Black spot is one of the most serious diseases of roses. It is caused by a fungus, Diplocarpon rosae, which infects the leaves and greatly reduces plant vigor.
The name is very appropriate, because this fungal disease forms black spots all over the foliage of rose bushes and if left uncontrolled, it can cause a rose bush to totally defoliate.
The disease does not kill the plant outright, but over time, the loss of leaves can weaken the plant making it more susceptible to other stresses and damages.
The fungus is genetically very diverse and new strains arise rapidly.
DISEASE CYCLE
The fungus overwinters in resting structures on fallen leaves and also in dormant infections on young stems and buds.
The produced fungal spores germinate and are disseminated by splashing water. These spores on the leaf surface must be continuously wet for at least seven hours for infection to occur.
Once infection is established, the fungus develops fruiting bodies called acervuli in black lesions which, in turn, produce spores that splash to new tissue, spreading the disease.
Disease development is favoured by cool, moist weather.
SYMPTOMS
The symptoms are usually variable, depending on the rose variety and the strain of the fungus.
Typically, rapidly enlarging purplish or black patches appear on the upper leaf surface, with diffuse and radiating strands of the fungus sometimes visible.
Leaf tissues may turn yellow around the spots and the leaf often drops, even though other parts are as yet unaffected. At other times, the yellow colour does not appear, but infected leaves still drop.
Sometimes, the spots remain relatively small and the leaf does not drop.
Small, black, scabby lesions may also appear on young stems.
Badly infected plants can shed almost all their leaves and their vigour is greatly reduced.
DISEASE CONTROL
Chemical control
The following fungicides are recommended for use in preventing and curing black spots of roses.
- RANSOM 600WP 15g/20l
- EXEMPO CURVE 250SC 15ml/20l
- DOMAIN 250EC 10ml/20l
- BRADLEY 500SC 10ml/20l
- DUCASSE 250EC 20ml/20l
- MEGAPRODE LOCK 525P 30g/20l
- MILLIONAIRE 690WDG 40g/20l
- PROVIDENCE 400WP 50g/20l
- ABSOLUTE 375SC 10ml/20l
- EXPLORER 3SL 10ml/20l
- LONGSTAR 400EC 10ml/20l
- MONUMENT 400EC 40ml/20l
Non-chemical control
- Collect and destroy fallen leaves or bury under a layer of mulch. Do not compost them.
- Plant resistant/tolerant varieties
- Use disease free planting materials
- Avoid overhead irrigation to keep the foliage dry.
- Prune plants to allow more air circulation and facilitate drying of foliage
- Grow plants in an open sunny location to promote rapid drying of the foliage
Note
- Whenever spraying, it is advisable to mix the fungicide with INTEGRA 3ml/20l, which improves its efficacy. It is a sticker, spreader, wetter and penetrant.
- In order to prevent resistance build-up over any of the fungicides by the fungus, it is recommended that several fungicides be alternated throughout the crop season.
- Fungicides should be reapplied every 7–14 days.
- Timely control of the disease reduces losses due to disease infection.
