growth, smaller leaves, and premature fall color or leaf drop, especially during periods of drought.
  
  
• Cracked or Oozing Bark
  The lower trunk may develop splits, cracks, or areas where sap is weeping, often accompanied by foul odor or decay.
  
  
• Sudden Collapse in Advanced Stages
  In severe cases, infected trees may topple without warning due to root failure, especially after storms or heavy winds.

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How Root Rot Spreads to Other Trees

Armillaria Root Rot is especially problematic because of how easily and quietly it spreads underground. Because Armillaria spreads slowly but persistently beneath the surface, trees in close proximity to infected ones should be closely monitored—even if they appear healthy. Unlike some tree diseases that rely on airborne spores, Armillaria moves through root systems and soil in the following ways:

• Root-to-Root Contact
  The most common method of transmission. When an infected tree's roots grow into contact with healthy roots nearby, the fungus can move directly between them, silently infecting new hosts.
  
  
• Rhizomorphs (Fungal “Shoestrings”)
  Armillaria produces thick, black, cord-like structures known as rhizomorphs. These can grow several feet through the soil, seeking out new root systems to infect. Once they reach a susceptible tree, they penetrate the bark and begin colonizing the root system.
  
  
• Contaminated Soil or Wood Debris
  The fungus can survive for years in dead roots, stumps, and woody debris left behind in the soil. New trees planted in the same area are at high risk if infected material is not properly removed.
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• Environmental Conditions
  Moist, compacted, or poorly drained soils increase the likelihood of infection and promote the spread of rhizomorphs. Trees already weakened by drought, construction, or other stresses are especially vulnerable.
  
  

Common Trees in Missouri at Risk from Root Rot (Armillaria)

Armillaria Root Rot is a threat to a wide variety of hardwood and coniferous trees across Missouri, especially in landscapes where trees are stressed by poor soil, drought, or prior damage. Because this fungus thrives in hidden root systems and woody debris, any tree showing signs of decline—particularly in groups—should be assessed by a professional arborist.

 The following species are among the most commonly affected in the St. Louis area and throughout the state:

• Oak Trees (especially White Oak and Red Oak species)
  Oaks are some of the most susceptible trees to Armillaria, particularly those with pre-existing stress or growing in compacted soil.
  
  
• Maple Trees (Sugar, Red, and Silver Maple)
  Maples are vulnerable when exposed to prolonged wet conditions or root damage from construction or improper planting.
  
  
• Elm Trees
  Already under pressure from Dutch Elm Disease, elms can also fall victim to root rot, especially in urban environments.
  
  
• Hickory Trees
  Native hickories can become infected when surrounded by declining vegetation or after physical root injury.
  
  
• Walnut Trees
  Although somewhat more resistant, walnuts growing in low-lying or wet areas can be affected over time.
  
  
• Ash Trees
  Trees weakened by Emerald Ash Borer are at increased risk of secondary infection by Armillaria fungi.
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• Conifers (Pine, Spruce, and Fir)
  In some areas, ornamental and native conifers can also develop root rot, especially under prolonged moisture stress or poor drainage.
  
  

Treatment and Prevention for Armillaria Root Rot

While Armillaria Root Rot can be challenging to eliminate completely, there are proven strategies to manage the disease, slow its spread, and protect surrounding trees. Here’s how professional arborists approach treatment and prevention:

• Remove Infected Trees and Stumps
  If a tree is too far gone, removal is often necessary. Uprooting the tree, stump, and as much of the infected root system as possible helps eliminate the fungal source and reduce the risk to nearby trees.
  
  
• Improve Soil Drainage and Aeration
  Armillaria thrives in moist, poorly drained soil. Amending soil to improve drainage and aeration can help reduce fungal growth and support tree health.
  
  
• Avoid Replanting in Infected Areas
  Do not plant new trees in the same location without fully removing infected stumps and roots. Even then, avoid using highly susceptible species for several years.
  
  
• Mulch and Water Properly
  Use mulch to retain soil moisture, but avoid piling it against the trunk. Water deeply but infrequently, allowing the soil to dry between watering to discourage fungal growth.
  
  
• Limit Tree Stress
  Keep trees as healthy as possible through proper pruning, fertilization, and protection from mechanical damage. Stress from drought, compaction, or construction can make trees more vulnerable to infection.
  
  
• Monitor Nearby Trees Regularly
  Trees near a confirmed Armillaria infection should be closely observed for signs of decline. Early action can help limit the impact of the disease.
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• Fungicides Are Generally Ineffective
  Unlike some fungal diseases, Armillaria cannot be reliably treated with commercial fungicides once infection is established. Focus is placed on prevention and supportive care.