Answer: The best defenses are to plant them in an airy location in full sun with good air circulation, thin the plants so that air can circulate through the interior, avoid overhead watering, clean up and remove old foliage each winter, and then add or top up with a fresh layer of clean new organic mulch over the root zones each spring. Also, oversupplying them with nitrogen can contribute to making them more susceptible to fungal problems so make sure you are not overfeeding them.
Following the above steps should go a long way towards avoiding mildew. If you see a foliage problem developing, immediately remove and destroy any infected leaves. Next, the best thing to do is to take a sample to your county extension and have the problem identified precisely. They should also then be able to suggest additional steps to take or, if a spray is required, give you the most up to date recommendations on what to use.
Finally, some varieties of rose are simply more prone to problems than others. If mildew is an ongoing thing in your garden and these roses are repeatedly affected, then you might want to look into trying a different variety with better resistance to foliar problems.
I hope this helps you avoid any ongoing problems.
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