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Gardening: Daylily rust: What it is, what you need to know

Daylilies are a popular perennial that are favored by gardeners, landscapers and producers in the U.S. as a low maintenance plant with few insect or disease problems. In 2000, daylily rust was first detected in the U.S. and quickly spread throughout much of the country due to widespread movement of the pathogen on infected stock and subsequent establishment in the landscape. It is important to realize that daylily rust can be introduced at any time to new areas on infected plants.

Daylily rust caused widespread panic among daylily producers in the U.S. after it was initially detected. The presence of rust at a grower’s facility initially resulted in stop-sale orders by regulatory agencies – at large cost to growers. Many plants were destroyed in attempts to eradicate the fungus. Quarantines were used, without success, to limit the movement of the fungus throughout the U.S. Daylily rust is currently endemic in the southern regions of the U.S.

Daylily rust, is caused by the fungus Puccinia hemerocallidis and affects the leaves and scapes (flower stalks). It is a relatively new disease to the United States. The pathogen was first described in 1880. It is native to Asia, commonly found in China, Japan, Korea, Taiwan, and Russia. In the U.S., it was first identified in August of 2000 in the southeastern part of the country. The disease moved swiftly throughout the country and by the fall of 2001 it had been identified in over 30 states including Minnesota. The reason for such a fast movement of the pathogen is most likely due to the widespread, fast shipping of plant material throughout the country.

Like other rusts, daylily rust is spread by spores carried by the wind, by humans via clothing, hands and tools. There is little you can do if your neighbor’s plants have it. If you have a susceptible plant, your daylilies will eventually be infected. The good news, though, is that daylily rust only affects daylilies – not other plants. The easiest and quickest way to rid your garden of daylily rust, is to destroy the infected plant before it has time to spread. Don’t place them in the compost pile, because the spores on the pulled plants still have the potential to spread to healthy daylilies. The preferred method of disposal is to seal the diseased material in a garbage bag and dispose of it at the landfill. When it comes to daylily rust, doing nothing is not an option. If this disease is allowed to thrive, the daylilies in your garden will become unsightly. Daylilies are mainly appreciated for their colorful blooms, but paying close attention to the foliage is important to catching an infection.

Symptoms of the rust infection may not be present early in the development of the disease. The most obvious symptoms of daylily rust are yellow to brown streaks on the leaves. The plant veins tend to hold fungal growth making streaks. You may also see very bright, small, yellow spots on the surface of the leaves. The undersurface of the leaves will have numerous small, orange to yellow spots, called pustules, that poke out of the undersurface of the leaf. The pustules grow and release numerous dusty, orange-colored spores. The orange spores are very dusty and if you rub the leaf surface, will notice the dusty spores lofting about. As symptoms progress, leaves turn yellow and dry up. Severity varies with the daylily cultivar.

It’s easy to confuse daylily rust with a couple of lookalike daylily problems. Daylily leaf streak, caused by a different fungus, also causes similar symptoms of brown to yellow leaf streaks, but it lacks the orange pustules that develop on the undersurface of the leaves. Aphid feeding damage can also look similar but again there are no pustules.

Little information is known about the biology of daylily rust, but it is known it needs two distinct hosts to complete its life cycle. The primary host is Daylily (Hemerocallis sp.) and alternate hosts are Patrinia spp. perennial plants of which six species are known in the United States (species of golden valerian). Patrinia is an herbaceous perennial with tall stems bearing umbels of bright yellow flowers and can be used as an accent plant in gardens. Patrinia allows sexual reproduction of the fungus. Although Patrinia is a required host to complete the lifecycle of the pathogen, the fungus is capable of producing spores that are aggressive and virulent on daylily alone.

The University of Minnesota recommends the following:

Use resistant varieties: Different varieties of daylily have different susceptibilities to the rust. Select a variety that has some known resistance. And don’t grow both hosts at the same site, keep daylilies and Patrinia apart. A limited number of cultivars have been studied for susceptibility to the rust pathogen.

Sanitation: Clean up all plant residue in the fall. Remove all remaining foliage from the plants and remove it from the site.

Newly purchased plants can be pruned back in the spring, this may lessen your chances for introducing the pathogen into your garden from new plant material.

Fungicides: that may be useful for the home garden include some products that contain the active ingredients neem oil, potassium bicarbonate, sulfur, or Bacillus subtilis, or Plant Guardian. Some of these fungicides are currently registered to treat rust diseases and/or rusts on flowering landscape plants. Once an infection has become established, applications may be needed weekly because new infections are constantly beginning while plants are actively growing. Be certain any product purchased is appropriately labeled. Read the application instructions carefully.

You can find out more information about Daylily Rust at the University of Minnesota website: http://www.extension.umn.edu/garden/

Resource: M. Grabowski, University of Minnesota Extension; Cornell University: The Plant Disease Diagnostic Clinic

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