Mold on roses is a vexing problem that can often be solved by a few key changes to the physical care of your roses. When significant colonies of mold have established themselves on your roses chemical intervention in the form of a rose safe fungicide is called for and will help to keep the spores in check. While mold is not typically life threatening to the rose plant, but it is unsightly and can ruin the appearance of both foliage and flowers.
Clean up and discard all fallen petals, leaves, canes, off cuts, weeds, plants and anything else except the soil from under and around your rose plants. Mold spores thrive and multiply in the moist, nutrient rich environment that plant litter provides. When the spores are splashed up onto the plant canes and foliage during rains or watering the mold spores colonize on even more plant tissue.
Refrain from watering your roses overhead or with a gushing stream that will wash or spray soil laced with mold spores up onto the plant leaves and canes. Keep the rose foliage dry, except for when it rains, at all costs as mold spores need moisture to survive. Water gently and slowly at the soil line only and use a soaker or drip hose if you do not have the patience to slowly and carefully water by hand.
Spray affected foliage and canes with a rose fungicide according to product label directions. This will kill the mold spores it comes into contact with. While it won't reverse the damage already done, it will check the spread of mold provided other cultural habits are improved to not exacerbate the problem.