Black spot of roses
Also known as Rose black spot, Diplocarpon rosae
In short
Black spot is a fungal disease of roses caused by Diplocarpon rosae. It produces dark spots on leaves, yellowing and early leaf fall, and weakens plants year after year. The fungus needs wet leaves to infect, and it overwinters in fallen leaves and stem lesions.
How it looks
Purple to black spots on the upper leaf surface, round with a fringed or feathery edge made of radiating fungal strands, reaching about 14 mm across. Spots can merge into large dark areas. The leaf tissue around them often turns yellow. Small, black, scabby lesions may appear on young stems.
Signs
Yellowing leaves with black spots, heavy premature leaf drop that can leave bushes nearly bare, and progressive weakening and poorer flowering.
Life cycle
Spores form in the black lesions and are spread by water droplets from rain or watering to new leaves. The fungus survives winter in fallen leaves and dormant stem infections, releasing spores in spring onto emerging foliage. Infection fails if leaves dry within about 7 hours after spores start to germinate. It develops best at about 20–27 °C and is checked above about 29 °C.
What favours it
Wet weather and long periods of leaf wetness, overhead watering, and fallen infected leaves left under the bush.
Prevention
- Plant disease-resistant cultivars, though resistance may not last because new strains of the fungus arise to overcome it.
- Water at the base in the morning and avoid wetting the leaves.
- Collect and destroy fallen leaves, and remove leaves remaining on the plant over winter.
- Prune out stem lesions before spring growth starts.
Treatment
Start with the first step and move down only if the problem persists.
- Remove infected leaves as they appear and clear fallen ones.
- Keep foliage dry and improve air flow around the bush.
- Fungicide groups such as dithiocarbamates and chloronitriles are used against black spot; the RHS recommends avoiding fungicides for environmental reasons. If used, follow the product label.
Pros and cons
Cons
- Repeated defoliation weakens roses over successive seasons.
- Hygiene alone gives only partial control because spores also blow in from other gardens.
Look-alikes
- Fungal leaf spot
- Other leaf spots usually have a defined margin rather than feathery, radiating edges.
- Rust
- Rose rust forms orange or black powdery pustules on leaf undersides.
- Downy mildew
- Downy mildew makes purplish angular blotches with fuzzy growth below.
Affected plants
Roses (Rosa species and cultivars).
Related
Sources
Written in our own words from these sources. Check a product label before using any treatment.