In general, daylilies (Hemerocallis) have few insect problems, but an aphid attack can cause leaves to curl, yellow, and be distorted. You should be able to see a sticky, shiny substance on the leaves if aphids are present. The yellow, brown, or green insects are visible to the naked eye and are usually found in clusters in the fold near the leaf base. Control aphids by spraying them with an insecticidal soap. Spray only if you are sure you actually have them; soap sprays kill the good bugs too. Daylily foliage can also turn yellow from leaf spot, or leaf streak. This disease starts out as spots on the leaves that gradually turn the entire leaf yellow. It's most severe in warm, humid weather.
It can also be a problem if you are overwatering the plants. To combat leaf spot or leaf streak, you'll need to spray the plants with a fungicide containing basic copper or copper sulfate. Do this every week during periods of warm, humid weather. Another relatively recent serious problem that has hit daylilies is daylily rust. This fungal disease produces yellow-orange pustules on the foliage and can cause rapid plant decline. Rust is difficult to control. If you have a susceptible variety, it may be better to replace it with a rust-resistant one. Look for one of the All-American daylily winners such as 'Red Volunteer', a 7-inch midseason bloomer, or 'Miss Mary Mary', a double yellow repeat bloomer. Both have good to excellent rust resistance. Master Gardeners at your local cooperative extension office, or garden store professionals, may also be able to help diagnose your daylilies' ills and provide you with recommended cultivars for your area.