Disease is seldom a problem with daylilies, although fungal and bacterial infections can be bothersome if weather conditions are right. The greatest enemy of this hardy perennial is overly wet conditions that weaken the plant to the previously mentioned pathogens. The cure is prevention, plant daylilies in well drained soils whenever possible. A number of commercial products are widely available if infections do occur. In most cases the daylilies survive the infection, with only partial loss of the plant, and they out-grow the problem with no lingering effects. If you feel it is necessary to treat your plants with a chemical for bacterial or fungal infection, please consult your local agricultural agent.
A new disease called daylily rust showed up in gardens around the world in 2000. The disease manifests itself as orange/yellow powdery spores that destroy leaf tissue of the daylily plant. Daylily rust has a complex life cycle and much information is yet to be gathered. It does appear that conditions in the southern part of the United States are more conducive to its reproductive cycle. Many gardens in the northern tier of U.S., which reported rust in 2001, found no rust in the summer of 2002. Studies have recently found that daylily rust does not typically survive winters in USDA Plant Hardiness Zones 5 and lower. The Daylily Rust Information Page is an excellent resource for information and links to other sites concerning this disease.
A number of insect pests can and do like daylilies. The most problematic for gardeners in northeast Wisconsin is thrips (particularly during years with warm winters and dry springs). These little insects are seldom seen, but can cause bud damage during early scape formation. The injured buds grow into flowers with color distortions, small bumps, and sometimes deformed petals and sepals. Dark colored flowers, such as reds and purples, are often most drastically influenced (due to the way pigments are carried in the flowers). Fortunately, thrips seem to do little damage to the overall health of the plant. A spray mixture of Avid™ and Orthene™ is very effective in combating thrips, if sprayed on plants in the spring when they are approximately six inches tall. Avid™ is very expensive, and must be purchased in a quantity which most gardeners would not use in a lifetime. Once again, we recommend contacting a local agriculture agent to find a product that is more economically feasible, unless you have fields of daylilies. Old leaves and plant material in the garden is suspected of helping thrips overwinter. We have noticed a marked difference in gardens which are cleaned of foliage in the fall. Many gardeners prefer to put up with the typically small amounts of damage thrips cause rather than using a pesticide.
Rabbits and deer typically do not feast on daylilies foliage, but deer are known to nibblele on the foliage when it first emerges in early spring Deer will also nibble on the flower buds and seed pods. The diets of these animals, however, can vary greatly from locale to locale and a starving animal will eat almost anything. If rabbits and deer do become pests, a number of chemicals are available on the market. Sometimes these chemicals may be applied to a physical barrier, such as a fence or wall, with positive results.