Black Spot
Diplocarpon rosae
A common fungal disease affecting roses. It weakens the plant and makes it unsightly, but is rarely fatal.
Symptoms
- Circular black spots with fringed margins on the upper side of leaves.
- Yellowing around the spots.
- Leaves may drop prematurely.
- Reduced plant vigor.
Causes
- Fungus spores that are spread by water splash.
- Wet and humid conditions for extended periods (at least 7 hours).
- Spores overwintering in fallen leaves and infected canes.
Prevention
- Plant resistant rose varieties.
- Keep foliage dry by watering the soil, not the leaves.
- Prune plants to improve air circulation.
- Clean up fallen leaves and debris around the plants.
Treatments
- Remove and dispose of infected leaves.
- Apply fungicides labeled for black spot, such as those containing chlorothalonil or myclobutanil.
- Neem oil can be effective as a preventative and treatment.