Grow disease resistant roses. At least in this part of "zone 6" that is the best preventative.

what Diane said.......
Sammy

Just plant your roses in an area where there is a great deal of flowing air. Stagnant air encourages the fungus to grow and spread, so if you make sure that your roses are in a breezy area, this will help your rose plants to stay free from any fungal disease such as black spots.

Make sure the soil your plants are growing is a good, healthy soil, well balanced in nutrients with a soil pH that allows the plant to properly utilize those nutrients. Nutrient imbalances create plants much more susceptible to insect pests and plant diseases then is a plant growing in a good, healthy soil. The soil should be well endowed with organic matter so that soil is evenly moist but well drained and well mulched which will aid in suppressing "weed" growth, retaining soil moisture, keeping the soil cooler, and adding more organic matter to the soil over time.

It occurred to me to check in here today. I normally post on the regular Roses Forum.

I would guess that this forum is abnormally slow because people are getting their questions answered on faster forums such as Roses, Antique Roses, Organic Gardening or Soil, Compost & Mulch.

That being said, the others have given great advice as far as getting varieties that resist diseases common in your climate, planting with good drainage and air circulation and using organic mulch to improve the soil.

I would add that seaweed/kelp products build better foliage and alfalfa is another growth booster. A soil test can help you find pH and any soil deficiencies you might need to correct.

All of the above is excellent advice. One of the things that I have had luck with in the past for treatment of black spot if my roses developed it was a simple mix of water with a tablespoon of cider vinigar in it. I would spray the infected rose with it every other day for a week or so and it would clear it up. Black spot wasn't much of a problem for me however because I followed the previous advice of the other posters. I miss my roses and all of my gardens. I moved to another state and don't have any gardens at all at least temporarily.