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 cultivars usually fails to last because new strains of the fungus arise to overcome it.

Symptoms

These are variable, depending on the rose species or cultivar 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.

Control

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.

Non-chemical control

Collect and destroy fallen leaves in the autumn, or bury under a layer of mulch. If practical, pick off and dispose of any leaves that remain on the plant over the winter months. 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 cultivars of hybrid teas, floribundas, climbers and patio types are often susceptible. Gardeners may gain a few years' respite by planting the newest cultivars which claim resistance, but as discussed above, this usually does not last. Older species types are little affected.

Chemical control

The fungicides tebuconazole (Provanto Fungus Fighter Concentrate), tebuconazole with trifloxystrobin (Provanto Fungus Fighter Plus, Toprose Fungus Control & Protect), and triticonazole (Fungus Clear Ultra) are labelled for the control of rose black spot.

The following products contain a combination of both insecticide and fungicide, enabling the control of both damaging invertebrates and disease: triticonazole containing acetamiprid (Roseclear Ultra, Roseclear Ultra Gun). When a proprietary product contains an insecticide as well as a fungicide it would be preferable to use an alternative product if invertebrate damage is not a problem on the plants treated.

Inclusion of a product does not indicate a recommendation or endorsement by the RHS. It is a list of products currently available to the home gardener.