Daylily Rust Research Paper

A fungal pathogen that attacks daylilies.

Daylily rust is caused by the fungus Puccinia hemerocallidis and affects the leaves and scapes. It is not a new disease of daylilies, having been reported previously in China, Taiwan, Korea, Japan and Russia (Sakhalin, Kuriles and Siberia*). Unfortunately, the disease has now arrived in North America, and was first identified in the southeastern United States in August 2000.  In nature, the main method of rust spread is by wind borne spores.

Large image of infected plant (click to enlarge)

Infected daylily plant (click to enlarge)

Close up of pustules on leaf (click to enlarge)

Close up of pustules on leaf (click to enlarge)

Above photos courtesy of the Florida Department of Agriculture and Consumer Services Division of Plant Industry, used with permission. All photos by Jeff Lotz, DPI.

Rust pustules on daylily scape. (click to enlarge)

Enlarged rust on leaves. (click to enlarge)

Top side of infected leaf. (click to enlarge)

Bottom side of infected leaf. (click to enlarge)

The four photos immediately above by Jay Laundre, used with permission.

Reports from AHS members indicate that in North America daylily rust has successfully overwintered in USDA Hardiness Zone 7 and milder. At the current stage of our knowledge, it is safest to assume that if any daylilyfoliage remains green through the winter in an already infected garden, there is a possibility the rust could survive inside those leaves. In very mild winter climates the familiar yellow-orange powdery "summer spores" (urediospores) produced from the spots (pustules) on daylily leaves may cause repeated infection cycles year round. A spore is similar to a plant seed, and like a seed requires moisture and appropriate temperatures in order to germinate. It is not yet known how long daylily rust urediospores can remain alive on leaves until conditions become suitable for germination and infection, or exactly how far they can travel on the wind to new daylily leaves. Rust spores can often travel many miles on air currents.

Rust diseases may also survive winter as mycelium (the strands which form the body of the fungus inside the leaf) but this can only take place where some infected leaves remain alive through the winter such as in milder climates, or where there is protection from greenhouses, deep snow cover, heavy winter mulch, or proximity of infected plants to the warming walls of a house, for example.

Another means by which rusts can overwinter is in the form of "winter spores" (teliospores). These are dark colored spores which most commonly appear at the end of the growing season in response to shorter days and cooler temperatures. The spots and streaks containing teliospores on a daylily leaf are blackish, in contrast to the more familiar orange pustules of urediospores. "Winter spores" are hardier and more durable than the "summer spores" and lie dormant on dead daylily leaves over winter. In the spring they germinate to produce another type of spore but this cannot infect daylilies. Instead, these new spores must be transported by the wind or other means to a plant of the alternate host, Patrinia, which is a perennial plant also of Asian origin. After completing the next stage of the life cycle on a Patrinia plant, the rust can then pass back to daylilies. Thus it is possible that in climates where the rust cannot survive the winter as mycelium or urediospores, it may still be able to continue the infection in the subsequent year if there is a plant of Patrinia in the vicinity.

Patrinia is not common at the moment in North America, but several species are being offered for sale both as plants and seeds. Not only does it pose a threat to daylilies as far as overwintering of the rust is concerned, but the rust life cycle stage on Patrinia is a form of sexual reproduction which may increase the chances of daylily rust becoming able to infect currently resistant daylily cultivars. However to date we are not aware of any patrinia plants having been infected with daylily rust in North America. At one time, Hosta was also thought to be susceptible to daylily rust, but further research has shown this is not the case.

When acquiring new daylilies, whether by trade or purchase, it should be born in mind that these plants could be carrying a rust infection even though it isn't necessarily visible at the time of receipt. There have been numerous reports of rust first becoming apparent several weeks or months after the plants have arrived. The fungus may be alive inside a leaf, or spores be hiding on a leaf, without showing any obvious signs externally. An early infection may go undetected until the rust begins to form more spores, at which time it produces the characteristic yellow-orange pustules. From this point on, the infection may soon spread throughout one's garden or nursery. It is advised as a precaution to isolate all new daylilies well away from existing daylily plantings for several months or a growing season to minimize rust spread if they turn out to be infected.

On receipt of new daylilies, some growers are peeling the outer layers of leaves from new plants right to the crown, and then cutting the remainder about one to two inches above the crown. The plant may then be soaked in fungicide if desired. This procedure may add to the transplant and shipping shock of the divisions, and that risk should be weighed against the desire to protect one's garden and neighborhood from possible rust infection. Although there is no guarantee this treatment will prevent rust on new plants which have been exposed to the disease, it should significantly reduce the risk. Fungicide or disinfectant treatment prior to shipping does not guarantee a rust-free plant, and overseas purchasers should not assume a phytosanitary certificate indicates a clean plant.

Check new plants daily for rust especially the undersides of the leaves. Some gardeners report missing the rust because it was not visible when looking down onto the top surface of the leaves. Early signs of rust, prior to pustule development, are leaf spots which could be mistaken for some other disorder. Rust pustules are raised, and this can easily be seen with a small magnifying glass. A few other substances on leaves may appear raised, such as other plant seeds, pollen, and even fertilizer or pesticide residues. Yellow-orange powder (large numbers of spores) comes off on a white tissue when a rust infected leaf is wiped.

Differences in rainfall, humidity, temperatures and cultural practices will cause the severity of the disease to vary between gardens. Because fungicide availability and labeling differs regionally it is best to contact your local Extension Service or Ministry of Agriculture for the most recent advice on treating daylily rust. Trial results from Drs. Buck and Williams-Woodward at the University of Georgia were published in the Spring 2002 issue of the AHS Daylily Journal. The summary indicated that "daylily rust can be controlled under greenhouse conditions by fungicide applications, in particular, mancozeb, chlorothalonil, azoxystrobin and triadimefon." Products should be rotated to reduce the risk of fungicide resistance developing. Alternate between a systemic fungicide and a contact or protectant fungicide. Application may need to be repeated as often as every seven to fourteen days - follow product label instructions. It is often recommended to remove the foliage from all plants discovered to have rust at the first sign of infection, cutting just above the soil level. Similarly cut all apparently healthy plants in the vicinity. However, if the infection continues after this initial cutting back, it is not recommended to continue repeatedly removing all the foliage. For those who do not wish to use fungicides treatment may be limited to continuous removal of individual infected leaves as they are noticed. Removed leaves should be destroyed, preferably by burning or burying where this is possible. Spores can remain alive for a while even on a dead leaf and spread from there on the wind. Opinions differ regarding the safety of composting infected leaves. Some people are spraying the plants with a fungicide before cutting the leaves in order to reduce spore numbers around the remaining stubs of plants, and then spraying again afterwards. Fungicide applications will not be as effective if the foliage is not removed from the plants. Because leaf moisture for several hours promotes rust spore germination, overhead watering should be avoided wherever possible. If it cannot be avoided (by use of soaker hoses etc. which keep the foliage dry) then timing should be such that the leaves remain wet for the shortest possible time - i.e. do not overhead irrigate late in the day or in the evening. Also, plant spacing should be sufficient that leaves dry quickly following rain or irrigation.

While daylily rust may kill the foliage on some cultivars, it is unlikely in the short term to actually kill the infected plant. However, it is not yet known what the effects of continuous infection will be on individual daylilies. It is possible that some may become weakened and thus susceptible to other stresses and diseases, or that bloom could be reduced. Looking into the future, steps are already being taken by some hybridizers to monitor plants for rust susceptibility in the hopes that it will be possible to breed rust resistant cultivars. Unfortunately, many Hemerocallisspecies are reported to be susceptible to the rust in its native habitat, so whether any of these will be useful in the development of new cultivars is not yet known.

Other leaf problems may resemble daylily rust. To identify daylily rust, leaf streak, spring sickness and other non-rust disorders that affect Hemerocallis, see the following web site hosted by AHS member Susan Bergeron:

The above site also contains much more detailed information on the life cycle (including images of the rust on the alternate host, Patrinia), fungicide information, a list of reported host species, images of the leaf peeling process for new plants, contact information for local assistance, notes from presentations by plant pathologists about daylily rust and much more which is beyond the scope of this definition. The AHS provides this link as a source of information. The AHS does not have any control of the content on the site.

We do not have all the answers about this fungus yet, but we will try to keep this page updated as new developments arise that may change growing methods, prevention methods and practices related to rust.

*Hiratsuka, N., Sato, T., Katsuya, K., Kakishima, M., Hiratsuka, Y., Kaneko, S., Ono, Y., Sato, S., Harada, Y., Hiratsuka, T., and Nakayama, K. 1992. The Rust Flora of Japan. Tsukuba Shuppankai, Ibaraki, Japan. Pages 710-711.

Daylily is an important nursery crop and was relatively disease-free before the arrival of daylily rust, caused by Puccinia hemerocallidis in 2000. Rust-tolerant varieties are being identified and susceptible varieties are being phased out of production. Several fungicides are effective against rust, including azoxystrobin (Heritage), propiconazole (Banner Maxx) and chlorothalonil (Daconil Ultrex). Relying on fungicides to minimize rust symptom increases production cost and concerns on potential resistance of rust to these products.

Biofungicides are biological or bio-rational alternatives that can reduce symptom by using mode of actions different from systemic fungicides. If effective, biofungicides can be used as resistance-management tools, and they are usually less expensive.

Materials and Methods: We tested K-Phite (phosphoric acid, Plant Food System), Vital (phosphoric acid, Luxembourg-Pamol), Actinovate (Streptomyces lydicus, Natural Industries), Rhapsody (Bacillus subtilis, AgraQuest), MilStop (potassium bicarbonate, BioWorks) and Citrex (organic acids, Special Nutrients Inc.) in rotation or as tank-mix partners for their preventative or early-stage curative effects on daylily rust.

Stella de Oro, a popular landscape variety, was used in two trials conducted from 2005 to 2006. This variety is moderately tolerant to rust infestation. At the Hammond Research Station, we had observed two outbreaks per year of rust symptom, in April and September on this variety for two consecutive years prior to this study. We did not use highly susceptible varieties because they are very unlikely to be in production in the future.

In each trial, the experimental design was a randomized complete block design with 4 replications (clusters). Each cluster had 5 pots placed close to each other. The center pot had one 2-year old plants showing rust symptom. The four surrounding pots were newly planted and was either cut back to 2 inch tall (Trial 1) or left un-pruned (Trial 2).

Trial 1 was conducted from April 10 to June 5 2005. Plants were naturally infested and showing early rust symptoms (rated for an average 1 = minor symptom, according to a 1 to 5 rating scale). A total of 11 treatments were applied: K-Phite (4 pt/100 gal), Actinovate (12 oz/100 gal) and MilStop (2 lb/100 gal) applied alone or at the above rates in rotation with Banner Maxx (4 oz/100 gal). K-Phite was also applied at the above rate with 1ml/gal CapSil to see if adding an adjuvant would increase its efficacy. CapSil alone, Banner Maxx alone and Banner Maxx in rotation with water were used as controls in addition to the untreated control (water). Foliar applications were made with a hand-held sprayer. Treatment solutions were sprayed to runoff except Milstop, which was sprayed so as just to wet the foliage. All rotation or tank-mix treatments were applied every 10 days for a total of 6 applications.

Trial 2 was conducted from September 15 to October 30, 2006 with the same treatments. The center plants in each cluster showed rust symptom for an average rating of 1. Leaves on the four surrounding test plants were cut back to 2 inch, and new leaves growing from these plants were sprayed with the products.

Rust was evaluated using a subjective visual rating system with a scale of 0 to 5, where 0 indicates no rust infection, 1 = observable rust infection on lower leafs (about 20% foliage infected based on number of leaves showing any yellow spots), 2 = about 40% of the foliage showing yellow spots, 3 = about 60% of the leaves showing infection with orange rust spores present of the lower leaves, 4 = most of the foliage showing yellow spots with lower half of foliage showing orange rust spores and dry leaves and 5 = plants covered with orange spots.

An overall quality rating was assigned to each plant and correlated to rust ratings. Based on this correlation, rust ratings greater than 1 represent that plants start to lose aesthetic quality (data not shown). Rust ratings equal to or less than 1 was considered still aesthetically acceptable. Ratings were assessed at 4, 5 and 6 weeks after treatment began.

Results: In Trial 1, where foliage were not cut back and new plants were placed next to infested center plant, at 4 WAT, plants treated with Actinovate alone, Banner Maxx alone, Banner Maxx in rotation with water, or its rotation with K-Phite had less severe rust than the untreated control (Fig. 1). However, none of these treatments resulted in aesthetically acceptable plants due to moderate to severe rust symptom. The role of K-Phite in a rotation program is still unclear because its effect was less significant than rotation program with water.

In Trial 2, the same treatments were applied to plants that had been cut back for new growth. Plants treated with K-Phite, K-Phite plus CapSil, and MilStop had less severe rust ratings than the water or CapSil control (Fig. 2). Actinovate plus CapSil had similar rust rating as CapSil control. However, all these treatments resulted in rust ratings more than 2.3, therefore, when used alone, these biopesticides will not provide sufficient control to grow quality daylilies. Among rotation treatments, Banner Maxx in rotation with Actinovate or MilStop resulted in similar ratings as its rotation with water, thus did not contribute to the rotation. K-phite was the only material that contributed to its rotation program resulting rust ratings as effective as Banner Maxx alone (Fig. 2).

Overall, biofungicides evaluated in this study were effective compared to untreated control under high rust pressure when applied to plants that had been cut back for new growth. However, they were less effective than the current standard Banner Maxx except that, K-Phite showed promise as a rotation partner. More trials are needed with K-Phite and phosphoric products.

Economically, if consistent effects can be obtained, adding K-Phite into a rotation program can reduce overall cost and possibly delay resistance development. Results from this study also suggest that more effective control can be obtained when fungicides are used in combination with cutting back plant foliage for new growth after rust symptoms are detected, which provides a preventative control rather than curative.

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