Give your roses full sun and plenty of space between for good air circulation. Water at ground level, not with a sprinkler that leaves moisture on the foliage. Pick up and dispose of all dropped foliage. Pick off any leaves that appear to be developing blackspot.
Even with these best practices, your roses may still get blackspot. It is a fungal disease. The best prevention is buying roses that have been developed with immunity or great resistance to blackspot. There are several new varieties on the market now. Most of the older roses are not resistant to blackspot except for the rugosas.
Answered by Bobbie's Green Thumb from USA, Shaker Heights in January 2011.
Bobbie's Green Thumb is a Landscape designer and consultant, garden coach, garden writer.