When the weather is hot days and cold nights it becomes the idea condition for downy mildew. This is the worst kind as it will overwinter on the canes and show up again when next year. Spraying at the first sign with a fungicide weekly until all signs are gone will cure it. Some roses will get it if their roots are dry so a mulch is a good idea. Too close planting where air cannot circulate is another reason for this type of mildew.
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