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NAME: Black Spot (Diplocarpon rosae)
BIOLOGY: Black spot is the most serious and widespread disease of roses, thriving wherever there is adequate humidity and rainfall. The arid Southwest is the only region of the United States where black spot is rare. The disease overwinters in infected canes and on fallen leaves. In the spring, the spores are spread by splashing water, and they germinate wherever tissues have been moist for at least seven hours, at a temperature between 60 and 80°F.
SYMPTOMS: Round or irregularly shaped black spots with a feathery margin on the leaves are the most common indication. These spots can reach up to half an inch in diameter. Tissue around the spots turns yellow, and the leaflet eventually dies and drops from the plant. Leaf petioles, rose hips, and stems also can be sites of infection. Infected canes exhibit reddish purple spots that become blackened and blistered with time. Roses seriously infected produce few flowers as a result of defoliation and reduced vigor.
CONTROLS: Keep the foliage as dry as possible. Plant roses where they will receive at least six hours of sun, in well-drained soil, and spaced far enough apart to allow adequate air movement. When watering, use a soaker hose or drip irrigation to avoid wetting the leaves. Remove infected leaves to reduce the source.
Rake and mulch the ground under the roses in the spring, and remove any foliage that remains in the fall. Diseased canes should be cut out and pruners disinfected afterward to avoid spreading the disease (use equal parts of water and Lysol). Contact fungicides such as chlorothalonil (Daconil 2787) and mancozeb (Dithane) can be used, but these should be rotated with systemics such as propiconazole (Banner MAXX) and thiophanate-methyl (Cleary’ s 3336) to avoid the build up of resistance.
Finally, choose cultivars that are naturally resistant to black spot such as ‘Peace’, ‘Iceberg’, ‘Carefree Wonder’, ‘The Fairy’, 'Scarlet Meidiland’, and most rugosa roses. For more information on disease-resistant roses for your area, contact the American Rose Society (www.ars.org), or your local cooperative extension service.