Diagnosing Blackening Aucuba: Root Rot, Disease & Environmental Causes

Aucuba (Aucuba japonica) turning black from stems upward signals serious stress. Common causes:

Top Suspects

1. Phytophthora root rot: Waterlogged soil → blackened stems, wilting, death. Aucuba hates wet feet.
2. Southern blight (Sclerotium rolfsii): White fungal mats at base, black sclerotia (mustard-seed sized).
3. Nematodes: Root knot nematodes cause galls, decline, blackening.
4. Winter injury: Sudden freeze after warm spell kills cambium → black dieback.
5. Chemical injury: Herbicide drift or salt buildup.

Diagnostic Steps

• Check soil drainage: dig 12" down—standing water or sour smell = drainage issue.
• Inspect base for white fungus, sclerotia, or girdling roots.
• Scratch bark: green = alive; brown/black = dead to that point.
• Send sample to county extension plant clinic for lab ID.

Resources

• NCSU: Aucuba Diseases
• Crescent Bloom: Aucuba Profile
• UF/IFAS: Aucuba Fact Sheet

Immediate Action

• Improve drainage (raise bed, add compost, install drain).
• Prune out blackened wood to healthy tissue.
• Apply fungicide (phosphonate for Phytophthora) if caught early.
• Mulch 2–3" (keep off stems).

Grandmother's cutting is worth saving—act fast.