Canker on Knock Out Roses: Aggressive Treatment Needed

When 'Disease-Resistant' Roses Get Sick

Knock Out® roses are tough but not immune. Blackening canes spreading through a planting indicate cane canker, a contagious fungal disease that enters through wounds, winter injury, or stress.

Why It Spreads

• Contaminated pruners (always disinfect between cuts with alcohol).
• Splashing water spreads spores.
• Cool, wet spring weather favors the fungus.

Rescue Protocol

1. Hard prune – Cut each affected bush to 6 inches or until pith is pale green with no brown streaks.
2. Sanitize tools – Dip pruners in 70% alcohol after every cut.
3. Remove all debris – Burn or bag infected wood and fallen leaves.
4. Fungicide drench – Spray plants and soil around bases with a rose fungicide for black spot/canker.
5. Water carefully – Avoid wetting canes until weather warms; spores need moisture.

Prevention

Space plants for airflow, mulch to reduce splash, and prune only in dry weather. Warm summer temperatures naturally suppress canker activity.