Perennial phlox plantings are subject to powdery mildew disease, caused by the fungi Erysiphe cichoracearum and Sphaerotheca humuli. Signs of powdery mildew appear as a white, powdery growth on leaf and stem surfaces. Infected leaves may shrivel up and die. Although the disease generally does not kill plants, it affects their appearance and flower production. Further infestations that occur on a yearly basis can lead to a permanent decline in the overall vigor of the plant. It may be wise to move it to a sunnier location within the landscape.

Where possible, choose varieties that have resistance to powdery mildew. Reduce humidity in phlox beds through wider spacing, full sun exposure, and careful watering.

Fungicides are sometimes beneficial. Check the fungicide label for rates, timing, and approved use for garden phlox. Contact your local Cooperative Extension office for fungicide recommendations.