In general, daylilies are relatively disease and pest resistant. However, daylilies grown in large numbers or in unfavorable conditions may be susceptible to a number of problems. Some can be spot-treated with little difficulty, and others point to a larger problem with your daylily's environment or the viability of that particular cultivar.
Failure to Thrive
This is most likely due to unsuitable environmental conditions. In order to grow and flower to their potential, daylilies need full sunlight, well-fertilized soil and regular watering. Small, sickly looking daylilies are often missing one of those elements. Other culprits can be extreme weather conditions, or incorrectly applied pesticides or herbicides.
Daylilies are susceptible to Erwinia chrysanthemi, a bacterium found in many soils. When this bacterium enters daylily plants trough a wound or other natural opening, it causes what is known as soft or crown rot. When infected, daylilies will wilt, eventually become too soft to stand and emit a foul odor. The bacteria moves swiftly and can kill a daylily plant in a matter of days. Once you identify the infection, dig the plant up and destroy it. Sterilize the soil before planting anything else in that spot.
Leaf streak is a common daylily fungus that enters the plant through a wound or natural opening. The first signs of leaf streak are dark green, translucent spots on the leaves. These spots quickly spread over the entire leaf. Leaf fungus can cause your daylilies to look sick and unsightly, but it will not kill the plant. To get rid of leaf streak, simply isolate any affected plants and remove damaged leaves until the fungus is gone. Mustard seed fungus is a fungus that attacks the crown of daylily plants. Infected daylilies will have a mass of what looks like white cotton threads on their crown which may or not be covered in brown or black spots. Once you identify mustard seed fungus, uproot the plant. To save it, apply a fungicide to the crown, according to the manufacturer's instructions and replant it in another location. Or, re-plant the daylily in its original location after sterilizing the soil.
Root rot is caused by a fungus commonly found in garden soil and largely affects daylilies with damaged roots or those weakened by poorly-drained soil. A daylily suffering from root rot will look wilted and yellow, but will not perk up when watered. Affected plants often die within the week and should be removed as soon as root rot is identified. Before re-planting in that location, sterilize the soil and improve its drainage.
No one really knows what causes spring sickness, although it most often occurs in areas where daylilies are subjected to repeated freezing and thawing cycles. Affected plants have twisted or pleated inner leaves that may grow sideways and become brown and jagged at the edges. Mild cases often go away on their own without too much damage to the plant. Severely affected plants will not flower.
Daylilies can be infested by the regular lineup of garden pests. Yellowed leaves covered in sticky residue and tiny white flecks are a sure sign of aphids. Leaves covered in light green, yellow or brown spots on the surface and covered with tiny webs underneath mean spider mites are afoot. If you spot big, multi-colored bugs on the plant itself, those are tarnish plant bugs. Slimy trails all over your garden mean snails or slugs, and petals that stick together are a sign of thrips. Whatever pest your daylily is afflicted with, avoid using toxic pesticides. Most daylily pests can be handled by organic means. Aphids, thrips and tarnish plant bugs can be handled by the introduction of ladybugs, stink bugs and pirate bugs respectively. Spider mites can be deterred by nightly misting, and snails and slugs cannot crawl over diatomaceous earth sprinkled over the soil.
Scape Blasting or Blasted Scape
Blasted scape often occurs when a heavy rain is followed by a period of drought, or when daylilies are fertilized just before a period of heavy rain. The scape then bursts, splits or completely detaches due to internal pressure. Partial ruptures can be splinted with Popsicle sticks and duct tape.