Looking at your picture again, first thing I'd do is clean up - cut off all that dead foliage and put it in a plastic bag in the trash, don't compost it! That way if it is a fungal disease or insect infestation, you won't keep the bad stuff around.
After you do that, take the hose on a warm day and hose the whole plant off thoroughly. Then inspect it with a magnifying glass, especially looking in the leaf axils, and on the undersides of the healthy leaves. Insects especially like to feed on new tender growth, not the old dead stuff. But they use the dead foliage for hiding areas. You have what looks like a nice healthy new plant right on the bottom of that picture. Check it over carefully!
