Armillaria Root Rot

Scientific Name: Armillaria ostoyae

Armillaria is a root rot fungus also called stringy butt rot or white root rot. As its name suggests, the fungus creates mycelial threads that appear on infected tissue like strings. These structures are called rhizomorphs and can grow, under soil, to distances of up to ten feet. Armillaria may live for decades on its host plant and can infect healthy trees through root contact. Vigorously growing trees are less susceptible to infection. This is a pathogenic organism causing mortality, wood decay and growth reduction.

Attacks: Many tree species

What you will see:

• White fungal mats can usually be found under the bark
• Black shoestring structures
  ◦ Rhizomorphs (mass of mycelia)
• Honey-colored mushrooms may appear at the base of the plant (Honey fungus)
  ◦ Mushrooms are edible
• Foliage thins and discolors, turning yellow, then brown
• Branches may die-back, shoot and foliar growth are reduced

Life cycle:

• Mushrooms present in late August through October
• Spores are distributed by the wind to exposed wood or injured bark
• Spores germinate and produce mycelia that infect the bark and sapwood killing the cambium
• Fungus can persist from several months to many years

Treatment and Applications:

• No labeled fungicides are effective against Armillaria
• Preventative
  ◦ Reduce moisture problems
  ◦ Reduce soil compaction
  ◦ Don’t damage bark tissue or roots
• Root Enhancement System (RES)
  ◦ Increases population of competitive soil organisms
  ◦ Increases growth rate of callus tissue
  ◦ Increases root growth and tissue resistance
  ◦ Increases soil porosity, decreases soil moisture

Cultural practices:

• Remove soil from the base to a depth of 9-12 inches
• Leave the trunk exposed to air
• Keep the upper roots and crown area as dry as possible
• Dry conditions will kill the fungus

 

An integrated approach

When caring for urban trees it is important to make a complete evaluation of all environmental conditions to accurately diagnose all stress factors and prescribe care based on specific circumstances. This prescriptive care will help your tree meet its full potential.