Your leaf appears to have some kind of chlorosis going on, which means your soil likely has a deficiency in some nutrient the plant needs. One of the most common kinds of chlorosis I've seen in daylilies is iron chlorosis, which often occurs in soils with PH that is too high. Applications of iron chelate can be helpful if it is an iron deficiency, but the first thing I'd suggest if possible is that you do a soil test to determine the exact problem. There are some self tests of soil that can be ordered, I believe.
Yellowing can also come from other pathogens or drainage problems, so those need to be ruled out first also. Good luck!
