Interesting. I spray weekly in Florida and I have blackspot invariably show up even with a rotated Bannermax, Funginex and Bayer fungicide routine.

pkapeck, you might try a heaping handful of corn meal every week instead of any other fungicides. You have to give the old fungicides 3 months to wear off. Then apply a light dusting of compost (minimum) and immediately some corn meal. Then just repeat the corn meal every week. Scatter a handful under the canopy of each of the roses.

Corn meal works by growing its own fungus as it decomposes. The fungus that decomposes corn is itself attacked by another fungus called Trichoderma. Trichoderma is a family of 'clean-up' fungi. They come in to clean up when there is too much fungus around. Within 10 days of applying corn, the Trichoderma population will blossom. Any black spot spores or fungi living in the soil will be destroyed by the Trichoderma. You could also make a corn tea and spray the roses. Soak the corn overnight and spray every surface of the plants. You could use that same corn meal under the roses if you want.