day lily chewed diseased

As I rearranged and moved a lot of things last autumn I might be mistaken about it being a daylily, but this plant is certainly under seige. It is in the bit of garden outside the front garden wall, with grass up to the pavement and I rarely get any slug damage or see a slug here, and it looks a lot worse than slugs. I have daylilies in the front and back, the closest about 20 feet away and the others unaffected A search says that daylilies have few pests, one reason I like to have them, but I have found "spring sickness" and it looks rather like that. The cause seems unknown and hence no treatment. It doesn't seem to be fatal, anyone had experience of this problem?