Tony Dickerson answers your questions
Q What’s the best way to keep black spot on my roses under control? Janet Fisher, Wragby, Lincolnshire
A Black spot is a fungal disease of roses where purple or black spots develop on the leaves. The damage can be quite limited, but in many cases the patches spread outwards and much of the rest of the leaf turns yellow before dropping. It’s the most common disease of roses and can occur from spring onwards. Where just the odd leaf is infected you can just pick it off and bin it. But where roses are badly infected you need to take steps to reduce the disease, as the vigour of the plant will be affected if too many leaves fall. Older, species types of roses often show good resistance, but black spot is a disease that develops new strains rapidly so even modern roses sold as black spot-resistant may become infected after two or three years. Drier summers restricts the spread of the disease, particularly in southern Britain. The fungus spends the winter as resting spores on fallen leaves, so collecting and destroying fallen leaves in autumn will help prevent the spread. Also check for, and prune out, any stem lesions in spring and don’t compost prunings. Most fungicides available to the home gardener will help control the problem, together with other common rose diseases such as rose rust and powdery mildew. It’s probably not worth spraying now so late in the season, but next year you should spray susceptible roses on a regular basis following the manufacturer’s instructions.
There’s also a range of products for roses that combine a fungicide and insecticide, but only consider using these if pests are actually present on the roses to avoid harm to beneficial pollinating insects. Popular brands include Bayer Multirose 2 and Scotts Rose Clear Ultra. These will also control common rose pests, such as aphids and sawfly.
Most roses are best pruned in late February. Follow this up with an application of a rose fertiliser. If possible, mulch round plants with well-rotted, farmyard manure or garden compost. Repeat the application of fertiliser in late May or early June.