Armillaria Root Rot vs Fusiform Rust: How to Tell the Difference
Compare two significant fungal diseases of trees: armillaria root rot, which attacks roots, and fusiform rust, which deforms stems and branches of southern pines.
At a Glance
Armillaria Root Rot
HIGH- Type
- Fungus
- Severity
- HIGH
- Affected States
- 36 total (36 active)
Symptoms
Clusters of honey-colored mushrooms at the base of the tree in autumn, white mycelial fans under the bark of roots and lower trunk, black shoestring-like rhizomorphs in soil, crown dieback and resinous bleeding at root collar
Fusiform Rust
HIGH- Type
- Fungus
- Severity
- HIGH
- Affected States
- 11 total (11 active)
Symptoms
Spindle-shaped swellings (galls) on main stem and branches, bright orange spore masses on galls in spring, stem breakage at gall sites, pitch seeping from galls, canopy dieback above girdling galls
Similarities
What makes Armillaria Root Rot and Fusiform Rust easy to confuse:
- Both are caused by fungal pathogens that affect trees across large regions of the U.S.
- Both can lead to structural failure and tree death in severe cases
- Both are native diseases that have been present in North American forests for centuries
- Both can affect tree plantations and cause significant economic losses
- Both are influenced by site conditions and tree stress levels
Key Differences
| Feature | Armillaria Root Rot | Fusiform Rust |
|---|---|---|
| Pathogen | *Armillaria* species (honey mushroom complex) | *Cronartium quercuum* f. sp. *fusiforme* |
| Target area | Roots and root collar of trees | Stems and branches of pines; requires oak alternate host |
| Host range | Extremely broad; affects hundreds of woody species | Primarily loblolly and slash pine; oaks as alternate host |
| Key symptom | Crown thinning, resin flow at base, honey-colored mushrooms at trunk base in fall | Spindle-shaped galls (swellings) on branches and main stem |
| Geographic focus | Found throughout North America in all forest types | Southeastern United States pine forests |
| Underground signs | White mycelial fans under bark at root collar; black rhizomorphs (shoestring-like structures) in soil | No underground structures; disease is above ground |
How to Tell Them Apart
- Check where the disease is located: Armillaria attacks at or below ground level, affecting roots and the base of the trunk. Fusiform rust produces obvious swellings on branches and stems well above ground.
- Look for mushrooms: Armillaria often produces clusters of honey-colored mushrooms at the base of infected trees in fall. Fusiform rust does not produce mushrooms.
- Examine for galls: Fusiform rust creates distinctive spindle-shaped swellings (galls) on pine branches and trunks. Armillaria does not cause galls or swellings.
- Peel back bark at the base: Armillaria produces white, fan-shaped sheets of mycelium between the bark and wood at the root collar, with a strong mushroom odor. Fusiform rust does not produce mycelial fans.
- Identify the tree: If the tree is a southern pine with a branch or stem gall, fusiform rust is likely. If any species of tree is declining with basal decay and mushrooms, armillaria is the suspect.
Frequently Asked Questions
Can a pine tree have both diseases?
Yes. Southern pines, especially loblolly and slash pine, are susceptible to both armillaria root rot and fusiform rust. A tree weakened by fusiform rust galls may be more vulnerable to opportunistic armillaria infection.
Which disease is more common?
Both are extremely common in their respective niches. Armillaria is one of the most widespread tree pathogens in the world. Fusiform rust is the most economically important disease of pines in the southeastern United States, affecting up to 50% of trees in some plantations.
Can armillaria be treated?
There is no reliable cure for armillaria root rot once a tree is infected. Management focuses on maintaining tree vigor, improving drainage, removing stumps of dead trees, and planting less susceptible species on sites with a history of the disease.
Do fusiform rust galls always kill the tree?
Not always. Branch galls may not be fatal and can sometimes be pruned out. However, galls on the main stem are serious because they weaken the trunk structure, making trees prone to wind breakage, and can eventually girdle and kill the tree.
Is armillaria the same as honey fungus?
Yes. Armillaria is commonly called honey fungus due to the honey-colored mushrooms it produces. It is one of the largest and oldest organisms on Earth, with individual colonies spreading across acres through underground rhizomorphs.