DAYLILY RUST – AN AVOIDABLE NUISANCE

GRAIN VALLEY, MISSOURI Spring 2004 ... Since its discovery in 2000, daylily rust (Puccinia hemerocallidis) has been found in daylilies in over half of the United States. The infection does not kill daylilies and, according to the All-American Daylily Selection Council (AADSC), can be avoided by proper selection and horticultural practices. In a worst case scenario, daylily rust can be controlled by treatment of the susceptible varieties. The University of Georgia, led by Dr. Jean Woodward, has done most of the key work on daylily rust so far. She has identified chemicals that are appropriate for control and performed greenhouse testing of varieties to determine their susceptibility or resistance to the new pest. The AADSC, in the course of its normal multi-year field testing of daylily varieties, has made some findings that should complement the work of the University of Georgia. As can be expected, the results of long term, multi-year field testing and observations are not always 100% correlated with the results of greenhouse testing. Field conditions provide additional variables that can affect how severely a plant responds to the presence of rust.

Daylilies infected by Puccinia hemerocallidis show unsightly rust spots and yellowing, mainly on older foliage, similar to rust symptoms on roses, geraniums and other garden plants. The rust is confined to the foliage and bloomstalks (scapes) and does not enter the crown or roots. No daylily plants have been killed by the rust. Puccinia spores spread quickly
by wind, on clothing and infected plants, but do not infect other plant species. Roses have their own specific rust species, as do most other ornamental plants. Just as with roses, symptoms of daylily rust vary greatly depending on growing conditions and the susceptibility of each variety. Among the 48,000-plus daylily cultivars are varieties that are unusable in some gardens and those that are virtually symptom-free.

The worst rust symptoms will appear on daylilies grown in conditions of high humidity, poor air circulation and nighttime overhead watering. Daylily rust spores require 100% humidity and temperatures between 40 and 90 degrees for five to six hours to germinate. If germination does not occur within two to three days, the spore dies. If germination occurs, infection can lie dormant within green tissue until optimal conditions arise. It appears that spores do not survive outdoors in winters colder than USDA Zone 6, making daylily rust less of a problem in colder areas.

Many gardeners simply cut off the unsightly, rust-infected foliage, which is quickly replaced by clean, new foliage. More aggressive action is also an option, which includes cutting the infected plants to the ground, disposing of foliage, and treating the plants with a fungicide specifically for rust prevention, such as Daconil.

Since 1989, the AADSC has operated a network of daylily test sites throughout the United States and has collected data on over 50 performance characteristics. In 2001, rust resistance was added as one of the key test criteria. In selecting for "bulletproof" performance, the AADSC has eliminated many of the highly susceptible varieties from its program and focused on identifying and promoting the most rust-resistant daylily varieties.

The AADSC does not release conclusions about varieties until it has three to five years worth of testing information. In the beginning the AADSC simply rated cultivars as "more susceptible" or "less susceptible." At present there are four scoring systems that the AADSC is aware of:

A. AADSC (multi-year, outdoor field tests)
S = Susceptible;
MS = moderately susceptible;
MR = moderately resistant;
R = resistant

B. University of Georgia (greenhouse, two test cycles of 4 weeks each)
S = Susceptible;
MS = moderately susceptible;
MR = moderately resistant;
R = resistant

C. Cornell University Plant Disease Diagnostic Clinic
VS = very susceptible;
MS = moderately susceptible;
VR = very resistant

D. USDA National Plant Board
HS = highly susceptible;
MS = moderately susceptible;
LS = least susceptible

More than 700 varieties have been or are being put through rust trials by the AADSC and University of Georgia, as well as Cornell University and the USDA. Here are the results on some of the most commonly available varieties:

Susceptible:
Pardon Me
Ming Toy
Russian Rhapsody
Always Afternoon
Mary Todd
Pandora's Box
Strawberry Candy

Resistant:
Little Business
Mini Pearl
Butterscotch Ruffles

Among the AADSC's "All-American Daylilies," Black-Eyed Stella, Lullaby Baby, Bitsy, Frankly Scarlet, and Plum Perfect have been reported as rust resistant; Judith as moderately resistant; Star Struck as moderately susceptible; and Leebea Orange Crush as susceptible.

There are reasons that daylilies are America's favorite perennial. They are an amazing plant, available in a rainbow of colors, shapes, sizes, and varieties that can be easily grown anywhere in the U.S. With proper selection, there's no reason for gardeners to be intimidated by the rust challenge. For regularly-updated information regarding the benefits of gardening with daylilies, and comparisons of daylily variety performance (including rust resistance), visit www.daylilyresearch.org.

For more information about the All American Daylily Selection Council, please contact Mary McLoughlin at (616) 698-0748.