Aphids do little damage in small numbers. They are extremely prolific, however, and populations can rapidly build up to damaging numbers during the growing season. Damage occurs when the aphid sucks the juices from daylily leaves. The aphid is unable to digest fully all the sugar in the plant sap, and it excretes the excess in a fluid called honeydew, which often drops onto the leaves below. Ants feed on this sticky substance and are often present where there is an aphid infestation.

A fungus (Puccinia hemerocallidis) causes this disease. Native to Asia, where daylilies originated, it was discovered in this country in Georgia in 2000. It is now found in many eastern states, Canadian provinces, and in southern California. Like many rust diseases, its life cycle includes two host plants. In Asia, the alternate host is golden valerian (Patrinia), but the disease also spreads from daylily to daylily. The orange spores are spread by the wind, tools, clothing, and hands. The disease develops and spreads very fast. Within 1 or 2 weeks, infected plants produce spores that infect other plants. Because of its virulence and the popularity of daylilies, this disease is apt to spread across the continent. Daylily varieties have differing susceptibility to rust: 'The Pardon-Me' variety is highly susceptible and may die. Damage to other varieties varies greatly.

This plant disease is caused by a fungus (Aureobasidium microstictum, also known as Collecephalus hemerocalli). Besides daylilies, it also infects iris, lily-of-the-valley (Convallaria majalis), and true lilies (Lilium). Dormant bodies called sclerotia are produced on dead and dying leaves in the fall. Sclerotia survive the winter to initiate the disease when new foliage appears in the spring, but symptoms don''t appear until later. Spores are made continually throughout the growing season, but the fungus suffers above 90°F, so it is most active in spring. Spores are spread to new plants by splashing water, gardeners, or animals. In warm climates with evergreen daylilies, the disease is active all year.

The cause of spring sickness is unknown. Although informed speculation suggests that cold snaps, insect damage, and pathogens cause spring sickness, there is no consensus. The problem is sporadic, striking one or a few fans in a clump and not others. Different fans are affected each spring, and some gardens have no problems with it at all. No daylilies seem to be more susceptible or resistant than others. The problem seems to be more severe in northern regions, but occurs in many areas.