Plantwise Factsheets for Farmers
Black Spot disease in Roses
Recognize the problem
The disease is caused by a fungus and can initially be recognized by small dark brown to black spots on leaves and stems (a few mm to 1.5.cm). The more or less round spots have wavy margins. In an advanced stage the spots enlarge to form dark patches surrounded by yellow rings (see picture). Symptoms are seen on both sides of the leaves. In the worst cases the entire rose bush gets defoliated.
This problem can occur year round and is most severe during rainy seasons; a wet humid environment with temperatures around 24 degrees C for about 7 hours is all the fungus needs to grow abundantly. So it can develop overnight.
Fungus spores (seed-like structures) are spread by insects, splashing water or mechanically by pruning shears etc.
The fungus continues to grow on the stems and decaying leaves when these are on the ground.
1. use resistant varieties;
2. choose the right location for roses; a spot well ventilated and 6 hours sunlight during the day;
3. use correct spacing between plants (3ft apart for smaller roses and 4 ft for bigger ones);
4. prune regularly;
5. water preferably in the morning; no overhead irrigation when watering in the afternoon;
6. prune off infected parts and burn;
7. remove diseased plants, gather dropped leaves and burn.
In wet periods use preventative measures:
Spray weekly with chlorothalonil 3 ml/l water.
When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval.
The recommendations in this factsheet are relevant to:
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