Phlox can get several leaf spot diseases.* They also get spider mites.* Look for those as well.
Southern blight could be the cause of your problem.* From this site:
"Plants with Southern blight have lesions on the stem at, or near, the soil line. These lesions develop rapidly, girdling the stem and resulting in a sudden and permanent wilting of the plant. High temperatures (above 85 degrees F, 29 degrees C) favor the disease[/b]. The fungus attacks a wide range of plants and survives for long periods in soil. To control, treat with a recommended fungicide according to label directions."
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.