Ask Mr. Smarty Plants

Please follow this plant link, Phlox paniculata (Fall phlox),  to our webpage on the plant and see if the growing conditions in your garden match those of this plant. From that page, we extracted this: "Conditions Comments: The plant needs at least 6 hrs. of sun per day. Powdery mildew is a frequent problem."

Next, please read this article from Colorado State University Extension Powdery Mildew. From that source, we want to emphasize this paragraph:

"Conditions That Favor the Disease

The severity of the disease depends on many factors: variety of the host plant, age and condition of the plant, and weather conditions during the growing season."

Powdery mildews are severe in warm, dry climates. This is because the fungus does not need the presence of water on the leaf surface for infection to occur. However, the relative humidity of the air does need to be high for spore germination. Therefore, the disease is common in crowded plantings where air circulation is poor and in damp, shaded areas. Incidence of infection increases as relative humidity rises to 90 percent, but it does not occur when leaf surfaces are wet (e.g., in a rain shower). Young, succulent growth usually is more susceptible than older plant tissues."

Scroll down that page to the area on "Control," which includes some suggestions on chemical controls.

Those of us who live in blazing, drought-stricken Texas don't tend to think of Pennysylvania as "warm, dry," but everything is relative, and we believe the growing season for  your phlox probably is warm and dry. However, as noted above, if you don't have  6 hours or more of sunshine directly on the plant it is probably going to be very mildew prone. Possibly when you first planted your phlox, shrubs and trees around it were small, and the phlox was not very thick. You can certainly try thinning out the phlox, possibly even some pruning of overhanging woody branches, but NOT IN VERY HOT WEATHER!