fungicide might help. they are notoriously famous for mildew. Some cultivars are less susceptible. David for one, but there are others.

I don't know for certain, but looks like it could be a fungal infection or spider mites or an awful combination of both. Maybe you can try spraying with neem oil at 10-day intervals, making sure to coat the undersides of the leaves? This will knock out any fungal or mite problem and do a minimum amount of harm to beneficial insects. Just be sure to spray late in the evening or on a cloudy day or the neem oil could magnify the sunlight and cause burned foliage.

do you have a local extension service you could take a leaf to?

before spraying anything. identify the problem for sure.

you can always plant new ...

but check w extension before you attack chemically

dave

I read to throw them away and start over when that happens. It will spread - & also to make sure the air flow is good around them... There are better phloxes for humid conditions

no, don't throw them away.....you can try a Chelsea Chop - cut them back by half, water and feed and inspect the new growth - which will be bushier and more able to support itself. It will be later in flower and the flowers will be a little bit smaller than usual....but this is a very good treatment for many late flowering perennials. One of the most notorious diseases of phlox is eelworm.....which this is not. With an eelworm infection, the leaves tend to curl and grow in very thin strips, looking almost like there is only a central vein and no green substance. This does look like a fungal disease so rather than using copper sulphate (an accepted organic spray for fungal disease), I would be inclined to try a systemic fungicide.