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Get Ready for Black Spot

May 30, 2001

No, we have no reports of black spot yet, so there is still time to make plans to fight this disease in 2001. You know it will be here as usual. This fungal disease is common in rose gardens and causes nearly circular, black leaf spots with fringed margins. They are usually seen on the upper leaf surface but may also appear on the underside of leaves. Black spot may also appear on the flowers (look for red spots and possibly some distortion), the stems (purplish red to black lesions), and the fruit. The major concern with this disease is that it causes early defoliation and weakens the plant to infection by other pathogens. Infected plants are often predisposed to environmental and site stress as well.

The black spot fungus thrives in warm, humid weather. It infects when the leaf surface is wet. As soon as temperatures rise again, we will begin to see this disease. Because many secondary cycles of infection occur, the problem can persist throughout the summer.

Ideally, the management of this fungus should begin in the dormant season when plants are pruned of old cankers and winter-killed stems. Remove the material from the site because the causal fungus may overwinter on those tissues, as well as on old leaves. Most of us take this step in normal spring cleanup.

Fungicides are very effective (and time-consuming) in controlling the fungus. Sprays should begin as soon as the disease is seen. Generally, sprays are continued on a weekly basis per label instructions until hot, dry weather occurs. Then sprays can be limited to just after each rain until weather is cool again. Yes, this is a high-maintenance disease for rose growers. The good news is that rose cultivars vary from highly resistant to very susceptible, so you can choose a cultivar with resistance. Because other diseases also bother roses, look for plants that are resistant to black spot, powdery mildew, and possibly rust. Prune surrounding vegetation to allow better air movement in the garden. This helps plants dry more quickly and reduces the time that the fungus can infect the plant. For the same reasons, avoid overhead watering where possible.

To see a photograph of this disease and for more information, consult Report on Plant Disease no. 610, “Black Spot of Rose.” There are too many chemical options to list here. Consult the 2001 Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide (formerly Illinois Homeowners’ Guide to Pest Management).

Author: Nancy Pataky


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