PREVENTION
Plant varieties that are resistant to diseases.
Keep water off of foliage if black spot or rust is a problem. Water when there will be enough sunlight to dry off leaves before dark.
Powdery mildew needs dry foliage, so rinse off leaves. Wash leaves every three days. If black spot and powdery mildew are a problem, rinse leaves off when there is enough sun to dry leaves quickly.
Prune roses so that the middle of the bush is free of stems and there is lots of space between stems so there will be plenty of air movement.
Too much nitrogen produces lots of new tender and weak growth. This growth will be susceptible to disease. Use compost or slowly released fertilizers.
Rake up fallen leaves.

CONTROLS
Prune out canes that are diseased; the disease is stored in the canes. If you had severe damage, prune down to 4 inches of the ground in the winter.
Remove all fallen leaves; they are a source of disease.
Mulch about 3 inches deep to form a barrier.
If your roses do not go dormant due to warm winters, remove all leaves and infected canes in mid-January.
Surfactants will eliminate powdery mildew and rust. It will prevent black spot, but not kill it once it has begun. Follow label directions.
Antitranspirants are waxes that work as a barrier. They are effective on mildew and rust. Follow label directions.