What is black spot?
Black spot is the most serious disease of roses. It is caused by a fungus, Diplocarpon rosae, which infects the leaves and greatly reduces plant vigour. Expect to see leaf markings from spring, which will persist as long as the leaves remain on the plant.
The fungus is genetically very diverse and new strains arise rapidly. Unfortunately, this means that the resistance bred into new varieties usually fails to last because new strains of the fungus arise to overcome it.
These are variable, depending on the rose variety and the strain of the fungus.
You may see the following symptoms:
- Typically, a rapidly enlarging purplish or black patch appears on the upper leaf surface, with diffuse and radiating strands of the fungus sometimes just 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 affected plants can shed almost all their leaves and their vigour is greatly reduced. The symptoms are so severe that, anecdotally, the disease has been blamed for a decline in the popularity of roses in UK gardens in recent decades.
Collect and destroy fallen leaves in the autumn, or bury under a layer of mulch. Prune out all stem lesions in spring before leaves appear. These actions will help delay the onset of the disease, but are of limited value because spores are bound to blow in on wind-blown rain from elsewhere.
Popular garden varieties of hybrid teas, floribundas, climbers and patio types are usually susceptible. Gardeners may gain a few years' respite by planting the newest varieties which claim resistance, but as discussed above, this usually does not last. Older species types are little affected.
Fungicides containing difenoconazole (Westland Plant Rescue Fungus Control), myclobutanil (Bayer Garden Systhane Fungus Fighter, several formulations), tebuconazole (Bayer Garden Multirose Concentrate 2), triticonazole (Scotts Fungus Clear Ultra) and Plant and Fish Oil Blends (Vitax Organic 2 in 1) may all be used. The formulation of tebuconazole (Bayer Garden Multirose Concentrate 2) contains deltamethrin and some formulations of triticonazole (Scotts Roseclear Ultra and Scotts Roseclear Ultra Gun) contain acetamiprid to control insect pests.
It is advisable to alternate several products to maximise their effectiveness. Avoid products also containing insecticides unless there is also a pest problem that needs control.
Plant and Fish Oil Blends (Vitax Organic 2 in 1) may also be used and are permitted under some organic regimes.
Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)
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The fungus produces spores in the black spot lesions on the upper leaf surface and these spread in water to initiate new infections. Wet conditions are required for the disease to build up, but most summers in the UK are sufficiently wet. The fungus spends the winter in resting structures on fallen leaves and also in dormant infections on young stems and buds, producing spores in the spring to infect young foliage.