Treating Black Spot on Knockout Roses in Humid Houston Climate

Managing Fungal Disease on Disease-Resistant Roses

A Houston gardener's Knockout roses, planted for a 50th anniversary, show yellow leaves with black spots on older foliage despite healthy new growth. The site gets half-day shade in winter/spring, full sun in hot, humid summer.

Black Spot Confirmation

Symptoms match black spot, a common fungal disease. Even "disease-resistant" Knockouts succumb in humid, partially shaded conditions—ideal for fungal spores.

Control Strategy

• Remove and destroy (burn or trash) all infected leaves—don't compost.
• Apply any labeled rose fungicide per directions.
• Improve air circulation via pruning.
• Water at soil level, not overhead.
• Accept that spores persist in air; management, not eradication, is the goal.

Realistic Expectations

Knockouts are excellent roses but not immune. Consistent cultural practices and timely spraying keep them healthy.