Susceptible roses are showing moderate to heavy infection by the black spot fungus, Diplocarpon rosae. It causes black spots, with fringed margins on either leaf surface. As the disease progresses, foliage turns yellow, with black spots; early defoliation follows. Even the flower stems, petals, and canes may show spotting. Disfigured flowers and weak growth make this disease hard to accept in the garden.
The black spot fungus thrives on wet leaves under high humidity and warm temperatures. It overwinters on fallen leaves and rose canes, so fall and spring cleanup can help reduce inoculum. Practices that help air movement in the garden reduce disease spread because plants dry more quickly. Wet leaf surfaces are necessary for fungal infection. Even with these practices, you will likely see some black spot on susceptible plants.
If possible, use resistant rose cultivars. Fungicides may be used to supplement disease control on susceptible cultivars if used as protectants for the healthy new growth. Applications must begin when new leaves appear and weekly into the fall although often sprays can be suspended in hot, dry periods. Follow label directions of the product chosen. A spray of lime-sulfur in the dormant season (winter) may also help disease control. There are 28 products listed as options in the Home, Yard, and Garden Pest Guide and almost as many in the Commercial Landscape and Turfgrass Pest Management Handbook. Look at the end of the disease chapters in these manuals for information on mobility of the chemicals. The systemic products give longer control than the contact chemicals. Consider this when looking at options, costs, and number of applications needed. Active ingredients are listed, so you can see which products are offering the same active ingredient.
For more on black spot, consult Report on Plant Disease (RPD), no. 610, available in University of Illinois Extension offices or on their VISTA web site.