Rose
Black Spot
Diplocarpon rosae
Fungus
5 mins to read
In a Nutshell
- Small spots on upper leaf side.
- Surrounded by yellow halos.
- Premature leaf drop.
Can also be found in
Rose
Symptoms
Symptoms are described by small spots on the upper leaf side. These purplish or black patches can enlarge rapidly from 2 to 12 mm and show diffuse margins. The surrounding leaf area may turn yellow and drop prematurely. Sometimes also small, black, scabby patches appear on young stems. Under severe infestation the plant may shed almost all leaves and produces less flowers.
Recommendations
Organic Control
The following ingredients are recommended to control black spot: copper, lime sulphur, neem oil, potassium bicarbonate. Baking soda (sodium bicarbonate) can also be used: 1 teaspoon (5 ml) to 1 l water, plus a drop of liquid soap. A formulation containing a bacterium, Bacillus subtilis, is available. Trichoderma harzanium in combination with fungicides also gives good control.
Chemical Control
Always consider an integrated approach with preventive measures together with biological treatments if available. Fungicides containing tebuconazole, tebuconazole + trifloxystrobin and triticonazole are recommended to keep control of black spot.
What caused it?
Black spot on roses is caused by the fungus Diplocarpon rosae. The fungus tends to overwinter on fallen and decaying leaves and stems. Spores are spread by wind and raindrops, infecting openings of leaves in the spring season. The fungus is most severe during rainy season with temperatures between 20-26°C and wet humid conditions.
Preventive Measures
- Use disease-free planting material.
- Plant less susceptible varieties, either the newest or older species.
- Provide a sunny, well-drained, well air circulated planting site and leave 1-1.25 m between the plants.
- Apply a layer of mulch on the soil.
- Prune regularly, removing weak or dead stems.
- Water the soil around roses in the morning.
- Collect and destroy fallen leaves or bury them under mulch.
- Prune affected stems before new leaves appear.
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