Without seeing your lantana in person, it's difficult to be certain. We can tell you, though, some of the problems that lantana is susceptible to. Your verbal description sounds quite a bit like powdery mildew, though this is not something that troubles lantana very often. If your plant is overcrowded or growing in some shade, this could be an option. Spider mites can often take out lantana, and the lantana lacebug is becoming more and more of an issue in the South. This insect, which can be seen on the underside of the leaves, makes the upper side speckly/silvery/white-ish.

If you could send a good image, or pump up the volume on your description, we could narrow it down better.

Did you ever figure out what it was? I have the same thing happening to mine. There are no bugs, and it is happening to every single lantana on my property - in several beds on an acre. We have had lots of rain this season...could that be it? It looks just terrible, and has been such a terrific performer in the past, I'm just sick about it. Were you able to save it? What happened?