How to recognise it
The first indications of an attack are dark brown spots with irregular margins appearing on leaves from late spring onwards, which may then be followed by a general yellowing of the infected leaves which then drop. It can also spread to infect and weaken the shoot.
The signs of attack may not become clearly visible until July.
Why it’s a problem
Generally weakens the plant and can be unsightly. Infected stems can die back and are particularly susceptible to autumn/winter frosts.
Spores are released from infected plants in wet and warm conditions and readily spread on water droplets to other plants
Where you are likely to find it
On roses, particularly in warm, wet weather.
How to prevent it
Good hygiene practices (eg clearing debris, cleaning implements) can prevent the spread of the disease.
Some cultivars can be purchased which have a level of resistance. Generally, shrub roses and yellow flowered varieties are less resistant to attack than modern hybrid roses, but no rose is completely resistant.
These are some of the more resistant varieties available:
- Large-flowered bush roses (aka hybrid teas) – ‘Alec’s Red’, ‘Alexander’, ‘Blessings’, ‘Chicago Peace’, ‘King’s Ransom’, ‘National Trust’, ‘Peace’, ‘Picture’, ‘Pink Peace’, ‘Prima Ballerina’, ‘Rose Gaujard’, ‘Super Star’, ‘Sutter’s Gold’, ‘Troika’ and ‘Uncle Walter’.
- Cluster-flowered bush roses (aka floribundas) – ‘Allgold’, ‘Arthur Bell’, ‘City of Leeds’, ‘Korresia’, ‘Marlena’, ‘Queen Elizabeth’ and ‘Southampton’.
Some of the more susceptible roses include:
- Large-flowered bush roses (aka hybrid teas) – ‘Blue moon’, ‘Fragrant Cloud’, ‘Harry Wheatcroft’, ‘Piccadilly’ and ‘Wendy Cussons’.
- Cluster-flowered bush roses (aka floribundas) – ‘Elizabeth of Glamis’, ‘Iceberg’, ‘Orange Sensation’ and ‘Tip Top’.
Mixed planting between more and less resistant varieties can help to reduce the incidence of the disease in all the plants.
How to get rid of it
Spores overwinter in fallen leaves and wood produced in the autumn (particularly if it is black spotted), so clearing leaf debris and pruning well can gradually remove the infection. Ensure that this is destroyed rather than put on your compost heap (eg through municipal green waste collections or burning).
You can remove the leaves and shoots which appear early in the season which are infected; this can delay the overall onset.
A fungicide such as myclobutanil, mancozeb or penconazole applied at the appropriate times (generally when buds are opening, then 7 days later, and again when spots appear, repeating every 2 weeks from that point) will control and potentially remove an infection. Do not wait until spots appear to start spraying – by then it will be too late to control the infection. Proprietary fungicides can be purchased specifically to control rose black spot.
In professional horticultural situations, captan can be used as a protectant against the disease, although its effect is reduced as the season moves on since foliage growth will reveal untreated, and therefore vulnerable, leaf surfaces. The use of a wetter/spreader can improve the coverage of the leaf surfaces.
In some industrial areas, the levels of sulphur dioxide concentration in the atmosphere may be sufficient to help control the fungus.
Is it good for anything?!
Other useful information
There are other leaf spots affecting roses:
- Elsinoë rosarum causes small, round dark purple-brown spots with paler centres on leaves and stems.
- Sphaerulina rehmiana causes grey-brown spots which are similar to rose black spot but they have reddish margins.