It could be a number of things. Do you have a picture of the problem? Are you over watering? Are they properly planted? Have you over fertilized? Did you spray any weed killer near them recently? Slugs?
As you can see, there can be many reasons for the problem.

If you search this forum there are several posts about Rust and links to the AHS website. Just use the search word "rust". I don't think it is rust but you'll be the one to know after reading about it.

Ikea: (unusual handle btw)
Not that I can shed any light on the problem, but I have the EXACT situation. All of my finer leafed daylilies look fine, but my single broader leafed daylily(First Knight - that is located among the finer leafed daylilies) is yellowed at the center and looks "broken". I do see some slug damage. Doesn't look nearly as healthy as the others .

I sprayed a bit of spinosad and some other organic disease spray last night. Hopefully they will grow out of it! I suppose it could be the slugs that ate the tender new leaves but would this cause yellowing? No photos unfortunately.

Mine is doing the same thing. All the wide leaf dl's have yellow leaves and my thin ones like stella and ruby stella are just fine. They are all planted the same way and get the same fertilizers. We had a very dry April so I was wondering if this could be the issue?

We also had a very dry spring. I wonder if mitanoff in ontario has the same experience with weather there.

I have same problem for 2 years. Janice Brown I just transplanted from my back yard to the front lawn border started to have the same thing. It was fine when it was at my backyard. This spring is pretty warm and dry. Last spring was normal New England weather, they were worse than this year. I lost one daylily because this problem.

I bet ya a dollar it's spring sickness. Some of my daylilies have it. Usually just a few fans of stunted growth with yellow leaves and holes in the leaf margin.

What happens with spring sickness? Do they overcome?

I also think it is probably spring sickness, all of mine have overcome it in the past. This year I made note of who has it , just to see if it is the same ones, I do recall that Prairie Blue Eyes always gets a bad case of it. This spring I had a few that the fans actually laid down on the ground. So far so good, all but a few have recovered and are growing well. The few left with it have a few good fans growing , so I dont' think any will be lost. I didn't do anything to help them along, they seemed to manage on their own. I had one that was particularly bad looking, and then upon closer inspection I noticed that ants were starting to build an anthill. So I dug that daylily up, cleaned off the roots and potted it up. I noticed that the roots looked really good, so I don't think this sickness is doing anything to the roots.
cindy

It sounds like spring sickness to me too. A lot of my daylilies have spring sickness this year - they all seem to be outgrowing it. Party Pinafore always gets the worst spring sickness for me. Here's a link with more info.

Juliet

Here is a link that might be useful: Spring Sickness info

I was also going to say spring sickness ... my are already recovering.

Mine don't look uguly like pictures they just have yellow leaves. Can I cut these leaves off?