I don't think it's sunburn per se, and it would be unusually quick for a disease. There are a couple of (old) daylilies that are known to have foliage issues of undetermined cause although I think one might have been related to light intensity - I think I have a picture of one of them somewhere and will verify the light connection. Whenever you get a chance to do the pictures we'll see if we can figure it out. It'll be interesting to see if the new middle leaves that subsequently emerge are clean or not.
