We think you may be dealing with a type of root rot called Phytophthora. It is a "water mold' and it is favored by frequent irrigation and wet, slow draining soils. When the soil remains saturated, it stimulates the Phytophthora fungus to produce many infective swimming spores (zoospores), causing more severe disease.https://extension.umd.edu/resource/wilt-diseases-flowers

At this point, remove the wilted petunia and the soil in that area of the box. Put in fresh potting media in that area and look for a good healthy plant and replant. Try this but there are no guarantees. 

Marian