The damage appears to be due to Southern Blight, a fairly common fungal disease that can potentially affect a wide variety of plant species. (This doesn't mean that the entire yard is necessarily at risk or doomed to succumb, just that its host plant range is not very restricted.) Plants growing densely, like groundcovers or perennials spaced very closely, or plants which are mulched heavily are at greater risk of developing this blight, though the fungus only thrives during hot temperatures and high humidity. Remove dead growth and try to also remove the top few inches of soil from the zone(s) of dieback, in order to help remove any sclerotia (spore structures) that may be missed. The linked page provides a bit more information, but essentially, use of a fungicide is not effective.

Miri