Rose black spot

Rose black spot is a fungal disease of roses where purple or black spots develop on leaves, which often drop early.

Rose black spot

Quick facts

Common name Rose black spot
Scientific name Diplocarpon rosae
Plants affected Roses
Main symptoms Purple or black spots on leaves, leaves falling early
Caused by Fungus
Timing Spring onwards

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.

Symptoms

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.

Control

Non-chemical control

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.

Chemical control

Fungicides containing 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. Some formulations of myclobutanil, tebuconazole and triticonazole contain insecticides to control 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.

Download

Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

Biology

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.

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  • SueF avatar

    By SueF on 15/06/2014

    I have a bush rose Ruby anniversary it is nine months old and has black spot. I have read the write up for this but could someone give me the name of the best product to use to assist with this problem and when to use it. I am not the best gardener but my husband gave me this for our ruby anniversary and I do not want to loose it. Can anyone help with this.

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  • JazzyB avatar

    By JazzyB on 15/06/2014

    Hi Sue, I have a lovely climbing rose which was constantly getting this so I bought a product that I found in a garden centre called Fungus Fighter made by Bayer Garden. I sprayed it on at the beginning of the year once the leaves started growing and then gave it a light spray every two weeks. Touch wood it has been perfect and grown amazingly this year and looks shiny and totally healthy. If I find a suspect leaf I take it off and give it a light spray again and so far it has worked like a dream.

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  • JazzyB avatar

    By JazzyB on 15/06/2014

    Hi Sue I forgot to say that I had a small patio rose too that was suffering constantly with this black spot. In the end I cut it right down to nothing and started again. Once the new leaves came through I just started the two weekly spraying and it is amazing now and not a black spot in sight.

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