Understanding Armillaria Root Rot in Apricot Trees
The fungus Armillaria mellea, commonly known as honey mushroom or shoestring fungus, poses a significant threat to fruit trees, including apricots. This persistent pathogen is notoriously resistant to fungicides and often lurks in soil, hiding in old tree remnants or roots where treatments cannot reach. While there is no definitive cure for Armillaria root rot, maintaining plant health and reducing stress can significantly lower the risk of infection.
Global Impact of Armillaria
Armillaria attacks thousands of plant species worldwide and is a common pathogen in hardwood forests. Many home gardens and orchards are established on former hardwood forest land, increasing the likelihood of Armillaria presence in the soil. Some colonies of this fungus are thousands of years old, with a notable example in Oregon recognized as the largest living organism on Earth.
Symptoms of Armillaria Root Rot in Apricot Trees
Armillaria root rot can silently infect apricot trees, often going unnoticed until significant damage occurs. Key symptoms include:
- Delayed Detection: Symptoms may appear after the tree reaches 5 years of age, with small leaves and stunted growth.
- Sudden Collapse: Infected trees may collapse abruptly during summer due to compromised root systems.
- Physical Indicators: Look for white fan-shaped mycelial mats between the bark and wood at the tree’s crown, loss of bark, and root decay.
- Spreading Pattern: In orchards, a circle of infected trees often forms around the initial infected plant.
- Rhizomorphs and Mushrooms: Dark, thread-like rhizomorphs spread the fungus, while tan-colored edible mushrooms appear at the base of trees in summer or fall.
Photo Credit: Rebekah D. Wallace, University of Georgia, Bugwood.org & Steven Katovich, USDA Forest Service, Bugwood.org
How Armillaria Spreads
Armillaria root rot spreads through several mechanisms:
- Root Contact: The fungus often spreads when the roots of an infected tree come into contact with healthy roots.
- Rhizomorphs: These foraging fungal strands invade uninfected roots, persisting indefinitely in the soil.
- Infected Debris: Small pieces of infected roots or stumps can harbor the fungus and initiate infection.
- Windblown Spores: Though less common, spores from mushrooms can infect injured or dead wood.
Preventive Measures for Armillaria Root Rot
Preventing Armillaria infection starts before planting and continues with vigilant care. Follow these steps to protect your apricot trees:
1. Prepare the Planting Area
- Remove old roots and wood debris from the soil to eliminate potential fungal hosts.
- Girdle native trees (remove a strip of bark around the trunk) in the area and leave them for at least six months before removal to deplete starch reserves, reducing their appeal to Armillaria.
- Plant annual crops for several years before introducing stone fruit to reduce soil fungal load.
- Ensure proper soil drainage to prevent excess moisture, which can promote fungal growth.
2. Avoid Lawn Grass Near Trees
Do not grow sod or grass close to the base of your apricot tree, as it can encourage honey mushroom growth.
3. Maintain Tree Health
- Keep your tree in optimal health to resist infection, as Armillaria targets weakened plants.
- Use proper fertilization and promptly treat diseases or insect infestations that cause defoliation.
- Prevent drought stress by maintaining adequate soil moisture, while avoiding flooding, which can also stress the tree.
- Minimize external stressors like nearby construction or soil fill, which can increase infection risk.
Treating Early-Stage Armillaria Infections
If caught early, exposure to air can kill Armillaria. Here’s how to attempt treatment:
- Dig a trench around the base of the tree and main roots within a 2.3-foot radius, using high-pressure water if needed.
- Remove and burn diseased roots and bark, then seal cuts with plastic paint.
- Keep soil away from the tree base to prevent reinfection, and lay a tarp around the trench to block fungal spread, as recommended by the University of California Integrated Pest Management (IPM) program.
For severely infected trees, complete removal and burning of the stump and roots are necessary to prevent further spread.
Armillaria as a Widespread Pathogen
Armillaria plays a natural role in forests by clearing dying trees and shrubs globally. However, this behavior can be devastating for cultivated apricot trees. Proactive steps like clearing soil of debris, minimizing tree stress, and early intervention are crucial. If an infection becomes severe, removing and destroying the affected tree is often the only solution. Growing grasses on the site for several years before replanting can help cleanse the soil of fungal remnants, preventing the establishment of long-lived fungal colonies.
Share Your Experience
Have you encountered Armillaria root rot in your apricot trees? We’d love to hear about your experience, the health of your tree, and any prevention or control measures you implemented. Share your story in the comments below!
