Are your daylillies in pots? Do they get enough water?

The Daylillies I have are in the ground, but without enough water regularly, they will turn yellow and start to die down. They are resillient enough to come back, which sounds like your problem.

If they are in full sun, you can try moving them to a part shade location for the afternoon.

Hope this helps you some-
Candace

I was watching Paul James "the gardening guy" and he did a section on "rust"... I realized that this is something that may be occuring on by Daylillies. It is a fungus that causes the leaves to turn yellow with dark brown "dashes" in the direction of the leaves. This can be caused by overhead watering I expect.

It would be worth looking into.

--Candace