It could be leaf blight, a common problem with mature Phlox paniculata plants. Older stems on mature plants have difficulty getting enough water to the leaves, and the yellowing usually appears first at the bottom of the plant. Dividing the plant may help.

All my disease resistant phlox have been yellowing from underneath. I sprayed with Greencure fungicide which is supposed to be better than the homemade remedy but see photo Is this a fungus or something else?

I'm in the Dallas, Texas area and planted some small John Fanick phlox in April. It seems to be healthy, but it hasn't grown any taller since I planted it. One of them bloomed briefly last week, but at 8 inches tall compared to the 36 it's supposed to be I'm thinking somewhere I've gone wrong. They're in a location that gets sun for a little more than half a day. They are mulched, is that bad? I haven't "fed" them anything at all, but they are in amended soil.

Wow, it is puzzling. It doesn't look like an ower-watering issue to me, but since we're not coming up with any more likely possibilities, then it is something to consider. This late in the year, it has likely stopped growing, even in Florida temperatures. Pothos tolerates drying out pretty well, so let the top couple of inches dry between waterings. How long have they been in that soil? Did they grow well all summer? If so, they might be a bit pot bound now, which can cause some dry spots in the soil, or some roots to become so restricted that they aren't able to draw enough water. It isn't the best time of year to re-pot, especially if something else is causing the problem, but you should at least check this. Can you pull the entire plant out of the pot with the soil and rootball intact? If so, a photo would help us. Check the root ball and medium for dry or mushy spots, for discolored or necrotic roots, for root parasites, crowding, etc. A few, small disrupted places in the root ball where you dig in to check the condition won't hurt it, even this time of year, and if it's very pot-bound, you can plop the entire thing into a larger pot of soil until the spring when you can do a better job or repotting, detangling, root-trimming, and so on. What parts of the plant are affected the most? Were the oldest/lowest leaves on each stem affected first/worst/most, or is the damage worse on the younger, more tender leaves? Are there any parts of the plants that aren't affected? When was it last fertilized, with what, and how often? (Do not fertilize it now- that's the worst thing one can do to a sick plant if the illness isn't due to fertilizer nutrient deficiency.) It might just be lighting, but the plant in the upper photo appears to have an overall yellowish pallor. That would support Tiffany's hypothesis of too much water. Also, I see a yellow leaf near the base of a stem in the second photo near that little sprig of Acacia or whatever it is that's taken root there. Some yellowing is normal, but check through the lower leaves for more yellowing. Too much water or nitrogen deficiency both tend to affect older/lower leaves first. Evenly yellowing lower leaves is more indicative of too much water or poor drainage, while yellowing that begins at the leaf margins is more often nitrogen deficiency. Other things can cause yellowing though, and that curious staining appearance isn't typical of either issue, so don't fertilize at this point. More photos wouldn't hurt: Entire plant Root ball free from the pot Close-ups of the "stains"