Well the first two links are for a common fungal disease of daylilies, "leaf streak". The fungus is a black yeast, Aureobasidium microstictum, and most likely needs an injury of some kind to infect the plant. There is more info on how to ID it, and which fungicides are effective to treat it (if you wish to) on the AHS page (the second link I gave), see the two PDF links at the bottom of that page. There are differences in cultivar susceptibility to leaf streak disease, some being more resistant than others.
If it could be nitrogen deficiency then you'd obviously need to give them some N. How likely is it that they are short of nitrogen? If it is unlikely then it would more likely suggest leaf streak. It's obviously not drought, which can also look like leaf streak!
