Research

Rose black spot

How rose black spot affects the garden

Rose black spot is all too familiar to most gardeners. About mid-June on heavily-pruned hybrid tea (large-flowered) roses, the characteristic brownish-black or purplish spots, up to 10mm across, appear on the leaves. These may coalesce into larger patches, and black scabs may appear on the stems. In rose species, which are less strongly pruned, these symptoms may appear in spring. The associated fungal growth releases spores to spread the infection. Eventually the leaves turn yellow and fall, creating a reservoir for potential infection throughout the winter.

Prevention

Roses in poor condition are more susceptible to black spot. Attention to providing good drainage, soil conditioning, and feeding will all help. Foliar feeds may benefit afflicted bushes. Organic mulches help conserve moisture, and are particularly beneficial on light soils.

Remedy

Each autumn, fallen leaves should be collected from around diseased bushes, to be burnt or buried deeply, and all shoots bearing scabs should be pruned out. Bushes should be sprayed with a fungicide immediately after spring pruning, as the leaves unfold, and at least three further times at fortnightly intervals. The use of different fungicides will help to identify the most effective and also avoid the development of resistant strains.

Work on rose black spot at Wisley

This was part of the research in the Pathology section at RHS Garden Wisley.
For more information on the rose black spot research project, click here.