Rose black spot

(Diplocarpon rosae)

Rose black spot is a fungal disease that attacks roses, causing black or purplish leaf blotches. Attacks usually lead to premature leaf drop, weakening the rose.

1. Spreading black or purplish patches on leaves
2. Leaves may turn yellow around these patches
3. Leaves drop prematurely
4. Reduced growth and vigour
5. Reduced flowering
6. Black, scabby lesions may appear on stems

Rose black spot is a fungal disease of roses – the commonest and the most serious and devastating.

The symptoms can vary from rose to rose and from year to year.

It usually starts as black or dark purplish spots or blotches on the leaves, which rapidly spread across the leaf. These are usually followed by the leaf turning yellow around the spots/blotches. The affected leaves usually drop prematurely from the plant, and badly affected roses can shed nearly all their leaves. If lots of leaves drop, then this seriously weakens the plant and subsequently can reduce flowering.

Attacks year after year will seriously weaken the plant, causing severe dieback and greatly reduced glowering performance.

• Roses

The rose black spot fungus produces spores in the spots or blotches that then spread further to cause new infections on other leaves and plants.

Moist conditions are needed for the disease to build up and spread, and most summers in the UK are sufficiently wet enough for this to occur.

The fungus spends the winter on fallen leaves and on the soil, and on stems and leaf buds. In spring, it produces spores that re-infect the new, young leaves.

Rose black spot is worse on plants that are under stress, especially a lack of moisture at the roots, and during warm, moist weather.

Try to keep plants growing as strongly as possible to help them fend off disease attacks – watering, feeding and pruning as necessary.

Keep a regular check on roses and deal with any symptoms of black spot as soon as they are seen.

Carefully pick off and dispose of affected leaves when the disease first strikes to reduce or slow down its spread. This is highly unlikely to actually eradicate the disease.

Prune out badly infected growth and dispose of it by either burning it or put it in the council’s green waste bin. Don’t compost it. Correct, annual pruning will remove stems affected with the disease and help to reduce infection. Cutting back bush roses in late autumn, usually suggested to prevent root rock in windy weather damaging the roots, will have the same effect.

Carefully pick up or rake up all affected fallen leaves – especially in autumn – to try and reduce carry-over through the winter.

You can help protect new growth and prevent the disease – and to a slightly lesser extent, control existing diseases – by spraying with a good garden fungicide, or a combined fungicide and insecticide. Prevention is better than cure, so use these as early in the year as possible.

Sulphur is the natural enemy of rose black spot disease. Before the Clean Air Act 1956, black spot was relatively unknown as the sulphur in the air killed the spores. Sulphur is also a key nutrient needed for healthy plant growth and, if there is a sulphur deficiency, plants may become more susceptible to diseases.

Use plant protection products safely. Always read the label and product information before use. Plants in flower should not be sprayed due to possible danger to pollinating insects. Either spray early in the morning or late in the evening when pollinating insects are less likely to be active.

Since rose black spot is worse on plants that are under stress, especially those that are short of water at the roots, make sure the soil or compost is kept evenly moist – especially during periods of warm, dry weather. Adding a water-retention gel will help even out periods of soil drying.

Adding a 5-10cm (2-4in) thick mulch of organic matter to the soil around the plant will help conserve soil moisture and help prevent it drying out. It may also help prevent disease spores splashing back onto the foliage.

As roses are hungry plants, feed them with a rose fertiliser, which will contain all the nutrients the plants need to produce strong, healthy growth that may be able to fend off attacks.

Some rose varieties are listed as being resistant to black spot disease. Rose varieties with shiny, glossy leaves tend to have more resistance. Sadly, new strains of the fungus appear regularly, which can lead to a breakdown in this resistance in time.

• Rose fertilisers
• Fungicides
• Combined fungicides and insecticides
• Manure
• Mulches