This is typical of many of these in Florida. The hot, humid conditions lead to several diseases, with Cercosporidium causing lower foliage to die like this. The only thing you can do is to make sure there is no irrigation water hitting the foliage, apply fungicides and also use granite pebbles as a mulch to prevent rainsplash and increase drying of the foliage..

it looks like they are probably not in full sun.. that close to the fence .... and that isnt helping ...

ken

Also looks extremely close to the trees either side which would militate against air circulation.