Black Spot is caused by a fungus that remains in the soil from year to year. The only solution is to spray with a fungicide specifically formulated for this purpose. However, in order to prevent your roses from getting the disease, you must spray them before the signs of black spot appear. Once the characteristic yellow leaves with black spots appear, it is too late to get rid of it, and the leaves will eventually; fall off.
This disease is spread and carried to other roses by the leaves of infected roses coming in contact with soil and water that splashes onto them.
Infected leaves should be removed and not composted. If your roses have symptoms of the disease, spray them with fungicide for Black Spot to protect the canes and any new growth that may occur during the season.
For next year, I would recommend spraying roses with fungicide as soon as new growth appears, and then again during the growing season according to label directions. Black Spot will not kill your roses...it just defoliates them, and affects their ability to perform photosynthesis, which in turn affects their ability to produce flowers. Good luck...
Article By Melissa Medina
Gracefully Contributed To GrandBobsGarden on June 4, 2008