This blog is focused on one of the major diseases associated with poor water management, oak root fungus (Armillaria root rot). This blog is one of a pair of articles focused on managing root rot diseases that often lead to the death of trees and shrubs.
Landscape trees are subject to diseases often associated with poor water management. When you notice a tree struggling, one of the first things to do is to examine how the tree is being watered, environmental conditions, and the soil moisture levels over time. Symptoms are often like ones associated with drought-stress and other diseases, hard to distinguish one from the other.
Diagnosis of tree problems can be difficult, and trees are a valuable portion of our landscape, not easy to replace; so hiring a consulting arborist may well be money well-spent. Consulting arborists are tree experts whose role is to bring a comprehensive, objective viewpoint to the diagnosis, appraisal, and evaluation of arboricultural issues. The correct diagnosis can help save or prolong the life of the tree.
A Common Root Disease—oak root fungus
Oak root fungus is a common soil-borne fungal disease that affects many ornamental and crop plants and is not limited to oaks. This disease is caused by fungus in the genus Armillaria; oak root fungus is also known as Armillaria root rot, mushroom root rot, honey fungus or shoestring fungus.
Oak root fungus (Armillaria) is found in the soil of temperate regions and is native to many areas including California. Several species of Armillaria are known to occur in California, but Armillaria mellea is the species commonly found in home gardens, parks, vineyards, tree crops, and natural landscapes. Under natural conditions, Armillaria mellea usually inhabits the root system of most native oaks without ill effect unless the oaks become stressed.
When the entire canopy of a tree or woody plant is affected, we suspect a root problem, and oak root fungus should be one of the prime causes to be considered and investigated.
General Symptoms
Often the rapid death of a whole tree is the first symptom noticed. Trees may die quite suddenly without showing any symptoms.
The general symptoms of Armillaria include the following, with severity and appearance depending on the progress and pace of infection: leaf wilting, foliage yellowing, defoliation, smaller-than-normal leaves, twig and branch dieback, thinning of the canopy, early leaf loss or dormancy and leaf color changes, resin or gum exudates at the root collar.
How to Identify Oak Root Fungus
Mushrooms
Oak root fungus is often recognized by the presence of a cluster of honey-colored mushrooms that appear at the base of the infected trees or shrubs. Sometimes the mushrooms form along the buried root away from the infected tree or form around a stump. But sometimes the tree dies and mushrooms are not there.
Mycelium
Another diagnostic sign is the white fan-shaped mycelia (fungal tissue) that grows just under the bark. To find these on a dead or dying tree, dig down 12-18 inches below the soil at the base of the tree trunk, peel away the bark to find the white mycelia and rotted wood.
Rhizomorphs
Rhizomorphs, “shoestring root rot”, are black strings of fungal tissue that are the third sign of infection. Rhizomorphs are found under the bark near the mycelia, on infected roots, or in soil immediately adjacent to the infected tree.
Host Symptoms on trunk or canopy
Since rhizomorphs and mushrooms are sometimes absent, and the mycelial fans are hidden under the bark, the presence of oak root fungus is often revealed by host symptoms on the trunk or in the canopy, such as wilting or yellowing leaves, twig and branch dieback, thinning canopy, smaller than normal leaves. A flat lesion on the surface of the bark may develop since new growth of the bark and wood may be halted.
Managing oak root rot is difficult because it remains infectious in the decaying roots
Oak root fungus is both a pathogen that kills the living tissue of a tree and one that lives on dead or non-functional wood after the infected host dies. The infective tissue, the inoculum, can persist for years below ground, living in partially decayed woody roots long after the infected host plants have died or were removed.
Some biological control may be effective.
As stated in the UC IPM pestnote on Armillaria (oak root fungus),“... it is clear from studies that multiple naturally occurring soilborne fungi attack Armillaria and likely limit its effects in landscapes and in natural ecosystems worldwide.”
Trichoderma is one of the fungi that attacks Armillaria and it is a common fungus present in mulches and in landscapes with woody materials. Trichoderma readily colonizes small pieces of wood where it can also colonize Armillaria. The strategy of reducing Armillaria in the soil by removing, grinding or chipping infected stumps or dead roots also has the added benefit of making it easier for Trichoderma to attack the remaining Armillaria. Higher soil temperatures and dry soil inhibit Armillaria while those same conditions promote the growth of Trichoderma; so soil solarization may be beneficial, though solarization would not affect any residual roots/stumps at depths below 4 inches.
Managing oak root fungus for affected trees
Luckily, the fungus is incapable of causing severe damage in many instances unless the tree is first subjected to substantial stress. Thus, keeping the tree healthy and avoiding severe stress is one important approach in preventing the loss of trees to oak root fungus.
Stressed trees are affected quickly. Tree stress can be reduced by preventing waterlogging of soils around trees from overwatering, especially during the summer.
Poor soil drainage should be corrected. Ensure that the root collar stays dry (i.e., do not let water spray hit the base of the trunk). While there is no cure for Armillaria, keeping trees vigorous can slow growth of the fungus.
Deeply planted trees with soil covering the root collar area are more susceptible. (More information here on planting trees and root collar and flares.)
Removing dirt to expose large structural roots and the root flare can slow existing infections and perhaps even prevent them.
Take care to keep the bark undamaged and intact.
There are no registered fungicides for oak root fungus (Armillaria) control in California.
Managing oak root fungus in the garden
Infection is through direct contact with residual roots or when rhizomorphs grow and contact susceptible roots of healthy plants.
- Avoid moving infected soil or roots to other parts of your garden to keep this disease from spreading in your garden.
- Remove the stumps of affected trees and as many infected roots as possible oak root fungus spreads via root-to-root contact between adjacent hosts; so removal of inoculum slows the spread of infection.
- Do not plant susceptible plants in areas where infected trees with oak root fungus have died.
A critical aspect of management involves careful irrigation, avoiding waterlogging soils, especially in summer. Here is information on watering of trees.
Make sure the root collar (the base of the tree trunk) stays dry during the warm summer months.
A non-chemical approach to controlling oak root fungus
If planting a tree in an area where oak root fungus has killed previous trees, consider planting the tree in a very large hole with fresh organic material. For more details, see this blog post on Controlling Oak Root Fungus and https://www.plantsciencejournal.com/apdf/jpsp-aid1031.pdf
Fresh organic matter would include yard waste, tree trimmings, organic manure or compost, partly decomposed organic matter.
Usually it is not recommended to add organic material to the planting hole, but in this case, the addition of the fresh organic matter, along with the soil disturbance due to the digging of the hole, helps a common fungus Trichoderma attack oak root fungus.
Two cautions:
- Be sure to add nitrogen fertilizers if fresh organic matter is added to soil
- Be sure to allow for sufficient drainage for the decomposition of the organic matter.
The best defense is to select resistant plants
No known plants are completely immune to oak root fungus. Some plants, like peach (Prunus persica) are very susceptible to this fungus. See the resources below for information on selecting the right variety.
https://wiki.bugwood.org/Armillaria_mellea
http://www.foogod.com/oakrootfungus/
http://www.foogod.com/oakrootfungus/Armillaria-list-6-16-08.pdf
Managing oak root fungus is a combination of selecting the right plants, plant your plants properly with proper drainage, and then watering carefully to avoid overwatering and waterlogging your plants.
Resources:
- After Flooding, Buried Trunks a Problem by Ben Faber
- Controlling Oak Root Fungus by Ben Faber
- Center for Landscape & Urban Horticulture
- Oak Root Fungus - Armillaria mellea, UC Master Gardeners of Butte County
- Planting with a "flare" by Linda Chalker-Scott
- UC IPM pestnote Armillaria Root Rot
- Watering Landscape Trees and Shrubs, UC Master Gardeners of Alameda County
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