Armillaria Root Rot
Armillaria ostoyae
462 host plants · Fungal
Last updated
This profile synthesizes data from multiple published sources. Expert field review is in progress.
Affected trees show thin, chlorotic foliage, distressed cone crops, and abundant resin flow or brown liquid leaching at the tree base. Trees slowly decline and often die standing, though some may blow over in wind. Beneath the bark at the root collar, you'll find the diagnostic white, stringy mycelial fans adhering to the wood and inner bark. Black shoe-string-like structures called rhizomorphs may extend into surrounding soil. Bark beetles often hasten decline in stressed trees.
Armillaria root rot is difficult to manage once established, so prevention through stress reduction is critical. Plant only high-quality, well-acclimated stock and ensure proper planting depth. Maintain good drainage, avoid waterlogging, and irrigate deeply rather than with frequent light watering that wets the crown. In landscape hedges, remove healthy-appearing trees next to diseased ones because infection spreads through root contact. On sites with known disease centers, clear all stumps and roots greater than 1 inch in diameter and leave the ground fallow 1 to 3 years before replanting.
Quick Reference
Management
Infection can occur year-round via root contact with infected wood and rhizomorphs; risk is highest during cool, moist months (Oct–June in Puget Sound) when soil moisture is high and rhizomorph growth is active. Symptom expression typically appears mid-to-late growing season (July–Sept) when stress from heat and drought stress stresses already-compromised root systems. Visible decline often lags actual root colonization by 1–3 years; newly infected trees may appear healthy for an entire season before symptoms emerge. Replanted sites with infected residual roots present immediate risk to new trees. Diagnostic fruiting bodies (mushrooms) appear Oct–Jan but are not the primary infection vector. BBCH 10–70 (emergence through growth); symptom appearance BBCH 75–89 (development through dormancy entry).
Armillaria ostoyae infection is primarily an opportunistic pathogen of stressed or weakened trees. Stress factors include poor soil drainage (waterlogging), drought stress that reduces root vitality, poor planting depth (too deep), root injury from cultivation or pest damage, and competing roots. Trees on newly cleared land with infected stumps and decaying roots as inoculum sources are at highest risk. Cool, moist soil conditions (Oct–May in Pacific Northwest) favor rhizomorph extension and root colonization. Bark beetles often hasten tree decline once roots are compromised, accelerating visible symptoms. The fungus persists indefinitely in infected stumps and soil; replanting susceptible species in infested soil leads to rapid infection of new trees.
Cultural Controls
- Maintain good overall plant health and vigor.
- Avoid stress conditions—manage water carefully to prevent both waterlogging and drought.
- Ensure proper soil drainage and avoid planting in poorly drained sites.
- When clearing infected sites, girdle large trees before removal to hasten root decay.
- Remove aboveground vegetation and clear soil of stumps and large roots after tree removal.
- Deep-rip soil in multiple directions to bring large roots to the surface.