They may have streak disease or rust. Streak disease (yellow and brown section down the middle) looks bad but isn’t deadly. Cut off the dead leaves and put in the trash. Don’t put them in the compost bin as that will just continue the cycle. Rust is a more serious problem and the entire plant may have to be removed. Clemson University Extension has a helpful publication on daylily diseases and insects for more information and help identifying the two diseases.
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