First reported in Siberia in 1880. daylily rust is a fungal disease that attacks the leaves and scapes of daylilies. A pathogen native to China, Taiwan. Japan, Korea, and Russia, it was discovered at a nursery in Georgia and identified by University of Georgia plant pathologists in August of 2000. Since 2001 it has been reported in 30 states, as far south as Mississippi and Texas, west to Oregon and California, and north to Minnesota and Canada However, the disease appears to be most serious in southern plantings, where daylily foliage is evergreen or semi-evergreen.
BIOLOGY: Depending on the climate, the daylily rust fungus can overwinter either as fungal mycelium inside living tissues or as spores. The fungus produces two kinds of spores: uredospores and teliospores. Uredospores are summer spores produced by the fungus during the growing season. Teliospores are winter spores produced in late summer and early fall. Uredospores can only infect new daylily growth, while teliospores can only infect an alternate host, Patrinia, a genus of herbaceous perennials in the Valerianaceae family. Asian in origin, six species of patrinia are grown in the United States.
After completing its life cycle on patrinia, the fungus sporu-lates The spores are spread by wind onto daylily leaves. The spores germinate, grow inside the leaf tissue, and produce more spores, which erupt from the leafs epidermis. On highly susceptible cultivars, new infections can occur as often as every two to three days throughout the growing season.
SYMPTOMS: Bright yellow spots and streaks on the leaf surface are telltale signs, although on some cultivars the disease appears as smaller, water-soaked tan spots with darker borders. The undersides of the leaves have raised yellow-orange to rust-brown pustules, which rupture to release yellow-orange uredospores. Wiping the underside of the leaf with a white tissue will confirm the presence of these spores. For complete confirmation submit a sample to a diagnostic laboratory.
CONTROL: Cut the diseased leaves back to the crown and burn or bury them. When new growth emerges, apply a fungicide containing chlorothalonil, thiophanate-methyl, mancozeb, or propiconazole. Treat both the infected daylilies as well as those in the vicinity of the diseased plants. To reduce the risk of developing fungicide-resistant races of the rust, rotate use of different fungicides. Avoid overhead watering and keep plants spaced wide enough to allow for good air movement and leaf drying.
When purchasing new plants, you may want to grow them separate from existent daylily plantings for six months to be sure that they are disease free. There are rust-resistant cultivars. These include ‘Creole Blush’, ‘Femme Fatale’, ‘Mac the Knife’, ‘Raspberry Splash’, and ‘Yangtze’. For a complete list see http://plantclinic.cornell.edu/FactSheets/daylily%20rust/daylily rust.htm. For more information on daylily rust visit the American Hemerocallis Society Web site, www.daylilies.org.–B.P.