Growing perfect roses is much easier than most people think. One of the biggest problems with growing roses is disease prevention and control. Blackspot and powdery mildew are the two biggest fungal problems attacking roses. Through years of trial and error I've found that a little preventative care, avoids bigger problems in Spring.
Dormant oil is a wonderful product. It acts by smothering dormant fungi and the eggs of thrips and other critters hibernating on your roses canes. 2 ounces of dormant oil to a quart of tepid water is all it takes. In the Fall, after you've cleaned your rose beds up and pruned, simply apply the oil, thoroughly saturating the entire bush, canes and all.
In the early Spring, before leafing occurs, simply repeat the process, and you'll have healthy insect and disease free roses. If, in spite of your twice yearly spraying, you still get the occasional outbreak of mildew or blackspot, simply respray the affected areas with this oil.
Dormant oil is also used on fruiting trees where insect burrowing is common and very damaging. You apply this oil at the same rate, using the same method of saturating limbs and bark, Fall and Spring. I spray two weeks before I use dormant oil with a copper/sulfur mix to further strengthen my roses and trees against fungi. Using this two pronged approach to preventive maintenance in my garden has given me healthier, more productive roses and fruit trees.