Answer: Mildew is a fungus, so it can be carried over from year to year on old plant debris. A good fall clean up is very important for this reason. I would also suspect that, over the years, all of your plants have grown larger and as a result the air flow is not as open as it once was. You might try thinning the phlox by removing some of the stems in the spring and make sure the plants are divided frequently so they are not crowded. You might also try a different type of phlox; "David" is reliably mildew free in my garden.
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