Symptoms
Black patches appear on the leaves and stems of roses. Leaves fall prematurely.
Cause
Blackspot is a fungal disease most prevalent in wet weather
as it is spread by water-splash or wind-blown rain. Susceptible
cultivars can be severely defoliated if left untreated. The
disease is most prevalent in clean air areas and occurs less
frequently where atmospheric pollution is a problem.
Non-chemical control
Remove infected and fallen leaves promptly and regularly, particularly in the autumn. If this is not practicable, the leaves can be buried deeply, which has also been shown to de-activate the spores. Hard prune infected bushes in spring and burn the prunings. Check out catalogues and plant roses resistant to blackspot, such as Rosa bracteata, 'Bonita' (ground cover) and 'Mermaid' (climber), but do not place heavy reliance on resistance because the fungus has several races and newly launched resistant varieties often soon succumb. Other varieties and Rosa species often show more resistance than climbing and bedding types.
Chemical control
Spray with penconazole (Scott's Fungus Clear) or myclobutanil (e.g. Systhane Fungus Fighter or Roseclear 3, which also contains an insecticide) to control the disease, alternating these with the protectant mancozeb (Dithane) to prevent the fungus from developing resistance to the fungicides. Avoid using products containing insecticides unless a pest problem is actually present. Several sprays may be required during the growing season, starting immediately after late winter/spring pruning.