Arborvitae Browning: Seiridium Canker from Stress

Emerald Green arborvitae planted last July; one suddenly shows a large brown patch spreading downward from leaf tips. Adjacent tree now showing tip browning.

Diagnosis: Seiridium Canker

Caused by Seiridium unicorne. Affects Leyland cypress and arborvitae. Sunken, dark cankers on bark with resin flow; foliage turns reddish-brown. Spread by water splash and pruning tools.

Management (No Chemical Cure)

• Prune infected branches 1 inch below canker; destroy debris.
• Sterilize tools between cuts (alcohol or 10% bleach).
• Avoid water stress—irrigate deeply during drought.
• Prevent wounds; space plants for air flow.
• Remove severely affected plants.

Underlying cause is stress (weather, planting shock). Healthy plants resist infection.