Daylilies (Hemerocallis spp.) are popular perennials, well known for their easy care and prolific blooms. Each individual flower only blooms for one day before it wilts but many more blooms open each day to take its place. According to the American Hemerocallis Society, although daylilies are relatively pest and disease resistant, there are a few of each you should know about to keep plants healthy and blooming.
The daylily aphid (Myzus hemerocallis) is a soft-bodied insect, usually wingless, that is light green in color. They attack between the leaves of the plant and are evidenced by tiny white specks (which are actually cast-off skins after molting). In mild climates, aphids can produce many generations asexually by live birth. However, when the weather turns cold, females mate with males and lay eggs. Ladybugs are natural predators, and allium (a member of the onion family) is a repellant companion plant, but if more control is needed, you can use an insecticidal soap.
The cucumber beetle resembles a ladybug except that it can be either yellow and spotted (Diabrotica spp) or striped silver and black (Acalymma spp.). Adults are about 1/4 inch long and they chew on both foliage and flowers. There is only one generation per year in northern climates, but as many as four in the south. Infestations can be treated with insecticidal soap.
Bulb mites (Rhizoglyphus spp.) look like a drop of milky gel with reddish-brown legs but they are so tiny, you might see them only as white specks. You’ll need a magnifying glass to inspect thoroughly. They feed in colonies at or just below the surface on the rotting part of old foliage. They are, however, known to attack healthy foliage and can carry diseases to the plant.
Slugs are usually about 1/4 inch to 4 inches long and, unlike the snail, have no protective shell. Snails and slugs feed on foliage during the night and cause leaves to shred. A repellant companion plant is artemisia. Oak leaf mulch is also an effective repellent or you can put out a bowl of beer and they will drown while feeding on the liquid.
Although daylilies are known to be disease resistant, daylily rust is a disease that can do damage. Daylily rust is caused by a fungus and is spread by windborne spores. It will cause leaves to turn yellow and spotted with what appears to be "rust."
"Spring sickness" appears in early spring as the plant begins to grow. New leaves will emerge wilted, detached or decayed at the base. Spring sickness may stunt the growth of the plant, but the plant usually survives.
Leaf streak is a fungal disease that causes the leaves of the plant to turn yellow along the central leaf vein. This will be followed by the appearance of reddish brown spots. The affected leaves will eventually die, but the plant will survive.