Marssonina rosae and Diplocarpon rosae
Image above: courtesy of Elise Dando
Black spot on roses is caused by fungal pathogens (Marssonina rosae and Diplocarpon rosae – sexual stage). It is specific to roses and primarily attacks the foliage but can sometimes infect stems too. The fungus favours wet and humid conditions, and infections occur when water remains on the leaves for approximately 6 - 7 hours. The spores are easily spread by rain or water splash. Areas with high summer rainfall and high humidity usually have the highest incidence of black spot.
It’s important to understand that how you care for the plant, for example watering and pruning, can have an impact on disease occurrence. In addition, recognising the symptoms of black spot early in the season and treating at first sight, will help reduce the spread.
It can sometimes feel like an on-going battle with black spot, particularly in humid areas, but with the right care and treatment, roses will have a better chance fighting the disease.
Irregular black spots on the leaves that become feathery or fringed with yellow margins. In more susceptible varieties, black spots can also appear on stems. If left untreated, leaves will yellow and fall. This weakens the rose bush and if the disease spreads, will cause the plant to defoliate and eventually die.
Roses are susceptible to black spot in warm and humid conditions, and this can be exacerbated if light and air flow is poor. To improve on light and air flow, ensure plants are adequately spaced (check plant labels to see how wide they grow) and when pruning in winter, ensure you open the crown to allow light and air flow into the centre (see How to prune your roses). Also, avoid overhead watering or wetting the foliage, as this can encourage black spot.
In winter, rake up all diseased leaves and dispose of them in the bin – do not add to green waste or compost. Prune back diseased stems to healthy wood, ensuring you sterilise the secateurs between cuts by dipping them in bleach – this prevents any spores from being spread as you prune.
After pruning, spray stems with Yates Rose Gun Black Spot & Insect Killer or Yates Rose Shield Concentrate. These are systemic fungicides, so help control the fungus from within the plant, rather than just on contact. Once foliage emerges in spring, spray regularly with Yates Rose Gun Black Spot & Insect Killer or Yates Rose Shield Concentrate or at first sign of disease. Continue to spray throughout the season, especially if conditions are warm and humid.
If you did not spray early in the season and you notice black spot on the leaves or stems, remove the affected parts (including any fallen leaves) and dispose of them. Spray thoroughly with Yates Rose Gun Black Spot & Insect Killer or Yates Rose Shield Concentrate and repeat as required.
A healthy plant is less susceptible to pest and disease attack. To help maintain a healthy plant, feed regularly throughout the season with Yates Thrive Natural Roses & Flowers Organic Based Pelletised Plant Food. It is made with the right balance of nutrients to nourish roses and flowering plants, plus it contains organic matter, which helps improve the soil and can assist with recovery of stressed plants.