Black spot, Diplocarpon rosae, is probably the most common disease found on roses and the one most people will recognise as a problem. It is a fungal disease.
The rose leaves develop circular brown or black spots which are surrounded by yellowing of the surrounding leaf. Eventually the whole leaf will die and fall off the rose. In extreme cases the entire rose bush can lose its foliage.
Black spot affects all types of rose, ie bush, hybrid tea, climbers, rambling roses, standard rose. Longer term it can kill your rose as the loss of foliage affects the plants ability to photosynthesise.
Good gardening practise will reduce the likelihood of black spot and other diseases. For example:-
- prevention is better than cure – check the leaves when you deadhead your roses
- clear away and burn all diseased rose foliage as it falls or is infected
- tidy up the leaves in the autumn after they’ve fallen off the roses, either burn or put in the green waste collection where the higher temperatures will kill off fungal spores
Other ways to reduce the likelihood of black spot include:-
- adding mycorrhizal fungi to the planting hole when first planting your roses
- ensuring the soil is free draining
- adding plenty of organic matter on an annual basis
- planting your roses in the correct place, some cannot tolerate shade for example
- a foliar feed can sometimes help
Avoid spraying with chemicals that will kill beneficial insects, organic alternatives are available when needed.
There are some disease resistant varieties available. Rosa ‘Iceberg’, a white flowered rose is normally resistant and can be found both as a climbing rose and as a shrub rose. Whether any particular rose is disease resistant does depend on other factors, including soil and general health and the weather conditions so a lack of black spot cannot be guaranteed.