Answer: The first defense is to select a variety of rose known for healthy foliage and plant it in a location with good air circulation. Then thin it as needed to allow good air circulation through the plant and avoid wetting the foliage when you water, especially in the evening. Mildew can be predominant in a year when weather conditions are right for it and may not occur again the following year.
Some gardeners report success using a milk spray. (One part milk to five parts water up to one part milk to ten parts water; whole milk may work better than skim and you may use regular or powdered milk.)
Or you might try a baking soda spray. As suggested by the American Rose Society this consists of
"...either sodium or potassium bicarbonate at 3 teaspoons per gallon of water, combined with Sunspray at 2 tablespoons per gallon of water. The bicarbonates eliminated the fungi, but addition of the Sunspray provided a spreader-sticker action that increased its performance."
They also caution: "do not attempt higher concentrations of the solutions, as leaf burn may result. Rain or overhead watering may wash the solution off, reducing its effect."
Be sure to test either of these on a few leaves and wait a few days to see if there is any adverse effect before spraying the entire plant. You will need to apply about once a week or sooner if it rains.
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