Leaf spots attack the leaves. Remove infected leaves as noticed.
Powdery mildew is the most common diseases on this plant. The disease causes a white powdery growth on the leaves. Use sprays of sulfur or benomyl.
Verticillium wilt may kill phlox. Remove and destroy infected plants. No chemical control is listed.
Crown rot may cause rotting near the soil line. A white fungal growth forms on the stem bases. Remove infected plants and avoid infected soil. The disease is caused by Sclerotium rolfsii.