Q. What kind of spray program would you use to prevent black spot or other common rose diseases?
The management of rose disease is quite a complicated story. The very best thing to do is to start with roses that are disease-resistant. We replanted almost the entire Peggy Rockefeller Rose Garden at NYBG recently to prioritize that strategy. But naturally that is not the way most of us start with our roses and once you have them, disease-management needs to be considered.
Blackspot and powdery mildew are the most common rose diseases and both are fungal. This link to a great article on the American Rose Society website which discusses the issue of fungal diseases and roses is very helpful to understanding what various types of treatments are for and what is effective. Preventing fungal disease is the right approach to take but it is time-consuming. You accomplish that through plant hygiene and, possibly, a program of spraying.
The essentials of rose hygiene are designed to keep fungal-supportive moisture from lingering on rose leaves. That involves having plants far enough apart from each other, and other plants, that good air circulation is maintained; planting only in full-sun conditions where moisture will dry quickly from leaves; removing fallen leaf litter (where fungal spores may be present); and watering the roses from the ground level (rather than splashing water on leaves) and in the morning only (when it has all day to burn off). Make sure that you don't fertilize more than recommended or the rose will actually become less robust and more likely to be affected by disease.
Courtesy of NYBG Plant Information Service