Daylilies, or hemerocallis, are prolific perennials known for long periods of bloom, a wide range of colors and undemanding habits. The ever-growing number of hybrid types have made gardening with these lily-like flowers and their graceful, grassy mounds of foliage possible from the northernmost states throughout much of the Deep South. Although several pests and diseases may affect daylilies, crown rot is the only problem that can seriously threaten them.
Crown rot strikes susceptible plants at the nodule where the green leaf base meets the white roots. In addition to crowns, crown rot can spread to lower sections of leaves and roots, killing the plant. Leaves turn yellow from the tip down to the soil, and the crown grows mushy. Crown rot can spread, killing entire clumps of plants.
According to the University of Minnesota, crown rot may have three causes. Bacterial soft rot is carried by soil bacteria, creates mushy, smelly crowns and spreads rapidly. White mold results from an infection by the fungus sclerotium, which may spread through the crown and lower leaves in a white, cottony mold called mycelia that provides an environment for sclerotia cells that will preserve the dormant mold for years. Rhizoctonia crown rot results from the growth of a “felt-like” fungus from the surface of the soil that roots down into the crown.
Crown rot is one of the only afflictions that cannot be treated after it infects daylilies. The organisms that cause it spread easily and persist in soil, making replanting difficult. There are no known cures for bacterial rot and no fungicides effective on white mold or fungal infections. The only cure for crown rot is removal and destruction of affected plants.
Crown rot is more common where summers are continuously hot and humid. Some newer hybrids are more easily infected, particularly when they are planted too deeply, overfertilized or watered too frequently. Crown rot can be caused by a number of organisms and may be made worse by nematodes and bulb mites. Older varieties are less susceptible to crown rot than some newer hybrids.
Preventing Crown Rot
Good garden hygiene is the best defense and is most important in Southern growing zones where summers are sultry. Aerate soil thoroughly during cultivation and amend heavy soils with compost to improve drainage; water should drain easily within 15 minutes. Plant daylily crowns no deeper than 1 inch below the soil surface. Daylilies are light feeders; don’t fertilize them more than once a year to discourage the organisms that cause rot. Plant hemerocallis where they get their sun in the morning and avoid watering or allowing water to collect in daylily clumps. Avoid over- or underwatering; daylilies need an inch of water a week, but standing water in hot weather could lead to rot.