Daylilies are so-named because their colorful flowers last only a day, with a single plant producing more than 50 flowers. They come in most colors except for black, blue and true white. Although some consider daylilies the perfect perennial, they do face some problems with pests and diseases. They’re popular plants overall, however, because they require little fertilization and are able to tolerate a wide range of temperatures.
Thrips, one of the leading pests of daylilies, are small winged insects that bore into buds and then suck out liquid. Their wounds cause a flower’s petals to stick together, which prevents flowers from opening properly.
Aphids typically feed on buds, giving them a wart-like appearance. This causes infested plants to have a late start in growing, producing new foliage that's yellowed, looking as if the plant has a nitrogen shortage.
Spider mites, which are problems in hot climates, suck chlorophyll from foliage, which causes the leaves to become dull brown and sometimes die. Other common pests include bulb mites, earwigs, tarnished plant bugs, Hemerocallis Gall Midge, slugs and snails.
Leaf streak is a fungal infection which causes unattractive foliage, although it doesn’t kill the plant. The fungus is first seen as dark green transparent spots that gradually turn into dead tissue. They finally radiate as yellow and brown streaks over leaves. Overhead watering and rain can spread the disease. Although there aren’t any controls for the fungus, good cultivation and garden hygiene can make daylilies less prone to the problem.
Crown rot mostly affects daylilies in hot climates. Bacteria are believed to cause the crown and roots to rot. Excess watering, especially during long hot summer months, can cause significant numbers of plants to die from this disease. Other contributing factors may be poor air circulation, over-fertilization and high temperatures. Prevention of crown rot involves letting wounded plants heal and dry out before planting, as well as discarding rotted plant materials.
Daylily rust is a rather new fungal disease caused by Puccinia hemerocallidis. This fungus, which first appeared in the year 2000 in the United States, affects such popular daylily varieties as Pardon Me, Stella de Oro and Attribution. Signs of the disease begin as rust that has yellow and brown streaks on leaves, similar to leaf streak. Aphid insects that feed on leaves can cause similar symptoms. Raised pustules rising on a leaf’s surface (either front or back) are the diagnostic test for daylily rust.