Tall Phlox Covered with Mold or Fungus
Our tall phlox has suffered for years with this issue. A whitish, powdery substance appears on the upper leaves (it still grows quite tall); the lower stems turn dark brown with the leaves turning yellow then brown. It still blooms, it just looks incredibly unhealthy, like it's suffering.
Montgomery County Maryland
This is powdery mildew, which is a fairly common fungal disease in some types of phlox. The best way to minimize this problem would be to plant mildew-resistant varieties. (If you are interested, Mt. Cuba Center did a trial of different phloxes and notes the best performing varieties here: https://mtcubacenter.org/trials/phlox-for-sun/). For your existing plants, there is nothing that will cure the mildew that is already there. Anything you can do to increase air circulation and reduce humidity (thinning out the plants) can help to minimize the problem. You could also apply a horticultural oil labeled for powdery mildew control. Refer to our webpage for additional information. https://extension.umd.edu/hgic/topics/powdery-mildew-annuals-bulbs-groundcover-perennials-and-vines