Table of Contents
Daylilies (Hemerocallis sp.) are one of the top-selling herbaceous perennials in North America. As members of the Liliaceae family, daylilies produce showy lily-like flowers that bloom in clusters, for one day before senescing (Figure 1). Daylilies are native to the Old World from central Europe to China and Japan. Most selections available to North American gardeners are cultivated hybrids of original species. Daylilies are most often propagated by crown divisions in spring or fall. These divisions are transplanted into the field or containers for further growth until sale. Daylily cultivation and breeding has become exceptionally popular, with the formation of strong national and provincial/state chapters across North America. Originating in Asia, daylily rust (Puccinia hemerocallidis) was first discovered within North America in the southern United States during 2000. The disease quickly spread and was found in various locations throughout the United States and in Canada during 2001. Daylily rust threatens the production and cultivation of daylilies in North America. Proper identification and understanding the biology of the disease is critical for successful disease management.
Figure 1. Flower of 'Pardon Me'.
Daylily rust is caused by a fungus, Puccinia hemerocallidis. This fungus will only infect and colonize green, live host tissue (Figure 2). It can infect leaves and scapes (leafless, flower stems) of daylily plants but not roots or crowns. Under conditions favourable for disease development, symptoms appear 3-7 days after infection. Symptoms on very susceptible varieties appear as small, yellow-orange, oval-shaped pustules (Figure 3, immature uredia). The chlorotic areas between the pustules often coalesce and become quite pronounced on more susceptible varieties (Figure 3, severe foliar symptoms). On less susceptible varieties, pustules are not as numerous and are surrounded by tan-brown, dead leaf tissue. The pustules contain hundreds of rusty coloured summer spores (uredospores). These spores can easily be transported by wind or rubbed off onto clothing, boots or tools. It is these summer spores that cause repeat infections on neighbouring daylily leaves and scapes throughout late summer and into autumn. The fungus requires living green tissue to continue to grow and produce the rust-coloured summer spores. In the autumn, before leaves begin to senesce or die naturally, new infections will produce dark brown to black pustules that contain the resting or winter spores (Figure 3, teliospore).
Figure 2. The spores of daylily rust can be easily rubbed off onto fingers.
Figure 3. Lifecycle
of daylily rust (Puccinia hemerocallidis). The alternate
hose, Patrinia, is shown in the lower left. The dotted green
arrows indicate the uncertain role of Patrinia in the daylily
rust life cycle in Ontario. The solid red arrows show the repeating
summer spore stage, which is found in Ontario from mid-summer to
mid-fall. The solid blue arrow shows the transition from the summer
spore stage to the winter spore stage. The solid black arrow indicates
the overwintering of the winter spores. See text for more details.
The biology of the fungus P. hemerocallidis is complex and requires 2 different plant host species and 5 different rust spore types to complete a life cycle (Figure 3). In the spring, the dark winter spores (teliospores) germinate and produce another set of spores, which can only infect the alternate host, Patrinia spp. On Patrinia, 2 more spore stages are found with their own distinct symptoms (Bergeron, 2004). In summer, spores produced from Patrinia can infect daylilies. These infections result in yellow spots called uredia which produce the repeating summer spores (uredospores), that re-infect daylilies. Several cycles of re-infection can occur weekly if conditions for rust infection and development are favourable. With cooler temperatures and leaf senescence, the uredia stop production of uredospores and begin to produce the dark, winter spores called teliospores. During this transition, both teliospores and uredospores can be found in the same pustules, which begin to darken as more teliospores are produced. The masses of teliospores (called telia) overwinter, and germinate in the spring to continue the life cycle. Again, the telial stage of this fungus is likely not required for the disease to continue on into the next growing season. The fungus may be able to survive on varieties that maintain green leaf tissue during the winter, on plants brought indoors, or on plants overwintering in a minimum-heated greenhouse.
The alternate host for daylily rust is the herbaceous perennial Patrinia spp. Patrinia is found in the Valerianaceae family and is sometimes referred to as Elvis-eyes or Golden Valerian (Figure 3 Patrinia). Patrinia spp. have small yellow flowering clusters that grow from a clump of palmately-lobed leaves, and are often used in lightly or partly shaded gardens. The plants are native to Asia, but several species including P. gibbosa, P. triloba, P. villosa and P. rupstris are sold in the United States as well as in warmer regions of Canada. Patrinia is not a common perennial in North American gardens and likely poses little threat to the spread of this disease in the landscape. However, there is still some uncertainty about the role of Patrinia in the disease cycle of daylily rust in North America. There is very little information regarding the survival of daylily rust in northern temperate regions such as Canada. However, the disease may not require the alternate host, Patrinia spp., since the fungus may be able to survive on varieties which maintain green leaf tissue during the winter, on plants brought indoors, or on plants overwintering in a minimum-heated greenhouse.
In Ontario, daylily rust is not usually observed until the latter part of summer and early autumn. Studies have shown that the optimal temperatures for summer spore germination can occur at 22-24oC under high humidity (although germination can occur from 7-34oC). Summer spores do not germinate in cold (<4oC) or extremely warm temperatures (>36oC) and this disease is not severe during hot, dry or cold conditions. A minimum of 5-6 hours of continuous leaf wetness is required for spore germination and leaf infection. In addition, summer spore germination decreases with high light intensity. Under conditions favourable for disease, hundreds to thousands of summer spores from each infected leaf can be produced quickly and spread rapidly, making this disease a serious threat to daylilies. This phase of the disease cycle can repeat itself many times during periods of warm weather with rain or dew periods, resulting in disease epidemics.
Daylily rust does not necessarily kill plants but can affect plant vigour, susceptibility to other pests and marketability. Differences in cultivar susceptibility to daylily rust have been observed in experiments conducted at the University of Georgia and the University of Guelph. However, a limited number of cultivars have been evaluated and more research into cultivar susceptibility and resistance is required. Varieties known to be very susceptible (many pustules containing numerous summer spores) include: 'Buttercup', 'Catherine Woodbury', 'Cherry Cheeks', 'Colonel Scarborough', 'Couble', 'Imperial Guard', 'Irish Ice', 'Ming Toy', 'Pardon Me', 'Karie Ann', 'Lemon Yellow', 'Little Gypsy Vagabond', 'Pandora's Box', 'Quannah' and 'Russian Rhapsody'. Moderately susceptible (fewer pustules frequently surrounded by dead brown-tan coloured leaf tissue and containing fewer summer spores) varieties include: 'Butterflake', 'Condon', 'Crystal Tide', 'Gerturde', 'Happy Returns', 'Prelude to Love', 'Joan Senior', 'Pandora's Box', 'Rosy Returns', 'Star Struck', 'Stella D'Oro', 'Summer Wine', 'Wilson's Yellow' (and Hemerocallis fulva). Low susceptibility (little to no infection) varieties include: 'Butterscotch Ruffles', 'Holy Spirit', 'Mac the Knife' and 'Yangtze'.
If you suspect your plants have been infected with daylily rust, reduce the chances of spreading this disease by placing a large, clear plastic bag over the symptomatic foliage and obtain leaf samples from within the confines of the bag. Close the bag tightly around the base of the plant and remove as much infected foliage as possible inside the bag. Take the samples indoors where foliage can be examined more closely. Raised, orange pustules can be seen with the naked eye on both the upper and lower leaf surfaces but may be more prevalent on the lower leaf surfaces of some varieties. Pustules are more easily viewed with a 10-20x magnification hand lens. Immature pustules are elliptical, raised bumps with a waxy sheen and a yellow-orange color. Mature pustules have a protruding mass of orange, powdery spores. The spores can be easily rubbed off onto fingers (Figure 2). Submit a sample to the local Pest Diagnostic Clinic for conclusive identification (see References).
Photos of teliospores and uredium were provided courtesy of George
L. Barron. The photo of the senescent leaf with telia was provided
courtesy of Susan Bergeron and the photo of the 'Pardon Me' flower
was provided courtesy of Suzanne Johnston. The authors provided
all other photos.
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