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.
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.
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.
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.