Armillaria Crown and Root Rot

Infection can occur year-round but is most active Oct–May when soil moisture is high and root growth is moderate. Symptom expression typically appears mid-to-late growing season (July–September) when heat and stress trigger sudden collapse of infected trees. Young transplants are vulnerable immediately upon planting into infested soil. Replanted sites (where infected roots remain after removing dead trees) present highest risk — avoid replanting susceptible species for at least 1–2 years and only after removing infected roots. BBCH stage 10–70 (root emergence through active growth); peak symptom appearance BBCH 75–89 (fruit development through dormancy entry) when infected trees cannot sustain water uptake.

Infection risk is year-round in moist, cool soil; Armillaria is most active during dormancy and cool months (Oct–May in Pacific Northwest) when soil moisture is high. Stressed, weakened, or recently transplanted trees are most vulnerable. Root injury from cultivation, herbicide drift, soil compaction, or competition from other roots increases infection risk. Excessive irrigation that keeps roots constantly wet, poor drainage, or trees planted too deeply all favor pathogen activity. Young, vigorously growing trees are less susceptible than mature or stressed trees. Disease initiation is slow (latent); infected trees may show symptoms only years after root colonization begins, particularly if trees are vigorous. Symptom expression is triggered or accelerated by drought stress, poor nutrition, pest damage, or repeated stress events.