Downy mildew is a highly infectious fungal disease that can spread to other roses very quickly, in serious cases can cause loss and failure. This can be confused with black spot but there is much less yellowing around the spots. It can also be confused with Potassium deficiency in the soil.
The defining sign is purple or black spots present on the stems as well as the leaves. In extreme cases the plant may defoliate.
Remove and dispose affected leaves and buds as soon as possible. Clean equipment afterwards.
Spray roses during the growing season with a fungicide that states the treatment of downy mildew. All roses in your garden should be treated. Follow up treatment will be necessary.
When winter pruning has been done and all waste removed, spray with Lime sulphur on the plant and ground around. This will clean the area of fungal problems giving you a fresh start in the new growing season.
Prevention is better than a cure
If you are living where roses are known to have this problem, make sure there is good air circulation around your rose and prune to take away dense foliage from the heart of the bush.
Start your chosen spray routine before you see the problem on the leaves.
Make sure your roses are well watered and fed regularly. A happy rose is a healthy rose.
Choose varieties that are strong growers and either newer varieties or those recommended by the rose society in your area.
Important: signs and symptoms will vary significantly between varieties, even within similar categories. The information provided here is a basic summery of the most common affects and will not always be applicable to all rose varieties.