Diseases of daylilies
Daylilies (Hemerocallis spp.) are considered one of the more disease-free flowering perennials. However, several diseases can affect daylilies grown under poor cultural conditions, especially if plants are overcrowded, planted in poorly drained soil, or subjected to prolonged periods of warm, wet weather.
Leaf streak, caused by the fungus Aureobasidium microstictum, is the most common foliar disease of daylilies. Although other leaf spot and leaf streak diseases do occur on daylilies, it is unclear whether any of these problems are caused by fungi.
The fungus Aureobasidium microstictum survives winter in infected plant material. In the spring, spores are released during wet periods and rain splashed to nearby leaves, where they create new infections. Infections may continue throughout the summer months during warm, wet weather. Spores are spread from infected leaves to healthy leaves by splashing water and the mechanical rubbing of two leaves.
Infected leaves initially develop water-soaked (dark green) spots, which later enlarge and brown, forming streaks along the midvein of the leaf. Severely infected leaves will turn yellow. Often, the inner leaves of the clump, where conditions are moist, are more seriously affected. Finally, the fungus produces black seed-like structures, called sclerotia, on dying older leaves. These structures survive winter and cause new infections in spring.
Aureobasidium leaf streak causes elongated brown streaks and yellowing of leaves.
Aureobasidium leaf streak can usually be managed through cultural practices. Divide daylilies as needed to prevent the planting from becoming overcrowded. Overcrowding slows leaf drying and favors the development of fungi. Water daylilies at the base of the plant to prevent water splashing. If leaf streak develops, remove infected leaves to slow the spread of the disease. At the end of the growing season, cut back and remove infected foliage to prevent fungal spores from surviving winter in plant debris. Susceptibility varies among daylily cultivars, but none are known to be resistant. No fungicides are currently labeled for control of foliar diseases on daylilies.
Crown and root rots
The most common above-ground symptom of crown and root rot is a rapid yellowing of foliage and dropping of flower buds. Leaves usually yellow uniformly from the tip down, without the presence of spots and streaks. Often an entire clump is affected. Close examination of the crown and root system is necessary for accurate diagnosis.
Bacterial soft rot causes soft, mushy tissue with a foul odor. This disease, caused by bacteria present in the soil, infects plants through wounds or natural openings near the root crown of the plant. The bacteria can spread quickly when soil moisture is high, causing rapid death of the entire daylily clump. There is no cure for plants severely damaged by soft rot. However, if only part of the crown is infected, the damaged tissue can be cut out and the healthy crown section replanted in a well-drained location. Let the cut surfaces dry in a shaded location before planting to prevent bacteria from infecting the fresh wounds.
White mold, caused by the fungus Sclerotium, causes a crown rot in daylilies. The crown and lower leaves of infected plants are often covered with white mats of cottony growth called mycelia. Small, brown to black, seed-like reproductive structures, called sclerotia, form in the mycelia. The sclerotia remain dormant in the soil for many years, making this a difficult fungus to control.
There are currently no fungicides available to homeowners for white mold control. To manage existing infections, remove infected plants and soil. Remove all soil within one foot of the infected plant, to a depth of one foot. Discard the infected plants and soil, do not compost them. Be careful not to spill any soil, which may contain sclerotia. Try to avoid planting daylily or other susceptible plants for several years. Since this fungus is able to infect many different garden plants, it is important to verify a plant's immunity.
Rhizoctonia crown rot of daylilies occurs in wet years or in poorly drained soils. Symptoms include crowns that rot from the soil surface down and dark, felt-like fungal growth on the plant crown. Sometimes, sclerotia can be observed in the soil around infected plants. Rhizoctonia-infected daylilies may recover if soil drainage is improved or plants are relocated to a well-drained location.
Gatlin, F.L., ed. 1991 Daylilies: The Beginner's Handbook, The American Hemerocallis Society, Jackson, MS, pp. 71-73.
Hill, L. and Hill, N. Daylilies: The Perfect Perennial, Storey Communications, Inc. Pownal, VT, pp. 89-90.