There are several possible causes of the cupped/curled leaves on the phlox. Phlox prefer moist, even-soil moisture. Leaves can curl to conserve moisture as a reaction to heat stress and inadequate moisture.

Spider mite damage is also a possibility. They are considered one of the most serious pests of phlox. They feed on the underside of the plant’s leaves, sucking sap from the leaf. As the feeding progresses, it causes the leaf to “cup” downward. Leaves may become speckled in appearance, turn yellow/brown, and eventually drop. Minute webbing may be apparent on the undersides of the leaves. Management of spider mites can include syringing the undersides of the leaves with a spray of water from the garden hose dislodging them. Insecticidal soaps applied to the undersides of the leaves can provide control.

Aphids feeding on the undersides of the leaves can also cause leaf distortion. They are very small pear-shaped insects if various colors. Aphids usually do not cause little long-term damage, but populations can explode so monitor frequently for them. Use the same management practices for them as for spider mites.

Phlox plant bugs are another possibility. They live under the leaves and also feed on plant juices. Their feeding initially causes white or light green spots on the leaves. Continued feeding can result in leaf curling, turning brown, and falling off. Use the same management practices as above.

Sources: South Dakota State University and University of Minnesota