Phlox Powdery Mildew Spreading to Neighboring Plants

White Mildew → Leaf Spots → Death

The progression on your ‘Eva Cullum’ phlox—white fuzzy patches, then spots, then browning from the bottom up—is textbook powdery mildew. The weed barrier and rocks may trap humidity.

Management

• Improve air circulation (thin stems, remove rocks/weed barrier if possible).
• Avoid overhead watering.
• Apply a labeled fungicide (see Extension bulletin for specifics).

Roses and the unidentified plant may have the same or different issues—get a sample diagnosed.