You spot some mushrooms at the base of your tree and shrub. Then, maybe you notice some white or black fungal growth. Your plant could be infected with armillaria root rot.
This disease is virulent around North America, primarily affecting oaks, maples, larches, beeches, aspens, apples, pears, cedars, spruces, firs, and pines, but can affect others as well.
Due to its aggressive mycelial spread, it can branch out and attack several different neighbor species along their interconnected root networks.
It can also spread to certain woody flowering shrubs, including azaleas, rhododendrons, and roses. We’re going to touch on how to identify this pathogen and how to deal with it effectively.
What Causes Armillaria?
Armillaria root rot is also called “shoestring root rot” or “oak root fungus.” It’s most commonly caused by Armillaria mellea, which is a basidiomycete fungus (i.e. one that forms and reproduces via filaments) and can affect both deciduous hardwood and coniferous species.
Various types of stress can all contribute to this insidious root rot. Unfavorable conditions such as heavy rain/flooding, drought, nutrient deficiencies, wounds at the root level, soil compaction, and insect presence can all contribute to its spread.
Let’s say a tree has been wounded low by passing machinery, leaving an open gash in the bark. Recent drought conditions have left the tree with diminished nutrients and water content.
Then, an insect that has previously fed on an infected tree flies over and carries some fungal spores over to the open wound or defecates on the soil surface nearby.
Once those spores find a welcoming embrace, so to speak, they spread out filaments until they gain purchase within the bark’s cambium, or in the tasty, healthy roots.
Remember that since Armillaria attacks plant roots, symptoms in the trees’ or shrubs’ aerial parts may not manifest until the fungus is well established. That said, there are usually some telltale signs that can tip you off to its presence.
Below are some of the key symptoms to look out for:
Failure to Thrive
Your plants may seem listless or otherwise “out of sorts.” Their trunks or stalks may be limp or curved instead of strong and firm, and their foliage may be sparse (or fail to emerge). Branches may also droop, and the tree may appear to age more quickly than is typical for that species.
Leaf discoloration and dropping
If your plants do develop leaves, they may show limpness and wilting despite adequate water and nutrition. Additionally, they may start to turn yellow, lose pigment, or start to drop well ahead of their autumn shed cycle.
If they do start to discolor, this pigment loss starts around the perimeter edges, gradually moving toward the stem center.
Stem and/or Branch Lesions
You may notice lesions and canker-like growths on your plant’s trunk, stem, or branch. These may differ in appearance, such as being bumpy and yellow or brownish-black with white centers.
This isn’t a term of endearment but refers to the honey-colored mushrooms that often develop on dead or dying plants.
These mushrooms are the fruiting bodies of the Armillaria fungus, lending it to have the additional monikers of “honey fungus” or “honey mushroom rot.” These mushrooms generally develop at the trees’ bases or adjacent soil areas.
Fungal Threads Between Bark Layers
In addition to the obvious honey mushrooms mentioned above, trees afflicted with Armillaria may have mycelial mats between their bark layers. These are damp, whitish-gray webs of thread-like fungus that most often appear between the outer sapwood and the inner cambium bark.
As this fungal issue is most commonly referred to as “Armillaria root rot,” you won’t be surprised to learn that the most telling symptom is decaying roots.
When and if you dig them up, you’ll find piles of yellowish-white, stringy fungus, which turns into brown-black rhizomorphs (thick mycelial strands) as the infection progresses. They may develop just above soil level, under and around the bark.
These rhizomorphs creep up into the living tree’s healthy tissues, spreading the fungus as it goes.
How to Treat Armillaria
This blight can be caused by several different Armillaria species, although Armillaria mellea is by far the most common.
Buy Resistant Species
Although only birch trees seem to be naturally more resistant to Armillaria, some other species have been cultivated to resist this pathogen. Furthermore, indigenous species to a specific area are often more resistant than those that have been introduced.
If you’re cultivating your land with new tree species, consider consulting with a horticulturist or arborist to discuss the best options for your area. Once you’ve done that, it’s time for the next tip:
Test Your Soil
Since an ounce of prevention is worth a pound of cure, it’s a good idea to get your soil tested to see if it contains Armillaria fungal spores. Even if this infection hasn’t been discovered in any nearby species, spores may still be present.
Most areas have a closeby University extension office that will run tests for a small fee.
Treat the Area if Needed
If spores are found, chemical fungicides may effectively treat or eliminate the issue. The most common ones are metalaxyl and fosetyl-aluminum, with the latter being far more harmful to many wild animal and pollinator species than the former.
In contrast, beneficial fungi and bacteria have proven to be incredibly effective at managing Armillaria without causing additional harm to other life forms. Some of the best include the following:
- Trichoderma spp.
- Pseudomonas fluorescens
- Phlebiopsis gigantea
- Burkholderia species
- Bacillus subtilis
- Streptomyces spp.
Additionally, if your soil is holding too much moisture, thus facilitating the fungal spread, try to improve soil drainage in and around your planting area. Dig down at least a few feet, and add a generous amount of aged compost as well as amendments such as gravel.
Practice Good Garden Hygiene
Keep surface soil clear of fallen branches, leaves, and other detritus, especially if there’s a risk that it came from any infected species. If there are any spores present, this will lessen the possibility of them spreading to healthy plants.
A Note on Edibility
If you’re a mycophile and like to explore tasty fungi species, note that the fruiting Armillaria honey mushrooms are edible when cooked, and apparently quite tasty.
There are some potentially harmful lookalike species out there, so ensure that you’re absolutely positive with your mushroom identification before cooking and eating them.