This fungal disease, caused by Diplocarpon rosae, is common in rose gardens and causes nearly circular, black leaf spots with fringed margins. It is usually seen on the upper leaf surface but may also appear on the underside of leaves. The spots are easy to see, getting as large as 3/4 inch in diameter, although most are in the 1/2 inch in diameter range. When these spots merge, they create large, irregular lesions on the leaves. A yellow margin may appear, followed by early leaf drop. 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 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. Because many secondary cycles of infection occur under these conditions, we should be having a booming summer for black spot. That does seem to be the case.
Ideally, the management of this fungus should begin in the dormant season when plants are pruned of old cankers and winter-killed stems. Remove this material from the site because the causal fungus may overwinter on those tissues as well as on old leaves.
Chemical options 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.
For more information, consult Report on Plant Diseases (RPD) No. 610, Black Spot of Rose. The many chemical options are listed in the Commercial Landscape and Turfgrass Pest Management Handbook 2000 or the Illinois Homeowners’ Guide to Pest Management.